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Common misconceptions about stainless steel

Everyone knows that stainless steel resists corrosion, but beyond that, an amazing range of half-truths and exaggerations have evolved - often misleading and sometimes simply wrong. This article examines some of the more common myths, explains why they are wrong, and more to the point, provides correct information.

MISCONCEPTION: There are only two types of stainless steel, 304 and 316. 

FACT: There are hundreds of stainless steels from high strength duplex 2205 supporting bridges, to furnace ducts of ferritic 3/5Cr12 and the high temperature 310, but the most common types are the austenitic 300 series.

Stainless steels were invented a little over 100 years ago. The corrosion resistance, ease of cleanability, and bright appearance of stainless steels meant its compound growth since 1950 has been about 5% year-on-year. Because of the ease of forming and welding, about 70% of stainless steel use has been within the austenitic family. Within 30 years the accepted chromium level for good corrosion resistance settled at about 18% and “304” was born. Then stainless moved to the seaside and corroded, which led to the development of “316” by adding molybdenum. This in turn created the popular myth of two readily weldable and formable stainless steels despite the hundreds of austenitic grades recognised in standards.

About 25% of global use is seen in (mainly) thin sheet ferritics for cladding. The remaining 5% sees strong duplex, extra strong martensitic blades and wear resistance, and the precipitating hardening grades where strength/hardness is the priority.


MISCONCEPTION: 316 stainless steel is a marine grade and is suitable for seawater immersion. 

FACT: Seawater has about 20 times the chloride level that 316 can withstand and it is worse if the surface is rough or has a crevice (such as a nut and bolt). Seawater suitable stainless steels are the super austenitic or super duplex grades.

316 is often referred to as the ‘marine grade’ but this simply means that, provided it has a good finish and is washed by rain or under a proper maintenance regime, it will remain bright and shiny. In seawater it will rust especially around hard fouling or crevices – think seashells or bolts – and even under deposits in the splash zone. Furthermore, in severe coastal applications where salty ocean spray is allowed to build up over time, 316 can visibly corrode. 


MISCONCEPTION:  If it has rust stains, it is not stainless steel.

FACT: Carbon steel contamination or choosing the wrong grade of stainless steel are the usual reasons for rust on stainless steels.

If the rust occurs within a few days or weeks, it is almost certainly due to carbon steel contamination from fabrication or the local environment. Longer initiation periods arise from surfaces that are too rough, aggressive environments (think 304 posts on a wharf), lack of washing (drainpipes under eves) or bar product that has not been passivated. 


MISCONCEPTION:  Stainless steel reinforcement will cause accelerated galvanic corrosion of carbon steel reinforcement.

FACT: In concrete, carbon and stainless steels have similar galvanic potentials. 

Galvanic interactions occur between connected metals that have different potentials when immersed in a liquid that will cause one of them to corrode. Hence 304 bolts in a 316 panel immersed in tap water will not show galvanic effects despite the difference in potentials. In contrast, carbon steel will corrode more rapidly when coupled to copper or stainless steel in water. It is different in alkaline concrete as both stainless and carbon steel are at the same potential. It is common practice to use stainless steel reinforcement in tidal and splash zones or around penetrations and couple it to the rest of the carbon steel reinforcement.


MISCONCEPTION:  Only non-magnetic stainless steels have good corrosion resistance.

FACT: Magnetism is not related to corrosion resistance.

Probably because the lower chromium stainless steels are all magnetic, e.g., the 3/5Cr12 utility grades or the 410 or 420 or 440 martensitic, a myth perpetuated that magnetism and corrosion resistance were related. And then along came duplex grades with their resistance to seawater (and more aggressive environments) plus a strong ferromagnetic effect. The weak magnetic effect of heavily cold worked 304 versus the negligible magnetic effect of cold worked 316 may also have contributed to the myth.


MISCONCEPTION: Low nickel in stainless steels means it will corrode.

FACT: Nickel only affects the microstructure form, NOT corrosion resistance.

Nickel is a friendly metal and is the predominant influence in turning  ferritic stainless steel into austenitic or duplex grades depending on how much nickel is added. It has no effect on corrosion resistance to initiation of corrosion, which is how the integrity of stainless steel is judged.


MISCONCEPTION: Well-polished stainless steel does not require maintenance. 

FACT: Maintenance is important for the long-term effectiveness of any product. Stainless steel requires minimal maintenance but relies on preserving its passive film with oxygen and water.

Maintenance of stainless steel is required i.e., cleaning to remove adherent deposits left after rain washing. High polish will ease maintenance cleaning, but in the long term, general grime can accumulate just like the detritus on coatings or concrete.


MISCONCEPTION: Using a 316 nut on a 304 bolt stops galling of fasteners.

FACT: Austenitic stainless steels are widely used for corrosion resistant bolting, but galling control requires consideration beyond materials selection, including hardness, design and quality control, lubrication and friction.

Galling of fasteners is simply the cold welding of clean stainless steel surfaces under load. It is worse with fine threads, tight clearances, poor profiles, lack of lubrication, accumulated dirt and over-tightening. Because 304 cold works more than 316, the rule of thumb that a 50HB difference in hardness would prevent galling leads to the 304 cold rolled bolts and 316 machined nuts combination as a “solution”. It may work but the list of caveats above shows its limitations. 


MISCONCEPTION:  Stainless steel is expensive.

FACT: The initial capital cost of stainless steel material may be a few percent more but, when considering life-cycle costing, stainless steel delivers long-term performance with minimum downtime and low costs associated with maintenance.

Using stainless steel does not require coatings, has reduced maintenance requirements compared to repainting or patch repairs of coated or galvanised steel, and can either be repurposed or recycled after its practical life. For example, replacing galvanised steel with 304/316 stainless steel in one particular wastewater treatment plant reduced downtime for refurbishment or replacement from 22% to a mere 2%.

There is a stark contrast between the maintenance of iconic structures built from different materials. The Eiffel Tower in Paris was constructed in iron and Sydney’s Harbour Bridge is the world’s largest (wrought) steel arch bridge, but both structures require regular repainting as part of essential maintenance. New York’s Chrysler Building is clad in stainless steel and has only required two washings in its 90-year history. 

The Schaffhausen Bridge in Switzerland was built in 1995 with duplex reinforcement in the lower 7.6m of the pylons and 304 in the longitudinal reinforcement because of concerns about road de-icing salts. A life-cycle costing over 80 years showed that with stainless steel used for about 5% of the steel tonnage, stainless steel delivered 13% lower life-cycle costs over carbon steel.

Looking at sustainability, the Tokyo Water Authority reduced leakage in their potable water distribution system from 15.4% in 1980 to 3.6% in 2019 primarily by replacing mains to meter connections with corrugated stainless steel tube. To put this into context, since 1994 Tokyo has reduced annual water leakage by nearly 142 million cubic metres, with savings in excess of US$200 million per year. 


 This article is featured in Australian Stainless Magazine issue 73, 2021.

Structural design of stainless steel

Stainless steel is used for a wide range of structural applications including:

  • Beams, columns, platforms and supports in processing plant for the water treatment, pulp and paper, nuclear, biomass, chemical, pharmaceutical, and food and beverage industries;

  • Primary beams and columns, pins, barriers, railings, cable sheathing and expansion joints in bridges;

  • Entrance structures, canopies, cladding and support systems for masonry;

  • Security barriers, blast walls, hand railing and coastal structures. 

Case studies of a range of structural applications are available at the case studies section of  www.teamstainless.org/resources/information-center-for-stainless-steel-in-construction.

This introduction to structural design in stainless steel aims to highlight differences between the material properties and structural behaviour of stainless steel and conventional carbon steel normally used for structural purposes, e.g. grade 350 to AS 3678 and AS 3679.

It should be noted that stainless steel structures should not be simply designed using design standards for carbon steel, such as AS 4100 and AS 4600, because of the significant differences between the mechanical properties of carbon and stainless steels.

Selection of an appropriate alloy of stainless steel is the first step in any design process.

Austenitic stainless steels are most widely used for structural applications, though the use of duplex stainless steels is increasing, where it is possible to exploit the benefit of high strength (around 460 MPa, compared to a strength of around 220 MPa for austenitic stainless steels). This can be particularly valuable in weight-sensitive structures like bridges or on offshore topsides. Duplex stainless steels are more likely to be used in heavier gauges. Ferritic stainless steels are also suitable for structural applications, offering a corrosion resistant alternative to many light gauge galvanised steel applications. They are generally used in gauges of 4 mm and below although the 12% chromium utility alloys are used in thicker sections (vehicle chassis or high temperature ducting) when minor rust staining can be allowed.

Material properties

From a structural viewpoint, the main property that distinguishes stainless steel from carbon steel is the stress-strain response. In contrast to carbon steel, for which the stress-strain curve may be modelled as bi-linear for most compression and flexural member design purposes, the stress-strain curve of stainless steel is generally highly non-linear and without a distinct yield point. Figure 1 compares the stress-strain characteristics of various stainless steels with carbon steel for strains up to 0.75% and Figure 2 shows typical stress-strain curves to failure. (The figures show stress-strain curves which are representative of the range of material likely to be supplied and must not be used in design.) The distinctive mechanical properties - considerable strain-hardening and ductility - make austenitic and duplex stainless steel particularly well suited for structures required to withstand accidental loading due to their high energy absorption characteristics.

In the absence of a clear yield stress, it is common practice to define an equivalent yield stress for stainless steel by using a proof stress, usually the 0.2% proof stress. (By definition, the plastic - or permanent - strain at 0.2% proof stress is 0.2%.) The proportional (or linear) limit of stainless steels’ deflection ranges from 40 to 70% of the 0.2% proof stress.

As a result of the non-linearity, stainless steel loses stiffness at low stress levels. This affects the design rules for members that rely on stiffness to transfer loads, notably compression members and unbraced flexural members.As well as nonlinearity, the stress-strain characteristics of stainless steel also display non-symmetry between tensile and compressive behavior and anisotropy, i.e. differences in behaviour of coupons aligned parallel and transverse to the rolling direction. In general, anisotropy and non-symmetry increase with cold work and so are more significant in the design of lighter gauge heavily worked sections, rather than thicker walled structural sections.

It is possible to enhance the strength of austenitic stainless steel by cold-working to a much greater extent than for carbon steel.

The initial modulus of elasticity (Eo) of stainless steel alloys is slightly lower than that of carbon steel.

The behaviour of stainless steel at elevated temperatures differs to that of carbon steel because of the metallurgical differences caused by the composition. Stainless steel retains a greater proportion of its strength at temperatures above about 550 °C and shows better stiffness retention at all temperatures, which is important in design against fire for components such as blast and fire walls.

The coefficients of expansion (CTE) of austenitic stainless steel alloys are larger than those of carbon steel. At the same time, the thermal conductivity is lower. While the CTE is important in determining thermally  induced stresses and deformations, the combination of larger coefficient of expansion and lower thermal conductivity has the effect of substantially increasing the risk and possible extent of welding distortions than those experienced in fabricating carbon steel structural member. The duplex grades have similar thermal conductivity to the austenitics but with 20% lower CTE so the risk of welding distortion is slightly lower than with austenitics. 

Specifications and reference documents for design of  stainless steel structures

American Society of Civil Engineers (ASCE) has revised ASCE 8 Specification for the design of cold-formed stainless steel, applicable to lighter gauge austenitic and ferritic material in the annealed and temper-rolled condition (Reference 1). The 2002 version has been substantially updated because of extensive research work and will be issued late in 2021. This includes alternative treatments of compressive loading, i.e. effective width and direct strength. The structure of AISC 8 will be familiar to those using AS/NZS 4673:2001 although 4673 has now been withdrawn as an aged standard.

Also in 2021, the American Institute of Steel Construction (AISC) will release a new standard (reference 2) AISC 370 Specification for Structural Stainless Steel Buildings to reflect the substantial increase in the use of heavy structural stainless steel sections. It includes hollow sections as well as welded, hot rolled and bar products. It will be accompanied by AISC 313 Code of Standard Practice for Structural Stainless Steel Buildings (Reference 3) and an updated 2nd Edition of the 2013 AISC Design Guide 27: Structural Stainless Steel.

The Eurocode for stainless steel design, EN 1993-1-4, covers welded, hot rolled and cold formed products made from austenitic, duplex and ferritic alloys, at room temperature and in fire (Reference 4). The Design Manual for Structural Stainless Steel (4th Edition) was published in 2017 and gives essential information needed by designers concerning alloy selection, durability, material properties, design rules and fabrication, in accordance with EN 1993-1-4 and other European standards (Reference 5). A Commentary explains how the design expressions in the Recommendations were derived and gives background information and references. Design Examples demonstrate the use of the Recommendations. Section property and member capacity software is also available, all aligned to EN 1993-1-4.

This Design Manual and these supporting design tools are freely downloadable from www.steel-stainless.org/designmanual.

This article has been extracted from the 2020 Australian Stainless Reference Manual, available for purchase at assda.asn.au

REFERENCES: 1. ASCE 8-02 Specification for the Design of Cold-Formed Stainless Steel Structural Members, SEI-ASCE 8-02.  \   2. AISC 370-2021  \   3. AISC 313-2021  \  4. EN 1993-1-4:2006+A1:2015 Eurocode 3. Design of steel structures. General rules. Supplementary rules for stainless steels.  \   5.  Design Manual for Structural Stainless Steel, SCI Pubilcation P413, The Steel Construction Institute, 2017 (available from www.steel-stainless.org/designmanual).



This article is featured in Australian Stainless Magazine issue 72, 2021.

Comparisons of hot and cold formed stainless steel

When comparing hot and cold formed stainless steel, the first question you would ask yourself is: are there any chemical differences between the two? ASSDA has previously published articles on the various surface finishes including the few hot and multiple cold finished processes, however this article concentrates on the differences. 

Since the 1970s, most stainless steel is produced by melting in an Electric Arc Furnace (EAF) and then the molten stainless steel is transferred to an Argon Oxygen Decarburisation (AOD) vessel or, less commonly, a Vacuum Oxygen Decarburisation (VOD) vessel. These processes control impurities such as carbon, sulphur, nitrogen, hydrogen and other impurities which could form oxide precipitates. For critical applications such as aerospace or precipitation hardening alloys, further refining is possible, but this is a smaller market. Critically, in AOD, using the injection of inert argon as the stirring agent into the melt allows control of nitrogen additions, e.g., for duplex or high austenitic stainless steels.  

Chemistry

The basic chemistry of a specific grade of stainless steel is the same regardless of how it is subsequently shaped, i.e., as hot or cold rolled, hot forged, cold drawn/shaped or simply cast –  with the proviso that cast stainless steels typically have more chromium and (often) more silicon than their wrought counterparts. In addition, even if cast products have been stress relieved, their microstructure is dendritic so that there can be significant composition differences (localised differences in corrosion susceptibility) between the early solidifying dendrites and the final bulk solidification.

Hot formed

Producing hot formed stainless steel is deceptively simple as shown in the graphic. Currently, for about 95% of production, the molten metal is decanted from the AOD into a cooled continuous caster and emerges horizontally as a slab. The microstructure in the slab is columnar from the outsides (because of the cooling by the caster walls) with a relatively uniform equaxial microstructure in the centre. It is also covered with heavy black oxide scale with a chromium deficient layer underneath – as with weld tints. Typically, the slab is then surface ground to remove solidification features and the scale which would both otherwise be incorporated into the surface during subsequent rolling. The slab is then charged into a reheating furnace and hot rolled to homogenise the microstructure and provide either plate or coil as the product form. The grains will now be more oriented along the rolling direction. The microstructure is then further refined (and internal stresses reduced) by annealing above 1000oC.

Unless the steel is to be used where appearance and maximum corrosion resistance is not critical, e.g., in a furnace, then after hot rolling it would be shot blasted to break up the scale and then pickled. The pickling causes the dimpled appearance of a Hot Rolled Annealed and Pickled (HRAP) plate surface because the pickling acid attacks the base metal at the defects in the black oxide scale. It also gives a typical surface roughness of 5 or 6 µm Ra. There is a potentially cosmetic corrosion issue if the (typically) fast pickle does not completely remove any shallow intergranular oxide penetrations, however this is unusual. For long product such as angles or channels, another visual distinction is that the edges meet at 90o compared to the radius of curvature determined by the thickness and ductility of a cold rolled product.

Cold formed

Cold rolled flat product is quite different because it usually starts at room temperature with a dimpled surface and finishes with thinner material – and watching the increase in speed of the sheet as it gets thinner is quite startling. There is a significantly more elongated microstructure along the rolling direction, and this enhances the anisotropy in transverse to longitudinal strength compared to the relatively slight effect for hot rolled material. Long product is not cold rolled but more correctly it is cold  shaped or formed.

The increase in strength with cold work can be substantial, especially in thin materials as the cold work increase can enhance the strength of the full depth. As an example, the table from ASTM A666 data shows the substantial change in mechanical properties for 304 from annealed to half hard, i.e. half the absolute maximum possible strength – which would have negligible ductility.

Effect of cold work on strength and ductility of 304

One effect of the increase in strength with cold work is that the limit of proportionality will increase with cold work, i.e., the linear deflection occurs up to a higher stress. The reduction in break elongation is simply reflecting the proof stress closing on the tensile stress from 40% to 73% as shown below.

Variation of thickness tolerances for cold and hot rolled materials

There are also differences in the tolerance between cold and hot formed material as shown by comparisons in A480 (flat product – sheet vs. plate) and A484 (sections). However, it is not as simple as “hot formed is less precise than cold formed” as seen below. 

Surface finish

Often it is important to consider appearance and corrosion resistance to the “bright and shiny” benchmark. Hot rolled material is always going to be dimpled, even when it is electropolished and exhibiting a brilliant lustre. It will be marginally more difficult to clean than a smoother cold rolled surface but abrading the surface to give a roughness of less than 0.5 µm Ra is counterproductive. You lose the passive stainless steel and an abraded surface potentially has sulphide inclusions exposed compared to the original pickled, sulphide free HRAP surface.

The inherent roughness of a cold rolled sheet decreases steadily as the plate is rolled thinner as shown in graph. It shows the decrease of Ra from the crushing of the peaks left from the hot rolling as the sheet gets thinner. The graph is for cold rolled annealed and pickled (2B) material and is useful when someone asks for a 2B finish for a thickness not in the band. However, thickness of pickled materials is not relevant to the corrosion resistance whereas the Ra is critical to an abraded surface both for reasons of cleanability and possible crevices from torn surface flaps plus, if not passivated, exposing sulphide inclusions.

 

This article is featured in Australian Stainless Magazine issue 71, 2021.

 

Seven ways to prevent tea staining of stainless steel

When used properly, stainless steel enjoys a strong and enduring reputation for visual appeal and structural integrity in a wide range of applications and environments. But, like all materials, stainless steel may become stained or discoloured over time, impairing the overall look. This brown discolouration - tea staining - has been identified in coastal applications in Australia and overseas.

In the late 1990s, the newly formed ASSDA Technical Committee researched the reasons for the brown discolouration. ASSDA’s work, in collaboration with the International Molybdenum Association, led to guidelines published in 2001 explaining the causes and remedial techniques. The work was later refined to ASSDA’s FAQ 6. This clarified some of the misunderstandings that have circulated about atmospheric exposure and possible corrosion of stainless steels. It does not deal with immersed exposures.

Corrosive chemicals in the atmosphere

In a clean atmosphere, the relative humidity is sufficient to maintain the thin self-repairing passive film which protects stainless steel. Given that chlorides are known to cause corrosion, locations within line of sight of the sea will tend to corrode, especially with onshore prevailing winds. 

Extensive research using corrosion coupons has classified corrosivity using carbon steel and zinc while measuring corrosives such as chloride and sulphur dioxide – as an indicator of industrial activity. AS 4312 presents maps with bands of corrosion rates (from extreme to low) and corrosion rates with distance from the sea or a bay.  For sheltered waters, corrosion rates drop to medium or lower within a few hundred metres, while for exposed areas like Cape Leeuwin in WA, Newcastle in NSW, or Cape Jervis in SA the rates can remain extreme to high for kilometres inland.  

The AS 2699 series of standards for ties used for brick fixing in walls uses deposited chloride measurements to arrive at similar bands of corrosivity. Those values define zones where increasing thickness of galvanizing or, 304 or 316 must be used successively depending on distance from the sea to achieve the design life in buildings. In rural or urban environments, corrosion of stainless steel is unusual although cosmetic staining can occur from vegetation, chemical spills or blown dust.  

Atmospheric conditions

Stainless steel will not corrode unless there is a sufficient concentration of aggressive ions (generally chlorides) and there is free water but not a sufficient flow to wash away contaminants. Water will condense on clean metal if the temperature falls below the dewpoint  but if there are chloride salts, then there is a lower critical relative humidity (RHcrit) which will form an aggressive salt solution by absorbing moisture from the air. This means that deposited sea salt can cause corrosion although the temperature is still below the dewpoint for a  clean surface.  

A secondary atmospheric factor is that corrosion rates roughly double for every 10oC rise in temperature; all else being equal. For instance, corrosion staining will be worse in Darwin than in Hobart.

Design, cleanability and drainage 

Open, bold exposures allow natural rainfall to wash away grime and potentially corrosive deposits. However, drying retained runoff on horizontal surfaces, surface tension at sharp edges, in crevices, or in horizontal abrasion lines on vertical surfaces, all cause increasing corrosivity and do not meet the “bright and shiny” expectations for stainless steel.

Surface roughness and standards

The widely publicised surface roughness of “no more than 0.5µm Ra” for a tea staining resistant surface is codified as 2K in EN 10088.2 which includes a requirement for a clean cut profile, i.e. no flaps or sharp edges. ASTM A240/A480 suggests that a good fabricator can achieve about 0.6µm Ra for a No. 4 finish.  ASSDA conservatively recommended finishing with 320 grit for 0.5µm. Ra is readily measured mechanically or by laser reflectance but, because multiple surface profiles can give the same Ra value, samples are frequently used for acceptance tests. Rough surfaces show significant tea staining.

Unfortunately, the 0.5µm Ra requirement is sometimes specified for HRAP plate with a typical finish of Ra ~6µm. This causes un-needed expense in removing metal. Both thick HRAP plate and thin 2B (or BA) sheet or coil were pickled as their last processing step. Even with Ra greater than 0.5µm these finishes have good resistance to tea staining. However, a rougher surface is still less cleanable and is generally not as bright. Smooth 2B or BA finished sheet has good resistance to tea staining.

There is ample evidence that smoother surfaces resist corrosion better and a mirror polish is the best possible mechanical finish. ASTM A240 and EN 10088.2 have descriptive definitions (for No. 8 or 2P respectively) which centre around high reflectivity and image clarity but do not specify Ra.

Electropolishing electrochemically cleans the surface, removes sharp edges and smooths microroughness. It provides a deep lustre but not necessarily a clear reflection. However, it only slightly reduces Ra, e.g. 0.7 to 0.5 µm for a linished strip. Electropolished surfaces have excellent resistance to tea staining.

Chemical cleanliness of the exposed surfaces

Stainless, carbon or galvanised steels may be fabricated in adjacent areas and carbon steel can contaminate the stainless steel either as floating grit or if tooling is used on stainless steel after processing carbon steel. Contamination during transport or handling is also possible. Any moisture will immediately corrode the carbon steel and cause a large brown stain compared to the contaminating particle - this is not tea staining. The illustration shows a rooster tail of carbon steel contamination on a stainless steel surface from cutting Colorbond® nearby. The contamination can be removed by pickling or, more slowly, by a passivating nitric acid application. The widespread pale spots are light tea staining.

In addition, an oxidising passivation treatment after the final metal removal substantially improves corrosion resistance, partly because the final passive film is thicker and has a higher chromium to iron ratio. However, for freshly abraded surfaces, the sulphide inclusions inherent in all steels are initiation sites for corrosion unless they are removed by a passivating acid. Bar product has more inclusions with at least 10 times the sulphur content of sheet or plate to aid machining. Nitric passivation does not change the appearance of even a mirror polished surface but does significantly increase its corrosion resistance. Further details of pickling and passivation are given in ASTM A380.

What alloy should be selected

An ASSDA/IMOA selection tool is available by searching for ‘stainless-steel-selection-system’ on www.imoa.info. It asks about environment, finish, orientation and maintenance before providing a recommended material from 304 for urban or rural exposures up to 2205 for severe marine. It does not include ferritics mainly because they are not as widely used in welded fabrications. Alloys with similar or higher corrosion resistance (as assessed by PRE) could be substituted. Super alloys are not usual for atmospheric exposures.

Maintenance – natural and/or applied

Stainless steel is low maintenance but not no maintenance. A rule of thumb is that if an adjacent window (or glass screen) needs washing, wash the stainless steel. FAQ 6 has recommendations including consideration of retained deposits and rain washing. One sub-tropical beachside council has implemented a 3 to 4-month cycle of a high pressure wash with low chloride water and detergent (and possibly a zero chloride solvent) before a fresh water rinse. Domestic cleaners, even non-abrasive ones, are not recommended as some have chloride activators and, because of their hygiene image, may have some bleach - which are both potentially detrimental.

This article is featured in Australian Stainless Magazine issue 70, 2020.

 

Coloured and patterned stainless steel

Think stainless steel, and most people think ‘bright, shiny and silver’. But did you know that specifying stainless steel is not limited by its silver appearance?

Coloured and textured stainless steel is an exciting material choice for designers and architects. In addition to offering a high quality and aesthetically-pleasing finish with choice of colour, stainless steel’s superior benefits when compared with plastics or anodised aluminium include resistance to heat, light, abrasion and corrosion, and overall increased durability and performance extending the service life of the application.

This article will take a look back at the development of coloured stainless steel, detail the electrochemical colouring and PVD coating processes, and explore the various surface textures available.

History and development

Back in late 1960s, INOX developed a process for uniformly colouring smooth stainless surfaces. The colour relied on the growth of a uniform oxide-based film in a sulphuric and chromic acid mixture. The colour changed because of the interference of reflections from the top of the layer and the metal underneath it. It is like the colours in a soap bubble or an oil film, except that the INOX film had a very uniform thickness. This is because it is grown under uniform temperature and flow conditions with tightly controlled chemistry. Because the colours were subtractive rather than additive, they were not the same as a rainbow spectrum, but colours ranged through bronze, blue, black, charcoal/grey, gold, purple and green as the film grew from 20nm to 360nm. The colours also varied slightly with viewing angle because of the interference process that gives the colour.

Initially, there were two limitations and two caveats. Firstly, the coatings were easily abraded so it should not be used in heavily trafficked areas because any mechanical damage could not be repaired. Secondly, it was initially only grown on 304. And the caveats? The tight thickness limits mean that batch-to-batch colours could have slight tint variations although, this also has been exploited to provide a softer colour image.

Electrochemical colouring

Within a decade, a dual stage process was developed with an electrochemical treatment that provided greater abrasion resistance. Research in Australia showed that, for 304 base material, the film provided a slight improvement in corrosion resistance although the change is not as significant as a passivation process. Further developments showed that coloured films could also be formed on 316. The necessity for a uniform film thickness still requires factory treatment which means that it is limited to sheets or round surfaces such as tubes. Nevertheless, building facades, shopping centres and smooth surfaced art works were able to display a variety of stainless steel colours, even when the coloured stainless steel has been carefully bent into shapes.

These colours are very durable, even in Australia, as they do not fade with UV exposure and, in a graffiti-infected urban environment, solvents can be used to remove tags and other unwanted additions to coloured facades and signs. However, they are not repairable if scratched and can only be mechanically fixed as welding locally destroys the coloured film.

Surface Blackening

Do the arms of your black windscreen wipers use this colouring process? Well, no. The rich, glossy black used to be from immersion of stainless steel in molten sodium/potassium dichromate at 400oC for about 30 minutes but is now usually replaced by a 180oC cured organic coating. Shorter immersion times were used for thermal solar water heaters but they are now either painted or plastic - although black chrome has had a place in the market. 

Physical Vapour Deposition (PVD) coating

The second major method of colouring stainless steel is PVD or Physical Deposition of a Vapour – hence PVD. The process is carried out in a high vacuum chamber with a small amount of (usually) argon gas. The gas is ionized by a high negative voltage on the target and forms a plasma of electrons and positive ions which bombard the source metal and ejects (or sputters) metal ions or atoms. These are deposited on the product to form a thin (typically 300nm) coating on the clean product. It is critical that the coated surface is free from contaminants or the coating will lack adhesion. It is routinely used to hard coat small objects like drills but, on a larger scale it produces coloured door furniture or objects whose size is only limited by the vacuum chamber. Coating larger objects and sheet material requires greater electron ionization efficiency in the plasma which typically uses magnetic fields parallel to the surface of the target. 

The source metal can also be generated by thermal evaporation but this is less common.

Unlike the electrochemical INOX process, the colour of the PVD coating is determined by the source material with a few examples shown in the table. It is also invariant with viewing angle. PVD coatings are much more abrasion resistant than the INOX system but are not indestructible. 

Patterned stainless steel: Surface texture and its effect

A range of embossed, patterned and textured stainless steel finishes are available. Hot rolled finishes are usually too dimpled for aesthetic finishes. Cold rolled mill finishes are smooth and either dull grey (2B) or very bright (BA – bright annealed) and provide differing basic appearances but the same mechanical properties. Both have significantly better corrosion resistance than as-abraded finishes. Aesthetic changes by abrasion or blasting will provide feature finishes but have only minor effects on the colour and mechanical properties although rough 

as-abraded surfaces are known to be less corrosion resistant, i.e. the 0.5 micrometre Ra criterion.

Mechanically embossed profiles on austenitic mill rolled finishes might reduce the cleanability, but they also increase the strength because of cold work strengthening - while retaining the base metal corrosion resistance. This strengthening means that thinner material can be used, such as the thin checker sheet used in toolboxes – good visuals and lower weight in the utility with security for tools. Profiled sheet for outdoor public seating is another application with thinner sheet because of the strength and a bright appearance without glare.

High wear areas such as airport baggage collection or hospital corridors often use rigidised stainless steels where a through sheet profile significantly increases strength and stiffness with a pleasing aesthetic. An added advantage for profiled finishes is that scratches only affect the peaks and are less apparent partly because they are not continuous. On a grander scale, the Petronas Towers in Kuala Lumpur, Malaysia is a more complicated Cambric profiled finish on a base 316 metal with BA stock. The profiled finish is to avoid blinding reflections while using a 316 base metal with a mill finish that has the highest corrosion resistance available.

 

Bus station seat with colourful anodised aluminium arms

Petronas Towers in Kuala Lumpur, Malaysia
Photo credit: Outokumpu

University of Florida (UF) Health Shands Children’s Hospital
Photo credit: International Stainless Steel Forum (ISSF)

Banner image - Westfield Doncaster, Victoria
Photo credit: Steel Color Australia

This article is featured in Australian Stainless Magazine issue 69, 2020.

 

AS 1528:2019 - A new edition pitched at food safety, consistency, useability and current practice

The aim of AS 1528: Stainless steel tubes and tube fittings for food processing and hygienic applications is to standardise hygienic tube and fittings for use in dairy, food and beverage manufacturing. It has been successful in maintaining the required food safety standards in Australia and New Zealand.

AS 1528 was first issued in 2001 and developed by an ASSDA group of stakeholders in the manufacture, supply, fabrication and use of stainless steel tube and associated fittings in the food manufacturing industries.

Changing industry practice, some existing errors, internal consistencies and expansion of sizes required a revision of the standard. The drafting journey to bring AS 1528 up-to-date began in 2015 and has been a challenge, but its successful outcome is significant for the industry and a testament to everyone involved.

The new edition of AS 1528 was published in four parts by Standards Australia in October 2019:

Part 1: Tubes

Part 2: Screwed tube couplings

Part 3: Butt weld tube fittings

Part 4: Clamp tube fittings

The revision of the AS 1528 suite of standards from the 2001 edition has brought the documents' technical coverage up to current practice and recognised the target industries in which hygienic tube is used. The suite is easier to understand and use, and facilitates verification of product compliance so that it achieves the required hygienic conditions.

 

What the revision achieved

The 2019 edition achieved all of the original aims, except one (see below). The suite of four standards now presents as a consistent coverage of all the tube and fittings regularly supplied in Australia.

  1. Addition of a consistent set of pressure ratings across all parts of AS 1528. Useful for designers.
  2. The wall thickness tolerance for tube has been changed. Previously it was +nil/-0.10mm for all sizes of tube. Widening it out to ±10% brings it into line with most other tube specifications and makes it more economical to manufacture without compromising product quality. It also then matches the tolerances of the fittings in other parts.
  3. The title now includes 'hygienic applications' in addition to food processing. This recognises the wider range of applications in which these products are already used.
  4. The reference to duplex stainless steels has been removed. In practice all tube and fittings referenced by these standards are austenitic.
  5. All tube and fittings can be produced without grit polishing the internal surface. Internal surface finish is specified by measurable roughness for hygiene cleanability.
  6. Inner tube surface roughness has been set as 0.8µm Ra maximum; this is consistent across all four parts of the standard and is also consistent with US and European specifications. From a gleanability perspective this is adequate. In addition there is now a specified maximum roughness for the inner weld bead, specified as 'Rt'. This is an unusual specification but it does address directly the requirement for cleanability of the remnant weld line.
  7. For the first time there is a stated limit for inner weld surface heat tint (no more than Level 3 in AWS D18.1M, commonly referred to as 'pale straw'). Again this aligns with US and European standards and much research work promoted by ASSDA and others.
  8. Consistent working pressures and temperature ranges have been given for all tube and fittings, with the exception of clamp fittings above 152.4mm.
  9. The range of sizes has been expanded generally up to 304.8mm or 12" diameter, but lesser maximum sizes for certain fittings, depending on market availability. Smaller diameter tubes have also been included as these have some niche applications. Additional wall thickness have been added. It is not anticipated that there will be a sudden move ways from the usual 1.60mm WT and the common OD range, but there were some industry requests for the expanded size range.
  10. Part 2 covering screwed couplings has been completely restructured. The two fundamental types - RJT and IDF/Trapezoidal - are clearly separated, with all dimensional specifications included in Sections 2 and 3. Section 1 deals with the requirements common to both types.
  11. Fittings not previously recognised have now been included. This includes both RJT blank hexagonal nut and an IDF blank cap in screwed couplings (AS 1528 Part 2). Butt weld fittings (Part 3) has addition of crosses, equal radius tees and 45 degree tees. In clamp fittings (Part 4) an end cap has now been included.
  12. The branch lengths of reducing tees and crosses (Part 3) have been clarified. The previous edition have a specification for this dimension that was in some cases contradictory and in all cases confusing. The new requirement is that the branch length, measured as the extension beyond the run surface, is the same as the branch OD.
  13. Reducers, both concentric and eccentric (Part 3), now include the option of a short extension to enable orbital welding.
  14. Reducers are now standardised as 'short reducers', with the 'full flow' reducers still specified but in the absence of request the standard type is short.
  15. New appendices in Part 4 cover a very useful description of clamp conditions for correct installation (App C), specification of grooves for expanded-type clamp liners (App D) and the method for expanding (App E).
  16. Correction of a long list of typos and inconsistencies in dimensions.

 

What was not achieved

The New Zealand market is already using AS 1528 and keen to have it branded as their own, but early discussions between the committee, Standards Australia and Standards New Zealand revealed the cost imposed by Standards Australia to make the project a joint cross-Tasman effort was prohibitive. As a result, the project became simply Australia, but the committee was able to co-opt a New Zealand member, and a tube manufacturer active in both Australian and New Zealand was also included as a Drafting Leader. The project therefore included New Zealand input, even though the document is branded Australian. The committee was mindful that there is substantial cross-Tasman movement of tube and fittings, of manufactured processing equipment, of engineering expertise and of food product, so joint output was essential to maximise all-round benefits.

 

Why this revision was important

The AS 1528 suite is the only fully integrated set of standards to the Australian industry's traditional dimensions for stainless steel tube and tube fittings for hygienic applications.

The Australian food manufacturing industry is critical both because of our high standards for domestic consumption and also as a very significant export earner. Australia has a clean and green reputation that only thrives if we can guarantee freedom from contamination. 

All the commonly used and some niche tube and fitting products are covered and all are consistent.

Food manufacturing plant is getting bigger, so this edition includes provision of larger size tube and fittings. The applications are also increasingly diverse, so an expanded range of products is appropriate.

This revision presents manufacturers of tube and fittings with a clear, consistent and measurable standard for these critical products. The standard offers a pathway to economical outcomes for tube and fittings manufacturers, designers, installers and asset owners.

 

This article was written by Technical Consultant and AS 1528 Committee Chairman, Peter Moore.

This article is featured in Australian Stainless Magazine issue 68, 2020.

Shielding gases for welding and their effects on stainless steel properties

Shielding gases form an integral part of all conventional welding processes. 

They serve multiple functions but are primarily there to shield the weld pool from the atmosphere and to provide a medium which can allow the flow of electricity from an electrode to a workpiece. Even processes that do not have an external gas supply such as Manual Metal Arc Welding (MMAW or MMA or SMAW) and Gasless Flux-cored Arc Welding (FCAW) all have a shielding gas which is generated by the decomposition of the flux in the presence of the welding arc.

The shielding gas can also have an effect on arc stability, weld shape and depth of penetration as well as the mechanical properties and metallurgy of stainless steel weldments.

The gas shielded processes such as Gas Tungsten Arc Welding (GTAW or TIG) and Gas Metal Arc Welding (GMAW or MIG) use shielding gases of a variety of compositions depending on the application. As the electrode in GTAW is made of tungsten, the shielding gas is typically argon or helium to prevent oxidation of the electrode. This restriction does not apply to GMAW and therefore the gas composition may include active gases such as carbon dioxide and oxygen. Small quantities of other gases such as nitrogen and hydrogen can be utilised with both of these processes as they are particularly advantageous for the welding of stainless steel. While neither gas is inert by definition, they can be used with GTAW as neither react with tungsten.

There are three key properties of the shielding gas which control the way the weld pool behaves; the ionisation potential (how easily an atom will give up an electron), the thermal conductivity of the gas, and finally the surface tension between the weld pool and the shielding gas.

Ionisation potential

The shielding gas allows transfer of electrons between the electrode and the workpiece. Upon arc initiation, electrons are emitted from either the workpiece or the electrode depending on which is positively charged. These electrons collide with gaseous atoms which results in these atoms liberating one of their electrons which results in a chain reaction that sustains the arc. The ionisation potential of the gas is the ease with which they will give up an electron. ‘Hotter’ gases are those which require more energy to ionise or release an electron. Helium has a higher ionisation potential than argon, so has a higher arc voltage and hence a higher heat input for the same current and arc length. 

A similar principle applies to molecular gases (H2, N2, O2, CO2) which dissociate in the arc into individual atoms and then recombine upon cooling, releasing energy in the process. Argon is often mixed with small amounts of other gases to improve weld penetration.

Thermal conductivity

The thermal conductivity of a shielding gas affects its ability to transfer heat across the arc. It influences the radial heat loss from the centre to the periphery of the arc column as well as heat transfer from the arc to the molten weld pool. Gases with low thermal conductivity such as argon will tend to have a narrow hot core in the centre of the arc and a considerably cooler outer zone. The result is a weld with a narrow ‘finger’ at the root of the weld and a wider top. On the other hand, helium has a high thermal conductivity, so heat is more evenly distributed across the arc, but as a result the depth of penetration is lower. Mixing gases allows combination of the advantageous properties of each gas while limiting the drawbacks.

 

 Surface tension

Surface tension affects the bead profile of a weld. Picture how water beads up on a newly polished car. This is undesirable in welding as it creates a steep angle between the weld and the parent which could lead to defects such as undercut, lack of sidewall fusion and decreased fatigue performance. This is another reason why pure argon is not used as a shielding gas for the GMAW process. 

Gas components

Oxygen

Though seemingly counterintuitive as it is well known that hot metals oxidise, small amounts of oxygen are often added to shielding gasses for the GMAW process. Small amounts of oxygen reduce the surface tension between the molten weld pool and the surrounding atmosphere. Lower surface energy results in a flatter and smoother weld bead with less tendency to undercut the parent metal. To minimise alloy losses by oxidation, oxygen content is typically limited to 2%. The heat tint will be more severe than for a weld without oxygen additions to the shielding gas.

Carbon Dioxide

GMAW also utilises CO2 as a constituent of the shielding gas. A common concern with stainless steels is embrittlement and corrosion through sensitisation due to chromium carbide formation, but the carbon pickup from CO2 has been demonstrated to be low enough that the resultant weld metal still achieves the required (≤0.03%) carbon content for L grade designation. The chosen CO2 content is therefore more about penetration and wetting than it is about carbon pick-up. Carbon dioxide contents in GMAW are typically 2-5% while flux-cored wires utilise 20% mixtures with argon or even 100% CO2

Hydrogen

Unique to austenitic stainless steels is its immunity to hydrogen cracking – except possibly in very heavily cold-worked material. This allows the addition of hydrogen to the shielding gas in quantities from 2–15% providing more heat in the arc and better penetration. Hydrogen quantity for manual welding is usually restricted to 5%, with the higher concentrations limited to automated process such as orbital GTAW. Hydrogen cannot be used as a component of the shielding gas for ferritic, martensitic or duplex stainless steels due to a risk of cracking. 

Nitrogen

Nitrogen is a useful shielding gas additive for duplex stainless steels which contain dissolved nitrogen. It is added to increase pitting resistance and in acting as an austenite stabiliser to create a balanced ‘duplex’ microstructure in the weld, especially for thin materials which cool too rapidly to allow sufficient austenite to form. Nitrogen can be added to both the welding gas and the purge gas to prevent the loss of nitrogen during welding.


This article has dealt with gases for the active side of a weld. When welding tube or pipe, it is normal to feed an inert gas such as argon or nitrogen into the tube or pipe to maintain low oxygen levels and minimise heat tint formation to no more than pale straw. This usually requires a sensitive oxygen meter or possibly previously proven purging practices. In thick sections, purging must continue for all passes. Nitrogen purging of duplex root passes will improve the corrosion resistance but may also upset the phase balance. Hydrogen additions have been used in purge gases for both austenitic and duplex welds to minimise heat tints. 

Stainless Steel and Fire Resistance

What is the fire rating of stainless steel? This is a common enquiry from ASSDA Members and the construction industry, especially with the current concerns about flammable cladding. The three major branches to this question are covered in this article.

Will stainless steel burn, and if it does, will it give off fumes or facilitate the spread of fire?  

This question is readily answered because stainless steels are steels. It is recognised that steels do not burn and only start to melt at about 1400oC. This means that stainless steels do not have a “fire rating” as such, so the tests of AS/NZS 1530.3 (or the equivalent tests in BS 476) are not required.

Heating in a fire will obviously have an appearance effect because, unlike the transparent nanometer-thick passive layer formed in moist air, stainless steels heated above about 300oC in air discolour as they grow a less dense oxide layer. This develops from the rainbow colours seen beside welds to a dark and non-protective oxide layer whose thickness depends on the time of exposure and temperature reached. The street rubbish bin shown suffered from a fire but remained functional for almost a year (until the repair cycle reached it) with a decorative rainbow oxide. By way of comparison, powder coated bins would suffer from unsightly burn marks and corrosion. 

For austenitic alloys such as 304 and 316, the temperature limits for lifetime section loss due to oxidation is about 870oC (with temperature cycling) so they are routinely used in high temperature furnaces and ductwork. The current trend to apply decorative coatings to stainless steels would require an assessment to determine the combustibility, potential fumes and flame spread of the coating. Tests to AS/NZS 1530.3 would be appropriate. 

Microstructural effects of a short-term heat cycle (less than a couple of hours of exposure, such as a fire) could include carbide precipitation (sensitisation) in an austenitic alloy which was not an L grade (i.e. carbon >0.03%). Duplex and weldable ferritic grades should not have sufficient carbon for sensitisation. Sensitisation would degrade the corrosion resistance but not affect mechanical properties. Both duplex and ferritic grades can suffer 475oC embrittlement, however data produced by the International Molybdenum Association (IMOA) shows that this requires more than two hours in the 400oC to 500oC range for a 50% reduction in toughness. This duration is unlikely in most fires.

 

Will stainless steel provide a barrier to flames and if it does, how rapidly will the heat penetrate the barrier sufficiently to cause damage (usually a specific temperature rise) on the far side? 

A satisfactory demonstration is supplied by reference BS 647 Part 22 tests carried out for a British Stainless Steel Association (BSSA) member, Stewart Fraser, who manufacture 316 framed doors which include a cavity filled with non-combustable boards. The results are given at www.bssa.org.uk/topics.php?article=106.

It showed slight discolouration and distortion on the flame impingement side with the sheltered side of the door reaching only 98oC after 60 minutes. The test was continued for another 80 minutes without the failure of flame containment or subsequent opening of the door in its frame. Similar testing was carried out on a 1.5mm thick 2304 duplex sheet fabricated into a simulated ship’s bulkhead with enclosed ceramic wool insulation. With a bright orange glow of an 1100oC metal temperature on the flame side, the “safe” side reached 30oC after 40 minutes and 110oC after 60 minutes. The test was terminated after 120 minutes with containment still satisfying IMO resolution A518 (XIII).

 

What are the effects (both during and after an event) to the mechanical properties of stainless steel? How do these compare with structural carbon steels? 

There are tests as well as a theoretical basis which demonstrate that both austenitic and duplex stainless steels have superior high temperature properties compared to carbon steel. The table below shows the deflection and failure modes of three metre long commercial electrical cable trays loaded to simulate actual loadings. They were heated with 18 LPG burners to obtain an average temperature of 1000oC  to 1050oC for at least five minutes. [Nickel Institute publication No. 10042]

    

 

The publication also considers the life cycle costs (LCC) of the use of aluminium, galvanised steel or stainless steel for stairways, handrails, gratings and firewalls, as well as cladding for corridors and accommodation modules on North Sea platforms. Fire risk controls are obviously a major concern although corrosion resistance is also critical. On an LCC basis, stainless steel was most economical especially when its reduced requirement for maintenance periods were included. 

In addition to the above testing in cable tray applications, substantial research and application work has since been carried out and codified. Installations include 2205 duplex hangers suspending the slab which forms the floor of the emergency ventilation duct in the CLEM7 tunnel in Brisbane [ISSF].

In short term fires such as on balconies or stairways, the temperature rise exposed to an ISO 834 fire temperature profile depends on thickness and emissivity. Polished stainless steels typically have low emissivity of <0.1 and hence a slower temperature rise. Conservatively, after 30 minutes a 12mm sheet of stainless steel with 0.2 emissivity would reach 620oC whereas steel (with no rust) and 0.4 emissivity would reach 750oC.   

When considering strength and deflection, the metal temperatures in a conventional fire do not reach levels to anneal the material so any cold work strengthening will raise the temperature for a 50% strength reduction. In addition, as shown in the graph, the reduction in Young’s Modulus, i.e. deflection from a specific load, is less than that of carbon steel for temperatures above ~200oC. By 600oC the modulus retention for stainless steel is 0.75 compared to 0.3 for carbon steel, i.e. less than half the deflection for a given load.

 

         

 

In summary, stainless steel has substantial advantages in structural use when fire risk is considered, and these advantages continue into higher strength and lower deflections at elevated temperatures.

CLEM7 image above courtesy of Ancon.

This article is featured in Australian Stainless Magazine issue 65, 2019.

 

 

 

Pickling and Passivation of Stainless Steel

One of the most common misunderstandings in specifications for stainless steel fabrication relates to the post-fabrication treatments to restore or enhance the corrosion resistance. 

The surface treatment processes invoked vary between pickle and passivate, passivate, or sometimes simply pickle. Needless to say, whilst pickling and passivation are two distinct processes, a lack of clarity can cause some confusion between the owner and the builder/fabricator about what is expected and required. 

This article briefly outlines the factors that affect the corrosion resistance of stainless steels, what surface treatments can be used and how they affect the steel’s surface to improve corrosion resistance.

Corrosion Resistance and its Controls

Stainless steel is resistant to aggressive environments because of a very thin, self-repairing, chromium-rich complex, oxide film present on the surface of the steel. It is not completely impervious, but it dissolves many orders of magnitude more slowly than it reforms. The passive layer is more resistant for alloys with more chromium, molybdenum and nitrogen. This is the reason for the empirical, composition based Pitting Resistant Equivalent (PRE(N)) index which is often used as a ranking tool in selecting which stainless steel will be used in new applications. However, the alloy composition is not the only control of the passive film’s strength, and hence its corrosion resistance. There must also be an adequate supply of oxygen and moisture to maintain the integrity of the passive film. This requires either good design or a maintenance program – and preferably both.  

For a specific alloy, i.e. a specific PRE(N), the passive film (and hence the corrosion resistance) can be improved by chemically oxidising the steel’s surface. Air and water are good and the ASTM standard dealing with passivation (A967 Standard Practice for Chemical Passivation Treatments for Stainless Steel Parts) advises that for many environments, no further treatment is required for satisfactory service. However, oxidising or chelating chemical treatments will provide better corrosion resistance.

Roughness

Corrosion resistance is indirectly improved if the surface is smooth and clean (free of contaminants) to facilitate the self-renewal of the passive film. For abraded surfaces there is also a critical surface roughness of 0.5μm Ra that should not be exceeded. This is recognised in surface finish 2K in EN 10088.2. It seems that for steels, the size of abrasives causing this roughness is too large to cut the surface cleanly and leaves rough edges and metal debris which can accumulate dirt and corrosives – hence more rapid corrosion with coarser polishing.

Contamination

The bête noir of stainless steel: carbon steel contamination. If it is not removed, the stainless steel will rust. In marine environments, it will collect chlorides and cause large rust stains and small pits in the stainless steel. If it is mechanically removed, it is likely that the smeared steel will leave a larger rust stain, although it may be less intense. Acid treatments can remove carbon steel deposits and have the added advantage that they can also remove surface breaking manganese sulphide (MnS) inclusions. These MnS inclusions do not have a passive film and act as initiating points for corrosion.

Welding

If you have welded your fabrication and there are rainbow coloured bands along the welds, they are zones where the passive film has been destroyed. Under the darker colours, there will be a wedge-shaped layer with a lower chromium content than the bulk stainless steel. Corrosion will initiate in these coloured bands. The weld tint colours can be mechanically removed provided the grinding is not too rough. Chemical removal by pickling is often a better option.

Pickling

Pickling uses a mixture of nitric and hydrofluoric (HF) acids. The wide range of concentrations and exposure times are described in ASTM A380 Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems. Typically the nitric concentration is up to 10 times the HF concentration, but pickling is slow unless the HF is more than about 3%. Longer pickling times are required at lower temperatures or if a high alloy is being used, i.e. super duplex takes longer than duplex which requires longer than 304. If a paste is used, the contact area acid gets exhausted unless it is stirred, e.g. with the application brush. Thorough washing is needed to remove all residue even from crevices and, to avoid stains, it is important not to allow acid or rinse water to dry on the surfaces.  

OHS and environmental considerations mean that using a pickling contractor is easier, safer and ensures the appropriate disposal of acid and pickled heavy metals. Contractors will often use a temperature-controlled, stirred tank or, sometimes, a spray pickling solution in an acid-proof and bunded bay. Unless an additional level of passivation is required for a very aggressive environment, the outcome is a pickled item that is passive.

Chemical Passivation

The traditional and very effective acid is nitric, typically between 15% and 25% for about two hours, although it is not uncommon to drop machined parts into a bucket of nitric acid for half a shift. The passive film is significantly strengthened and the ratio of chromium to iron in the surface layers can exceed 1 – compared to <0.4 in the bulk. Nitric acid will also remove rust stains and sulphide inclusions plus, more slowly, carbon steel smears. Phosphoric acid will remove rust and sulphide inclusions, but it is not oxidising and will not strengthen the passive film. Another method of strengthening the passive film of a chemically-clean surface is to use a hydrogen peroxide solution – lots of free oxygen and only water residue.

There are other acids that will strengthen the passive film and dissolve carbon steel and inclusions, but by a different method. Citric and oxalic acids and EDTA all have a carboxylic acid [O-C-OH] atomic structure, and once the acid dissolves the unwanted metal, the positive ion is trapped by the negative oxygen atoms in a process called chelating or sequestering. This process is used in wastewater treatment to remove metals. In passivation, it is important to rinse thoroughly. Chelating treatments are widely used in the food industry as formulations which include biocides, so the citric acid does not contribute as a food source.

There are a number of special cases detailed in ASTM A380 which require care when pickling:

• Sensitised or hardened (nitrided or carburised) areas may suffer intergranular attack.

• Free machining stainless steels requires an inhibitor or it will pit.

• Martensitic stainless steels can suffer hydrogen embrittlement.

All of the above methods are chemical treatments which are quite traditional and generally well applied. Further information is provided in ASTM A380, which also details test and inspection methods to confirm surface cleanliness.

Three Definitions

CLEANING Removal of contaminants such as soil, grease, oil, etc. using low-chloride detergents and/or solvents to allow free access for water and oxygen to grow the passive film.

The bulk material is not affected and the surface looks brighter. Chlorinated solvents may be a risk as residues can degrade if heated and may cause pitting. In vessels or pipework, it is important to drain and dry the surfaces.

PICKLING The removal of any high temperature scale and any adjacent low chromium layer of metal from the surface of stainless steel by chemical means.

It also removes embedded or smeared carbon steel, inclusions and loose flakes of stainless steel left from abrasives.

It will leave a matt finish, which may be paler if the pickling is extended. It provides a passive surface immediately on rinsing – hence you pickle and get a passive surface.

PASSIVATION The treatment of the surface of stainless steel, often with acid solutions (or gels), to remove contaminants and promote the formation of the passive film on a surface that was freshly created, e.g. through grinding, machining or mechanical damage. It will remove acid soluble inclusions such as MnS.

Clean humid air will form a passive film on clean stainless steel and the appearance will not change.

Chemical passivation strengthens the passive film and typically takes an hour or so at ambient temperatures. Air passivation is adequate unless the environment is very aggressive for the grade.

1. Rusting steel contamination from shearing stainless sheet. Photo courtesy of Graham Sussex.

2. Rainbow oxide from poor gas shielding during welding. Photo courtesy of HERA.
3.Before (left) and after (right) pickling of welded fitting. Photo courtesy of Graham Sussex. 4. Welded components after pickling to remove heat tint and possible steel contamination. Photo courtesy of Australian Pickling & Passivation Service.

This article featured in Australian Stainless magazine - Issue 64, Summer 2018/19.


Stainless Steel: Sustainability and Life Cycle Costing

Humanity’s use of materials has progressed over the millennia from natural resources such as plants and stone to manufactured materials such as ceramics, metals and plastics with a corresponding increase in consumption of energy and materials – and increasing waste production. In parallel, the world’s consumers have grown exponentially from about 1 billion in 1800, to 7.6 billion in 2018 and a predicted 9.8 billion in 2050 – all demanding more infrastructure, facilities and resources to support the expectations of higher standards of living. This has led to an increasing realisation that green production, recycling, waste reduction and more efficient use of resources are essential.  

The green or sustainable credentials of stainless steels largely derive from their corrosion resistance and consequent long life, without the need for more than cleaning by rain washing or routine water and detergent cleaning. A good example is the Chrysler Building in New York which was built in 1930. It has only been washed twice in 1961 and 1995 using low impact detergents and yet it still retains its bright appearance partly because of good drainable design, although the inherently smooth surface from its manufacture was also a factor.

In comparison, the Eiffel Tower in Paris is painted every seven years using 60 tonnes of paint in a 15-month campaign with 25 painters and their consumable equipment. Closer to home, the constant repainting of the Sydney Harbour Bridge provides a similar contrast to the penetration of stainless steel into the building and construction industry without the ongoing labour required for repainting and maintenance of carbon steel structures. At a smaller scale, current practice minimises maintenance in more aggressive environments by processing the surface after fabrication as shown by the bright surface of the electropolished railings beside the Brisbane River.

It is difficult to compare any corrosion (and therefore lifetime) of stainless steel with carbon steel or zinc because of the different mode of attack, i.e. stainless steel pitting vs. the general loss of copper or zinc. However, a South African 20-year atmospheric corrosion study of lifetimes used carbon steel as a baseline of 1 and found that zinc, copper, aluminium and 316 stainless steel had lifetimes of 25, 90, 170 and >5000 years respectively. 

A secondary benefit of the long life of stainless steel is that the carbon dioxide emissions and the embodied energy required in manufacture are amortised over a much longer period of time. Raw CO2/kg metal and MegaJoule/kg metal data is given in Table 1 for these materials. Stainless steel is not the lowest or highest in absolute terms of carbon dioxide emissions or energy required per kilogram of stainless steel produced, but when its long life is considered, its performance on these criteria is outstanding.

Stainless steel does not use volatile organic solvents in its production or use and does not contain lead, mercury or other leachable heavy metals. Stainless steel is routinely used in pharmaceutical, food and beverage processing because of this chemical stability due to the hydrated chromium-oxide passive layer.

In a confirmation study of the stability of stainless steel with water, a 3.5-year testing program of the hot and cold water in 316 pipework of a Scottish hospital found the chromium content was less than 1% of the 0.5ppb permitted for potable water and nickel content (a trace food requirement) was less than 3% of the 0.2ppb permitted.Looking at environmental issues, Table 2 shows the results of a Scandinavian run-off study, commissioned because of concerns about heavy metals in environmentally sensitive areas. The zinc and copper values will obviously vary with time as the oxide layers form and leach. However, the passive film of stainless steel is substantially stable so that run-off can be used for potable water. A first flush discard system may also be used.


REUSE AND RECYCLING

In a well designed and executed project, stainless steel will not degrade and therefore it is probable that the process or application will become outdated while the stainless steel is still operational as a pipe or vessel or tank or other component. Such repurposing may be on the same site or elsewhere in the same industry, e.g. from milk to wine or water or fruit juices or for a radically different process. However, it is rare for repurposing to move from chemical to hygienic industries. Since stainless steel has an inherently high value, there are multiple examples of building refurbishment where the stainless steel has suffered mechanical damage or the layout must be changed. The William Penn Place (Pittsburgh) rejuvenation shown was after 50 years of use but did not require material replacement.

Recycling may occur as part of the life cycle, e.g. re-melting of scrap, or at end-of-life. Table 3 indicates significant variations depending on the material and its proposed use.  A study of the recycling at 14 European mills covered 18 products across two ferritic, two austenitic and one duplex grade, i.e. all but the small volume of specialised, niche grades of stainless steel. For each of the 18 products, the mean recycled stainless steel content was significantly greater than 65%. The six ferritic products were all above 90%, the nine austenitic products were between 68% and 78% while the three duplex product forms had between 69% and 76% recycled steel input.

While some mills show significantly higher percentages, a nominal 30-year life of stainless steel combined with the almost 6% compound growth of stainless steel use means there is insufficient scrap available now to substantially increase the recycled content from general use.

LIFE CYCLE COSTING AND SAVINGS FROM DURABILITY 

The minimal maintenance required on stainless steel buildings and structures is a significant direct cost saving, and increased availability of equipment is also important. For example, in a waste water processing plant, a decision to replace the wetted parts of a galvanised distributor with 316 and the notionally dry parts with 304, reduced maintenance costs by 92% and increased availability from 76% to 98%.

A civil engineering example is the Progresso Pier as shown below where the original pier with carbon steel reinforcement is in ruins after 32 years exporsure. A Nickel Institute funded comparison between the 1940s construction using 304 reinforcement (right pier) and a theoretical pier constructed with carbon steel showed that the carbon steel would have contributed to a 44% greater overall life cost until 2020. It also showed that using stainless steel reinforcement had between 20% and 80% less environmental impact. This low figure was due to the predominance of the mass of concrete compared to the 240 tonne of stainless steel.

GREEN AND SUSTAINABLE

Green projects minimise energy use and one option is to reduce solar loading by installing perforated sunscreens or fixed slats in locations where insolation is high and ambient temperatures are not extreme. Design of perforated sunscreens is a sophisticated but well understood process with standard programs available. There are multiple examples that use stainless steel because it does not require more than rain or simple water washing to retain a bright appearance.  

Finally, increasingly the “green” label means growing plants or other flora along stainless steel wires or supports either in public places as a visual softening or as a deciduous sun screen where stainless steel is required because of the lack of maintenance access to the supports once the vegetation is mature.

In summary, the durability of stainless steel provides substantial reductions in maintenance costs, supports a considerable recycling and reuse process, and provides control mechanisms for energy use.

This article featured in Australian Stainless magazine - Issue 63, Spring 2018

Ferritic Stainless Steels

Ferritics account for approximately 25% of stainless steel use worldwide. The name arises because these alloys have similar properties to carbon steels when they are bent or cut and, unlike the well-known 304 and 316 austenitic grades, ferritics are strongly attracted to a magnet.

There is a major misconception that ferritic stainless steels are less corrosion resistant than austenitic alloys. On the contrary, for any required level of corrosion resistance (or Pitting Resistance Equivalent [PRE]), you can select a specific stainless steel from either the austenitic or ferritic family depending on the physical properties desired. Another similarity of these two families of stainless steel is that neither can be hardened by heat treatment. However, a significant difference is that, in common with carbon steels, ferritic stainless steels become brittle when used in sub-zero temperatures. The actual transition temperature depends on the specific alloy, but it increases for welded fabrications.

Often regarded as the simplest stainless steel alloy, ferritics are steels (iron and a small addition of carbon) with at least 11% chromium added to produce the passive chromium oxide film. This self-repairing chromium oxide layer gives stainless steel its corrosion resistance. The first stainless steels developed in 1913 were ferritics with a high carbon content. Today, those alloys are called martensitics and are used for high hardness blades or wear resistant surfaces. The alloys now known as ferritic stainless steels have been used commercially for many decades, primarily as sheet cladding up to about 3mm that do not require welding. The Fujitsu building in Brisbane for example is clad in profiled ferritic stainless steel sheet, and the use of perforated and solid ferritic stainless steel sheeting is featured in the ceiling and fascia paneling in Sydney’s Wynyard Walk.

Apart from the 12% chromium utility alloys, the sheet thickness limits for the supply and welding of ferritics are due to its metallurgical structure. Unlike austenitic stainless steels, the microstructure does not transform during welding, and so the initially microscopic ferrite grains can grow and embrittle the metal.

Ferritics have gained wider acceptance since changes in its alloy design and production methods allowed welding. The adoption of the Argon Oxygen Decarburisation (AOD) refining process in the 1970s also assisted, allowing both the reduction of impurity levels and, critical for welding, good control of both carbon and nitrogen content.

Table 1: Selected Ferritic Alloys

Common name

UNS

C%

Cr%

Mo%

Others

PRED

Main uses

409

S40900

0.03

11

-

0.3Ti

11

Car exhausts

4003, 3/5Cr12A

S40977

0.02

11

-

0.5Ni

11

Rail wagons, non-cosmetic structures

430B

S43000

0.03

17

-

-

17

Cladding – not marine

444

S44400

0.02

18

2

0.4(Ti+Nb)

25

Instant hot water units

446

S44600

0.15

24

-

-

24C

High temperature

447

S44700

0.01

29

3.8

0.1Cu,0.1Ni

42

Seawater tubing

Notes:
A. Balance of composition important to avoid welding corrosion issues
B. Also derivative grades with low carbon and Ti/Nb to allow welding
C. Not good indicator of corrosion resistance especially if welded because of high carbon
D. For comparison, the PRE of 304 is ~18.5 and 316  ~23.5.

Available Ferritic Alloys and Applications
The Ferritic Solution (TFS), published by the International Stainless Steel Forum, lists 71 ferritic alloys in ASTM, EN and JSA standards, although most are in sheet form. For example, A240 lists 26 alloys as flat product while ASTM A276 only has nine alloys listed as bar or shape. TFS classifies ferritic alloys into five groups based on chromium content:

- Chromium (10.5% to 14%)
- Chromium 14% to 18%)
- Titanium and/or niobium added to avoid sensitisation with welding
- Molybdenum additions for corrosion resistance
- Weldable group of alloys with higher corrosion resistance and chromium >18%, added molybdenum and low impurity content.  

Table 1 lists common names, UNS numbers, typical compositions and applications of representative alloys. There are also families of alloys derived from the same root UNS numbers. In addition, a growing number of proprietary ferritic alloys have been and are being developed especially in Japan. The PRE column is a measure of corrosion resistance based on composition, i.e. PRE = %Cr + 3.3% Mo. The 16%N term used for austenitic and duplex grades is omitted because nitrogen is virtually insoluble in ferritic alloys. 

Corrosion and Heat Resistance
These are not the same. Oxidation (or scaling) resistance of stainless steels in air depends on the stability of the oxide layer (or scale) on the surface. This is not the thin (nanometres) passive film formed in water but the thicker, high temperature oxide formed above about 250oC. Its protective properties depend on its bond to the metal surface below. In turn, this depends on the relative expansion of the oxide and the metal surface. 

As shown in Table 3, ferritic alloys have low thermal expansion compared to austenitics, which means the adhesion of their protective scale is better in thermal cycling conditions. In practical terms, this means that ferritic alloys have higher scaling temperature limits for intermittent service than in continuous service, whereas the reverse is true for austenitic alloys.

At temperatures in the high hundreds (oC), the relatively low strength of most ferritic alloys limits their use, although the niobium-treated ferritics have similar strength to the austentic alloys. Ferritic (and duplex) grades should not be used in the band around 475oC as metallurgical phase transformations cause embrittlement during extended exposures.

In oxygen-rich environments, the simple wet corrosion resistance of ferritic, austenitic and duplex alloys is well-described by the PRE index as given in Table 1. The predictions are for a passive surface and will be unreliable if the surface has been contaminated by carbon steel or if welding heat tint has not been removed.

PRE does not influence the spidery cracking that occurs in austenitic alloys that are stressed and exposed to warm or hot chloride solutions. Ferritic and duplex grades are effectively immune to this stress corrosion cracking attack and it is the reason why instant hot water tanks used in kitchens are ferritic alloys, usually 444.

Left: Fujitsu Building in Brisbane is clad in profiled ferritic stainless steel sheet. Right: Ferritic stainless steel sheeting featured in the ceiling and fascia paneling in Sydney's Wynyard Walk.

Mechanical and Fabrication Properties
Because of their microstructure, ferritic stainless steels behave very similarly to carbon steels in bending, roll forming, spinning and shaping. Fabricators can use the same techniques for ferritics when forming roofs or couplings.

Ferritics do not cold-work like austenitics and so, for the same thickness, they have less springback. Although deep drawing is easier for ferritics than austenitics, the higher chromium ferritics can suffer from ridging, so there are not many deep drawing applications. Stretch forming can only be to about 50% of that achieved with austenitics, as might be expected from the difference in ductility. Table 2 compares the mechanical properties of several ferritics with 304 and carbon steel. In broad outline, ferritic stainless steels have a higher yield (or strictly 0.2% proof stress) than austenitic stainless steels, lower tensile strength and about half the elongation at fracture. The modulus of elasticity is similar to carbon steels, so deflections under loading will be comparable.

Table 2: Typical room temperature mechanical properties

Common name

Yield MPa

Tensile

MPa

Elongation at break %

Modulus

GPa

409

170

380

20

220

4003, 3/5Cr12

L:320

T:360

480

18

220

430

205

450

22

220

444

275

415

20

220

304

270

650

57

200

Carbon steel

300

430

25

215

Welding

With the exception of the 12% chromium utility grades, welding of ferritics requires more skill than welding austenitics because of their sensitivity to impurities, which may cause cracking in the heat-affected zone. Very thorough attention to cleanliness is required as well as the use of high purity shielding gas and care in gas shielding – particularly outside the workshop where drafts can be a problem. Because of the risk of grain growth (and consequent low toughness) with extended periods at high temperatures, low heat input is required and pulsed welding equipment is a useful tool. This metallurgical sensitivity is the reason why ferritics are rarely available in thicknesses greater than 3mm. However, the low thermal expansion and better thermal conductivity of ferritics compared to austenitics means that welding distortion is less critical for all ferritics (refer to Table 3).

Like all stainless steels, the corrosion resistance of welded ferritics is restored if all heat tint is removed after welding, preferably by pickling. Mechanical abrasion is a good second best provided the surface roughness is not excessive.

The 12% chromium utility ferritics are widely used in welded thick structural sections in coal wagons, heavy vehicle chassis, high temperature exhaust ducting, fire proof fencing, low corrosion wear locations and multiple structures where aesthetics are not the primary consideration, i.e. where a brown adherent cosmetic haze is not considered a problem. The 12% utility ferritics are discussed in more detail in Australian Stainless #52 (available at www.assda.asn.au).

SUMMARY

Some ferritic grades have been in large scale commercial production for many years, but the variety of grades now available has only been possible because of new melting and refining technologies. A large number of grades now exist, and a great deal of active research and alloy development is continuing.

Ferritic stainless steels offer:
- Formability similar to carbon steels and can be readily bent, roll formed, pressed to shape or spun
- Higher yield strength and lower ductility than austenitics
- Comparable range of corrosion resistances to other stainless steel families
- A wide range of possible applications.

Table 3: Physical properties of Ferritic and Austenitic stainless steels

Property

Ferritic

Austenitic

Density (kg/m3)

7700

7900

Thermal expansion

(0-100oC μm/m/oC)

10.5

16.0

Thermal conductivity

(20oC, W/m.oC

25

15

Specific heat

(0-100oC, J/kg.oC

430-460

500

Electrical resisivity

(nΩ.m)

600

750-850

 

This article featured in Australian Stainless magazine - Issue 62 Winter 2018.

Thermal Expansion and Design of Stainless Steel Fabrications

Either while being welded or glistening in the summer sun, the three major families of stainless steel behave differently to each other, carbon steels, aluminium and copper alloys because, as shown in the bar chart, the coefficient of thermal expansion and conductivity - and their ratio - varies.  

While alloys of copper and aluminium have equal or higher coefficients of expansion than austenitic stainless steels, it is the unique combination of high thermal expansion and low thermal conductivity that necessitates special precautions and procedures in the design and fabrication of the most commonly used 304/304L and 316/316L grades of austenitic stainless steel in structures and vessels. Information on handling other families of stainless steels is given in ASSDA’s Australian Stainless Reference Manual.

Distortion during welding

Failure to address thermal expansion and conductivity can result in severe distortion during welding, as differential expansion causes the heat generated by the welding process to remain localised, causing steep temperature gradients  and high localised stresses or surface distortion. Standard welding procedures should be adopted to minimise heat build-up in the weld zone. These include using minimum amperage consistent with good weld quality and controlling interpass temperatures using guidelines provided in Table 5.10 of AS/NZS 1554.6. Clamping jigs with copper or aluminium backing bars as heat sinks on the welds may also be feasible. Other precautions to minimise distortion during welding include efficient jigging or the use of an ends and middle sequence of closely spaced tack welds rather than a straight run. The wrinkled guttering below illustrates the shrinkage problems of poorly planned welding.

The Design Manual for Structural Stainless Steel2 indicates that austenitic stainless steels suffer from the same types of distortion during welding as carbon steel, but the higher coefficient of expansion (17 μm/m°C versus 12 μm/m°C for carbon steel) and the lower thermal conductivity (approximately 30% of carbon steel) increase distortion of austenitic stainless steel weldments. Duplexes are between carbon and austenitic stainless steels in thermal expansion coefficient, but the thermal conductivity is similar to austenitics so heat control is still important. Ferritic stainless steels have similar thermal welding properties to carbon steel but require more skilled welders for metallurgical reasons.

The Design Manual also suggests that a number of additional actions can be considered by both the designer and the fabricator to minimise welding distortion and mismatches such as illustrated in the manifold. These include designing with symmetrical joints, designing to accommodate wider dimensional tolerance, reducing cross-sectional area of welds in thick sections (e.g. replacing Single ‘V’ preparation by Double ‘V’ or Double ‘U’), ensuring that good fit-up and alignment are obtained prior to welding, and using balanced welding and appropriate sequences such as ‘backstepping’ and ‘block’ sequences.

Expansion problems after installation

Another problem arising from the high coefficient of expansion of austenitic stainless steels compared to plywood is differential expansion – although water uptake may also be an issue.  In the illustrated case of stainless steel bonded to plywood by adhesive, a maximum length of 3m is recommended to avoid failure of the adhesive bond during thermal cycling. 

Another problem is when panels (even quite small ones) are in full sun and do not have expansion room for the movement since they were installed at (say) 20°C to the 40°C day plus 30°C overheated metal.

In architectural applications with long runs such as profiled roofing, expansion clips should be used to permit thermal movement without localised buckling and failures. As with other metal roofing and cladding systems with runs 3-9m or longer, there are limits to the maximum width of formed profile for the thickness of stainless sheet used. The formed profile must have sufficient columnar rigidity and strength to transform thermal expansion stresses into sliding movement in the expansion clips. For longer runs, expansion joints should be provided every 7-12m, with clearances of 6mm at vertical faces and 12mm where a gutter end abuts a wall. The publication Stainless Steel in Architecture, Building and Construction - Guidelines for Roofs, Floors and Handrails3 illustrates roofing fixtures for roll-formed profiles and the traditional standing seam and batten roll types. In contrast, ferritic guttering and roofing have similar properties to carbon steels with about 62% of the expansion of an austenitic structure.

In stainless steel piping systems, thermal expansion stresses can cause rupture of the support points, buckling of the pipe, or breakage of equipment connected to the piping if the changes in dimensions are not absorbed by expansion joints or flexibility of the piping installation. The Piping Manual for Stainless Steel Pipes for Buildings4 provides a guide to assessing thermal stresses and reactions at supports and anchor points, as well as a guide to determining if the flexibility of piping can absorb its expansion. The latter involves an empirical formula which requires that the piping anchor points are at the pipe’s ends, the piping system has no branches, and there are no changes along the length of the pipe (e.g. diameter, thickness, material quality, temperature, etc.). If the flexibility cannot absorb the thermal expansion displacement, then expansion joints, flexible joints or ball joints should be used (after a computer stress analysis of the joint).

Conclusion

Thermal expansion and conductivity are critical determinants when designing and fabricating austenitic stainless steel products and are still important with duplex stainless steels. Early consideration of these elements will ensure a better and longer-lasting product, both aesthetically and structurally.

 

 

REFERENCES

  1. ASSDA’s Australian Stainless Reference Manual, see also:

    Avery, R.E. & Tuthill, A.H. (1992) Guidelines for the Welded Fabrication of Nickel-Containing Stainless Steels for Corrosion-Resistant Services (NI 11 007)

    IMOA’s Guidelines for the Welded Fabrication of Duplex Stainless Steels, 3rd Edition (2014)

  2. Design Manual for Structural Stainless Steel, 4th Edition (2017): www.steel-stainless.org/designmanual 

  3. Cochrane, D.J. (1994) Stainless Steel in Architecture, Building and Construction - Guidelines for Roofs, Floors and Handrails (NI 11 013)

  4. Nickel Institute and Japan Stainless Steel Association (1987) Piping Manual for Stainless Steel Pipes for Buildings (NI 12 008)

This article is featured in Australian Stainless Magazine #61.

 

The Family of Duplex Stainless Steels

The use of duplex stainless steels has grown globally based on their strength, corrosion resistance and a range of properties that improve equipment life.

The name duplex is sometimes used to describe Alloy 2205 (UNS S31803 or UNS S32205), however duplex is a family of alloys ranging from lean duplex and standard duplex to super duplex stainless steel.

HISTORY

Duplex stainless steel was first developed in France and Sweden in the 1930’s, with the early grades becoming a forerunner for AISI 329, but a lack of control over the chemistry and lack of adequate welding products and techniques impeded development of the product.

Cast versions eventually became available and were subsequently used successfully in many industries where some corrosion, wear and strength were required.  

Areas such as pump components saw a raft of duplex grades developed in standard and super duplex. It should be noted that further work or welding was not required with these particular forms.

In the 1970’s Swedish manufacturers produced and marketed what could be described as a lean duplex called 3RE60 (UNS S31500) with lower chromium, nickel and nitrogen than grade 2205.

3RE60 had success with tubing and displayed excellent resistance in replacing 304 and 316 tubes that had previously failed due to chloride-induced stress corrosion cracking.  The use of 3RE60 in vessels was less successful due to issues such as inter-granular corrosion (IGC) from early welding techniques. The issue was not with the grade but with fabrication, as well as the melting technique to enable control of alloying elements to provide a consistent structure and provide predictable strength and corrosion control.

In the late 1970’s grade 2205 arrived in the market, initially as a tube, then in flat-rolled and other products. The point-of-difference from earlier attempts was well-documented welding technique control, which lead to the increased usage of duplex.

The grades displayed higher strength than standard austenitic grades, excellent resistance to stress corrosion cracking and improved pitting resistance. The other driver was the rising price of nickel, which added a commercial advantage over using a lower nickel duplex product.

GRADES OF DUPLEX

The grades are listed in three groups; standard, lean and super.

The major difference between each grade is corrosion resistance.  This is based on a Pitting Equivalent Number: 

(PREN) = %Cr + 3.3 x %Mo + 16 x %N.

This is a comparative rating that relates to the critical pitting and crevice corrosion temperatures in hi chloride environments (CPT and CCT respectively).

DUPLEX TYPE PREN
Standard Approximately 35
Lean 25-30
Duplex Above 40

USES OF DUPLEX STAINLESS STEELS

Stress corrosion cracking (SCC) is a form of corrosion that occurs with a particular combination of factors:

  • Tensile stress;
  • Corrosive environment; 
  • Sufficiently high temperatures: Normally above 60°C but can occur at lower temperatures (around 30°C in specific environments, notably unwashed atmospheric exposures above indoor chlorinated swimming pools). 

Unfortunately, the standard austenitic steels like 304 (1.4301) and 316 (1.4401) are the most susceptible to SCC. The following materials are much less prone to SCC:

  • Ferritic stainless steels;
  • Duplex stainless steels;
  • High nickel austenitic stainless steels;

 The resistence to SCC makes duplex stainless steels suitable for many processes operating at higher temperatures. Examples of the successful use of duplex stainless steel are hot water tanks, brewing tanks and thermal desalination vessels.

WHERE CARE IS REQUIRED WITH DUPLEX STAINLESS STEELS

Duplex stainless steels can also form a number of unwanted phases if steel is not given the correct processing, notably in heat treatment. Phases like sigma phase leads to embrittlement, meaning the loss of impact toughness, but sigma phase also reduces corrosion resistance.

The formation of sigma phase is most likely to occur when the cooling rate during manufacture or welding is not fast enough. The more highly alloyed the steel, the higher the probability of sigma phase formation. Therefore, super duplex stainless steels are most prone to this problem. Another form of embrittlement occurs above 475°C, and it can still form at temperatures as low as 300°C. This leads to the design limitations on the maximum service temperature for duplex stainless steels.

SUMMARY: DUPLEX CHARACTERISTICS

Compared to the austenitic and ferritic stainless steels, duplex can give:

  • Up to double the design strength;
  • Good corrosion resistance depending on the level required;
  • Good toughness down to -50°C;
  • Excellent resistance to stress corrosion cracking;
  • Welding in thin and thick sections with care;
  • Additional effort required due to high mechanical strength;
  • Up to 300°C maximum in service.

  

Author: Trent Mackenzie is a metallurgist with more than 35 years experience in the industry and General Manager of ASSDA.

Photos courtesy of Outokumpu.

This article is featured in Australian Stainless Magazine Issue 60 (Summer 2017/18).

K-TIG: A Quantum Leap for Welding

Innovation Design Set to Transform the Industry

For the past six decades, the welding process has only been tweaked and modified, but one Adelaide company has developed a new process set to save millions of dollars and forever change the way welds are performed.

DEVELOPMENT

In 2000, Dr Laurie Jarvis and his associates at CSIRO Adelaide studied the effect of surface tension within an active weld. It was noted that under certain conditions, namely narrow gaps and increased process conditions, that far greater speeds could be obtained when welding clean materials.

The team developed a brand new process involving a high speed, single pass, full penetration welding technology that significantly reduces the need for wire or edge bevelling and is not required where autogenous welds are acceptable.

The result is a flawless finish at a speed up to 100 times faster than TIG welding in materials up to 16mm in thickness.

By definition, clean materials include stainless steels, nickel alloys, titanium and zirconium. Other materials with high impurities (such as alloy steels) cause the weld arc to become unstable and the process becomes unmanageable.

NEXT STEP

With Dr Jarvis as technical leader, a group of experienced materials experts formed K-TIG. Today K-TIG has progressed into many world markets with the system, winning a number of awards along the way.

The K-TIG process involves a specially controlled high current arc which opens a full penetration keyhole in the joint between the two welding surfaces.

Featuring extremely high stability and operating over a wide range of welding currents, K-TIG looks set to become the next big thing in fabrication.

Since its inception, K-TIG has achieved enormous growth in the market, with the technology being exported to eighteen countries. Customers using stainless steels are typiclaly saving 90% on production costs.

THE PROCESS 

The process ideally suits non-corrosive and exotic materials with a thickness range of 3mm to 16mm for single pass welding, however thicker metals can be welded by multiple passes.

K-TIG easily handles the traditionally difficult material, super duplex.

As for energy consumption, K-TIG consumes as little as 5% of the energy and gas consumed by TIG/GTAW for the same weld, dramatically reducing its carbon footprint.

A typical K-TIG weld is performed in a fraction of the time of a conventional weld, in a single full-penetration pass using just one welding gas.

The resulting weld is with multiple fusion lines, dramatically reducing the potential for inclusions, porosity and other defects typical of many welding processes.

The K-TIG system can monitor and control the addition of wire to a weld if that is desirable.

This article in Australian Stainless Magazine Issue 59 (Winter 2017).

Guidelines to Using AS/NZS 1554.6 for Welding Stainless Steel

Using AS/NZS 1554.6 effectively means rather more than requiring “Weld finishing to AS/NZS 1554.6”. The standard is an effective way to get the finish you want or need on stainless steel structures. This guide should help you to nominate the quality of weld to the standard.

What is this standard?

This standard is for welding any non-pressure stainless steel equipment and when it was first drafted in 1994, its structure followed that of Part 1 dealing with carbon steel. A major revision in 2012 removed redundant text, expanded the good workmanship guidelines and brought the weld assessment and finishing processing up-to-date, while including guidance on precautions to minimise risk of failure from vibration. The assessment section includes mandatory limits to weld defects and now includes optional features such as level of heat tint and surface roughness that may be specified by the principal or owner.

AS/NZS 1554.6 is a mixture of mandatory requirements and recommendations with shopping lists of possibilities. In particular, the post-weld treatment provides a number of possible processes and results, and specifying the option desired will minimise cost and frustration and deliver the result required. As an example of mandatory requirements, there are strict requirements for personnel qualifications, which are difficult to address retrospectively.

The raw product of welded fabrication

Figure 1 (refer to banner image above) is typical of a routine TIG butt weld of two thin 316 stainless steel sheets and displays a rainbow of colours on the surface. The colours are caused by optical interference of reflections from the front and back of the heat formed oxide layer - just like reflections in an oil film on water. The unprotective iron-rich oxide layer can be seen in the dark colours and can reduce the corrosion resistance of a 316 to below that of a 12% chromium stainless steel. They must be removed along with a small amount of steel underneath them, where the chromium has been depleted during welding. Specifying their removal is covered later in this article. Let’s start with Section 6, because that is where the weld quality is assessed.

Classification of welds

Welds are classified as Category 1 (structural) or Category 2 (non-structural). Category 1 welds have a subset Fatigue Applications (FA), where vibration and fatigue failures may be an issue. The main difference is that Category 1 and Category FA welds require external visual inspection plus sub-surface inspection by radiography or ultrasonics. The permitted levels of sub-surface defects are listed in Tables 6.3.2(A) and 6.3.2(B) for structural and fatigue classifications respectively.
However, all of the Categories 1, 2 and FA are assessed against three levels of surface defects revealed by visual and liquid penetrant inspection.

The permitted defect sizes are set out in Table 6.3.2 and are grouped under three levels:
A:    No defects and used for critical structural, aesthetic or corrosive service;
B:    High quality for general and non-critical aesthetic uses but may have minor defects that allow corrosives to accumulate in very aggressive environments;
C:    Hidden locations or areas with low stress and benign conditions.

The temptation is to specify Level A for everything, but this may raise costs unnecessarily without adding to durability. Often Level B is very satisfactory. For instance, the ASSDA tea staining requirement of weld quality is Category 2, Level B.

Category FA welds require compliance to Level A assessment of surface defects plus restrictions on the angle between fillet weld tangents and the adjoining stainless steel surface. This restriction supplements the 1 in 4 slope in section thickness changes set out elsewhere in the standard. Table 6.3.1(B) gives the level of sub-surface defects permitted. It applies only for FA requirements.  

Post-weld surface finishing

The standard also provides options for post-weld and surface finishing. Welds may be treated mechanically with abrasives, or chemically (or electrochemically). Any of these finishes can be called up for Condition I and Condition II, but the defining feature of Condition I is that the weld bead must be ground flush. This strip polishing is common in tank fabrication for the food and beverage industries. It removes the heat tint and the chrome depleted layer beneath it without using pickling acids, but it also improves cleanability by removing the weld bead with its inherent unevenness. In vibrating applications, the mechanical removal also decreases the risk of stress concentration along the stiffening line of a weld bead.

The standard also allows stainless steel brushing to remove surface deposits or else for the surface to be left “as welded”. These options are included in Condition III.
Table 6.2.1 summarises the paths to the surface conditions and Table 6.3.3 provides the acceptance criteria based on discolouration, average surface roughness Ra and maximum surface roughness (Rmax). In the 2012 version, the criteria are largely “specified by the principal”, but Condition I and II for discolouration are tied to the AWS D18.2 colour charts of heat tint which match Sandvik and Nickel Institute work confirming that a pale straw colour caused no detectable change in corrosion resistance. There are non-mandatory notes that transverse surface roughness should be <0.5μm Ra and clean cut for corrosive service [as for surface 2K in EN 10088.2] and about the applicability of Rmax to cleanability in hygienic service. Amongst other variables, the grit size will determine the roughness (Ra and Rmax) and hence the as-abraded corrosion resistance and cleanability.

Condition III does not have acceptance criteria.

Tables 1 and 2 below are a guide to the use of category, class and condition (used both for treatments applied and assessment results) and relate them to post-weld processes.

Other treatments

While mechanical abrasion will remove heat tint and the chrome depleted layer, it will expose manganese sulphide inclusions which are points for corrosion initiation. It may also leave metal flakes on the surface, which provide crevice corrosion sites.

Pickling [Section 6.2.3(a)] using a nitric/hydrofluoric acid bath or paste will remove metal flakes and manganese sulphide inclusions. Pickling a non-abraded weld area will not significantly change the surface roughness, but will give similar corrosion resistance to an abraded and pickled surface. If the use of hydrofluoric acid is difficult, then a nitric acid passivation process of an abraded surface will improve the passive film, remove the inclusions, but not any metal flakes. A passivation treatment will strengthen the passive film even of a pickled surface. A nitric-only treatment is not effective on a heat tinted surface. Other modifications of Conditions I and II include electropolishing [6.2.3(b)] or, more recently, electrocleaning [6.2.3(c)]. Both apply a current which dissolves the surface either in a bath (electropolishing) or on site (electrocleaning). The mechanically polished bar illustrated in Figure 2 had an Ra of ~0.7μm before electrolishing, but 0.2μm less afterwards and with a much brighter appearance that also has a thicker passive film. Electrocleaning is a manual process, and while it can produce a very strong passive film, its results depend on the expertise of the operator.

Condition II finishes include simple pickling (HF/HNO3), electropolishing (although often with a prior pickle to remove non-conductive weld scales) and electrocleaning for site operations. The longitudinal weld in the pipe (refer to Figure 3 below) still has weld reinforcement, but is chemically clean. The black lines parallel to the weld have not been affected by the acid pickling and are probably due to cracked oils not removed by solvents prior to welding. Post-pickling passivation is also included in this Condition II suite of treatments.

The mechanical treatment of heat tint by stainless steel brushing [6.2.3(d)] simply burnishes the surface and does not remove the low chromium layer beneath, i.e. it will not restore the corrosion resistance. Abrasive polishing, linishing, grinding [6.2.3(e)] or even blasting [6.2.3(f)] can remove heat tint and the low chromium layer while leaving some weld reinforcement, but a nitric acid passivation process may be required afterwards. In addition, the surface may be too rough for good cleanability or smooth appearance. Under Condition II, one treatment to provide oxide-free welds for pipes and tubes is the use of inert gas purging with low (tens of ppm) oxygen levels.

Apart from the weld inspection, Section 5 of the standard has multiple recommendations for excellent fabrication including heat input, interpass temperatures, avoidance of arc strikes and welding under adverse weather conditions, to name a few. There are also mandatory requirements (the “shall” clauses) on tack weld size, weld depth to width ratio, thinning of metal when dressing welds and even chloride limits in leak test water. The standard is detailed and requires some study for those wishing to produce good welds compliant to the relevant sections of AS/NZS 1554.6 and applicable to the application or structure under consideration.

Conclusion

The specification of weld quality requires an understanding of mechanical and chemical processes used to produce a smooth and clean surface suitable for the specific application. The standard provides a shopping list to accurately specify exactly what you want. Respecting that intent will lead to the greatest productivity in delivering the structure.

This article is featured in Australian Stainless Magazine issue 58 (Summer 2016/17).

Revision of AS 1528: Fluid Transfer in Stainless Steel Tube and Fittings

Connections are vital

Any visit to a dairy, beverage or food processing plant will drive home the critical importance of the connections between the tanks, mixers, driers, pumps, etc. The image above (courtesy of TFG Group) showing an image of a brewery is a typical example. These tubes and/or pipes carry the process materials, the heating or cooling or wash water, gases, and also dispose of the wastes.

 

Getting the right standard

Except for high pressure or very aggressive environments, most tube is rolled into shape and welded longitudinally. For mechanical or structural service such as columns or handrails, the weld must penetrate and be sound although to perform its mechanical function, it may not need to provide a seal. This is reflected in the basic test requirements of standards such as ASTM A554 ‘Welded Stainless Steel Mechanical Tubing’ and is a reason why it is cheaper and is sometimes used, in error, for fluid transport. Despite these restricted requirements, the external finish is often critical for aesthetic reasons as seen on the handrails in the figure on the right.

Verification of leak tightness is the reason why tubing standards for carriage of fluids, e.g. AS 1528.1 or ASTM A269 or ASTM A270, all include either hydrostatic or 100% eddy current testing. Section 8.4 of the ASSDA Reference Manual summarises the test requirements of the plethora of tubing (and piping) standards commonly used in Australia. However, the food and sanitary industries also require surfaces that are readily cleanable. Hence, in addition to a lack of leaks, there are also requirements on the profile of the weld bead in the tubing, potential crevices in fittings and the surface finish of product contact areas. 

System design and installation

Quite apart from the manufactured components, the system design must include adequate slope for self draining (including across welded joins), simple cleaning procedures, velocities above ~0.5m/sec for low solids streams, at least double that for high solids content and avoidance of design features permitting stagnant zones or dead legs. Excess velocity (at least below about 40m/second) is not a concern for stainless steel, although it may increase noise and pumping costs. These are matters for another place.

Material selection

There are quite complete sets of corrosion resistance data for single corrosives (and some mixtures) at a variety of temperatures and concentrations but they are usually for continuous exposure.  For some acidic, hot and salty fluids or slurries such as sauces, high alloy stainless steels or even nickel-based alloys may be required and such components are rarely “off-the-shelf”. However, for apparently aggressive fluids processed in batches, the intermediate cleaning will arrest the initiation of attack and restore the passive layer so that standard 316(L) material is usually adequate especially with the highly polished finish often used to enable cleanability. One operational issue is that cleaning chemicals can be quite aggressive and the procedures must ensure that residues from cleaning do not remain and are not able to be concentrated and cause corrosion or hygienic issues.

Food tube and fittings – AS 1528

The weld bead is a potential source of crevices and for food tube, its effect must be removed without causing additional surface defects. AS 1528.1 requires the weld bead to smoothly blend without harmful markings. It also sets a nominal surface roughness (0.3 μm Ra) for the rest of the interior by requiring the use of fixed (1.6mm) thickness 2B material. ASTM A270 ‘Seamless and Welded Austenitic and Ferritic/Austenitic Stainless Steel Sanitary Tubing’ assumes a sophisticated specifier as it lists a mill finish as well as multiple alternative mechanical or other finishing techniques. Acceptance of minor surface imperfections is by agreement. The specifier may require a surface roughness (Ra) limit – which, of course, would override a grit size specification.

The manufacturing tests (eddy current or hydrotesting) ensure that food tube will hold pressure. For the essential quality assurance purposes, AS1528.1 requires line marking of tube. Finally, food grade tube requires a complementary set of fittings that will fit together. The AS 1528 suite achieves this with screwed couplings (Part 2), butt welding fittings (Part 3) and clamp liners with gaskets (Part 4). Aesthetics may be important and is in the hands of the specifier as the exterior of AS1528.1 tube may be as-produced or “buff polished as agreed”, i.e. polished with grit of a specified size.

The AS 1528 suite started life in 1960 as AS N32, was split into four parts in the mid 1970s and completely revised by an ASSDA driven working group to its present form in 2001. It has been widely accepted especially since the 2006 publication by ASSDA of what is now the Food Code of Practice for the fabrication and installation of stainless steel process plant and equipment in the food and beverage industries.  The New Zealand dairy industry has effectively adopted the AS 1528 requirements for dairy tube and fittings. Multiple overseas suppliers provide tube to the AS 1528 specification.

Food and beverage manufacture is obviously worldwide and this has resulted in national, regional and international standards which are different and locally focused. The sizes of the ISO alternatives (ISO 2037, 2851 – 3) are quite different. The European standard (EN 10357- which supersedes BS4825.1 and DIN 11850) covers similar tube but does not cover the range of sizes commonly used in Australia. The British Standard products (BS 4825) are similar in sizes to the AS 1528, but with a restricted range. The American 3A products also cover a restricted range. 

“As a result, ASSDA is spearheading an industry effort to revise the 15-year-old suite of AS 1528 standards”.

What is in need of review?

There are a number of typographical errors and inconsistencies between the parts, there are only some pressure ratings and the listing of fittings requires some revisions. The tolerance on the tube wall thickness has been narrow and one sided since inception and while the standard allows modification by agreement, the current wall thickness requirement will be reviewed.  Other issues for discussion will be the addition of larger sizes and assessment of differences for internal finishes between parts of the suite. And finally, it is intended that AS 1528 will be converted to a joint Australian and New Zealand standard to formalise New Zealand’s use.

If users of the AS1528 suite of standards have any suggestions for changes or improvements to the standards, ASSDA would welcome your emailed comments to This email address is being protected from spambots. You need JavaScript enabled to view it..

Acknowledgements

This article has drawn heavily on documents produced by the ASSDA/NZSSDA working group dealing with the proposed revision of AS 1528 and in particular Peter Moore from Atlas Steels, Kim Burton from Prochem Pipeline Products and Russell Thorburn from Steel and Tube in New Zealand.

This article is featured in Australian Stainless Issue 56 (Winter 2016).

Welding Dissimilar Metals

Welding the common austenitic stainless steels such as 304 and 316 to each other or themselves is routine and the easiest of fusion welding.

Nevertheless, there are many situations where it is necessary to weld stainless steel to carbon steel. Two common examples are balustrade posts attached to structural steel or doubler plates connecting supports to stainless steel vessels. There are differences in physical properties such as thermal conductivity and expansion, magnetic properties, metallurgical structure and corrosion resistance, which all require attention. This article outlines the necessary procedures for satisfactory welding, including reference to standards, and explains the necessary precautions. Appendix H of AS/NZS 1554.6:2012 has a more detailed technical discussion including advice on welding carbon steel to ferritic, duplex and martensitic stainless steels.

Welding process
The normal TIG and MIG welding processes are suitable for welding austenitics to carbon steel. As a guide, welding should be carried out at ambient temperature with no pre-heating required (except possibly for drying), unless the carbon steel has more than 0.2% carbon or a thickness of more than 30 mm and giving high restraint, in which case a preheat of 150 °C is usually adequate. Because carbon steels are susceptible to hydrogen cracking, the consumables and the weld area must be dry.

Weld area preparation
When welding galvanised steel (or steel coated with a zinc rich coating) to stainless steel, it is essential to remove the zinc from the heated zone because it is possible to get zinc into the weld, which will cause liquid embrittlement and cracking along the zinc penetration line. It is possible that fume from the zinc coating will cause Occupational Health and Safety (OHS) problems. The weld areas of stainless steel must also be clean and free from grease or oil, as the contaminants will cause carbon pickup and possible sensitisation, leading to intergranular corrosion.

In addition, because the nickel content of the austenitics makes the weld pool more viscous, the weld preparation must be more open (see Figure 1) and the root gap larger to allow wetting. Consumables with added silicon (Si) also assist with edge wetting. An additional effect of the nickel content is that the penetration into the no-nickel carbon steel will be greater than into an austenitic stainless steel (see Figure 2).

Welding consumables (filler metal and gases)
Carbon steel must not be welded directly to austenitic stainless steels as the solidified weld metal will form martensite, which has low ductility and which, as it contracts, is likely to crack. There is an easy way to select the higher alloy filler, which will dilute to give the correct austenitic microstructure with enough ferrite to avoid shrinkage cracks. Refer to Table 4.6.1 in AS/NZS 1554.6. Another way is to use a Schaeffler deLong diagram (see Figure 3) or the WRC 1992 diagram as described in Appendix H2 of AS/NZS 1554.6. The standard recommends that carbon steel to 304(L) uses 309L, and carbon steel to 316(L) uses 309LMo.

If nitrogen additions are used, care is required as it will decrease the ferrite content of the weld metal, which may cause hot cracking.

The shielding gas must not include the oxygen often used in carbon steel mixtures. If an active gas is desired, then low levels of CO2 can be used.

 

Thermal expansion
There is a degree of distortion inherent in welding a low thermal expansion carbon steel to a high thermal expansion austenitic stainless steel. The expansion coefficient for mild steel is approximately 12 compared to 17 μm/m/°C for stainless steel in range 0 – 300 °C. There is also the difference between the good heat conduction of the carbon steel compared to the poor heat conduction of the stainless steel (49 to 15 W/m°K at 200 °C respectively), which means that the stainless steel will cool (and contract) more slowly than the carbon steel, especially if the welded sections are thick. 

To control distortion, the heat input should be minimised and the joint tacked before making the full weld run. One trick is to tack the ends, centre, 1/4 points and possibly 1/8 points in that order. Heat input and interpass temperature recommendations for stainless steel welding are given in section 5.10 of AS/NZS 1554.6.

Post weld cleaning
After welding, clean the weld area to remove slag and heat tint to examine the weld integrity and also to allow the metal to be painted. If possible, blast the weld area with iron free grit but if that is not possible, grind along the weld line to avoid dragging carbon steel contamination onto the stainless steel. ASTM A380 has recommendations for passivation solutions for mixed mild and stainless steel welds. The formulations include peracetic acid and EDTA (ethylenediaminetetraacetic acid), but mechanical cleaning alone is the most common method.

Corrosion protection
It is assumed that the carbon steel will be painted for corrosion protection. When a barrier or insulating coating is used for painting the carbon steel, carry the paint onto the stainless for up to 50mm (depending on the environment’s corrosivity) to cover the stainless steel that has been heat affected. Figure 4 shows a carbon to stainless steel weld with an inadequate coating. Normally in a stainless to stainless weld, the welded fabrication would be acid pickled and passivated using a hydrofluoric/nitric acid mixture, but this is clearly not possible for a carbon steel to stainless steel fabrication because of the corrosive effect on the carbon steel. If the weld zone is to be exposed to corrosive conditions, and it is intended to use a zinc rich final coating on the carbon steel, a stripe coating of a suitable barrier paint is required along the edge of the zinc coating to avoid possible galvanic dissolution of the zinc coating adjacent to the stainless steel.

Stainless clean up
Quite apart from any weld to carbon steel, the stainless steel away from the weld area must be protected from contamination during fabrication. This includes weld spatter, carbon steel grinding debris and smearing of carbon steel on the stainless caused by sliding contact between carbon and stainless steels. If contamination occurs, then it must be removed either by mechanical means, followed by use of a nitric acid passivation paste or by the use of pickling and passivation paste. Passivation paste will not affect the surface finish of the stainless steel, whilst pickling and passivation paste will etch the stainless steel. All acids must be neutralised and disposed of according to local regulations. The surfaces must also be thoroughly rinsed after the acid processes.

Further reading
NI #14018 “Guidelines for welding dissimilar metals”
NI #11007 “Guidelines for the welded fabrication of nickel-containing stainless steels for corrosion resistant services”
IMOA/NI “Practical guidelines for the fabrication of duplex stainless steels” (3rd edition)
ISSF “The Ferritic Solution” (page 36) deals generally with welding ferritic stainless steels
AS/NZS 1554.6:2012 “Structural steel welding: Part 6 Welding stainless steels for structural purposes”
Herbst, Noel F.  “Dissimilar metal welding” © Peritech Pty Ltd 2002 (available for download from here)

This article is featured in Australian Stainless Issue 55 (Winter 2015).

General Corrosion Resistance
The normal state for stainless

Stainless steels resist corrosion because they have a self-repairing “passive” oxide film on the surface. As long as there is sufficient oxygen to maintain this film and provided that the level of corrosives is below the steel’s capacity of the particular material to repair itself, no corrosion occurs. If there is too high a level of (say) chlorides, pitting occurs. As an example, 316 works well in tap water (<250 ppm) all over Australia, but will rapidly corrode in seawater because seawater has very high chloride levels (20,000 ppm).

If there is not enough oxygen and the local corrosives are not high enough to cause pitting, then general corrosion can occur. This might happen in a crevice (which has very limited oxygen) or in a strong, reducing acid (such as mid concentrations of sulphuric acid). General corrosion can occur when there are stray currents flowing from stainless steel to ground. This can happen in mineral extraction if the bonding arrangements are inadequate during electrowinning. General corrosion may also occur from galvanic effects, e.g. if a conductive carbon gasket is used on stainless steel in an aggressive environment.

QUANTIFYING CORROSION RESISTANCE
For circumstances where general corrosion is expected, graphs are available called iso-corrosion curves. They plot the effect of a single chemical and corrosion rate for temperature against concentration. An example is the graph below of a 42% nickel alloy 825 in pure sulphuric acid with air access. This graph shows that the corrosion rate increases with temperature and that provided the temperature is less than ~45 °C and a corrosion rate of 0.13 mm/year is acceptable, alloy 825 would be suitable for any concentration of pure sulphuric acid. The boiling point curve is often included to show the limits of data at atmospheric pressure.

 

 

Most of the following graphs are from the Outokumpu Corrosion Handbook. The specific alloy compositions are tabulated in that Handbook and in the Appendix of the ASSDA Technical FAQ 8.

However, a series of graphs each showing the results for one material over the full range of concentrations and temperatures is cumbersome and so multi-material plots are used for the initial material selection. Titanium is frequently included because of the widespread expectation that it is the “super” solution – although the data shows this is not always correct.

The two graphs below show data for austenitic and duplex stainless grades in pure sulphuric acid. However, only the 0.1 mm/year lines are drawn for each alloy because it is assumed that a loss of 0.1 mm/year would be acceptable for continuous exposure during 365 days per year. This assumption may not be acceptable if, for example, the process using the acid required very low iron levels. For each material, the temperature and concentrations of pure sulphuric acid that are below the line would mean a corrosion rate of less than 0.1 mm/year.

 

WHAT ABOUT IMPURITIES OR ADDITIVES?
The graphs below show (and note the temperature scale changes from earlier graphs) the dramatic reduction in corrosion resistance when 200 mg/L of chlorides are added to sulphuric acid or ten times that amount, i.e. 2,000 mg/L. The heavily reducing range from about 40% to 60% acid concentration  defeats even the high nickel 904L and 254/654 grades.

Nevertheless, a number of grades are potentially suitable for concentrations below 20% sulphuric even with significant chlorides.  However, the graphs also show that at the other end of the concentration scale, the oxidising conditions, which occur for sulphuric acid above about 90%, are extremely aggressive if the acid is impure.

 

 

Some additives act as inhibitors to corrosion and this can be critical in selecting suitable materials for mineral extraction processes. For example, the graph below shows that adding iron ions to sulphuric acid improves the resistance of 316. Adding oxidising cupric ions has a similar effect but as with any inhibitor, attack can occur in crevices where the inhibitors may be used up. And despite the requirement for oxidising conditions to ensure  stability of the stainless steel’s passive layer, it is possible to add too much oxidant as shown by the positive effect of small additions of chromic acid followed by a reduction in corrosion resistance if more chromic acid is added. It is relatively common to refer to the redox potential (rather than concentrations of oxidising ions) if the chemistry is not simple.

 

The data in this section is intended to show that while these iso-corrosion graphs are useful in predicting corrosion rates for specific pure compounds, the addition of aggressive ions, oxidisers or crevice conditions require more detailed consideration.

MATERIALS SELECTION FOR OTHER CHEMICALS
A very common chemical is phosphoric acid, which is used in cleaning, pre-treatments, food preparation and a host of other applications.  It requires increasing chemical resistance with high temperatures and concentrations. For pure phosphoric acid, the iso-corrosion curves show a progression from ferritic 444, through the austenitic 304, 316, 317 to 904L. This is not an oxidising acid so although it removes iron contamination, it does not strengthen the passive film on stainless steels.

Phosphoric acid is frequently associated with chloride or fluoride ions especially in production from rock phosphate. The variation in composition in this wet process acid (WPA) means that iso-corrosion plots are of limited use. However, with thermally produced acid and various impurities, a plot of corrosion rate vs. contaminant ion concentration may be used instead of an iso-corrosion graph – in this case chlorides with the 2.5% molybdenum version of 316. This data is for exposure 24 hours a day, 365 days a year. Note that while the two graphs do not overlap, the trends of these different experimental plots do not exactly match, i.e. iso-corrosion curves provide trend data and not precise values.

 

 

ACIDS FOR CLEANING STAINLESS STEELS

Both the chelating oxalic and citric acids, and the oxidising nitric acid, are widely used on stainless steels both for cleaning and passivation as shown in ASTM A380 and A967. Nitric acid can be used at elevated temperatures and low to medium concentrations without concern for the standard austenitics. However, at high concentrations and above ambient temperatures, they can suffer intergranular attack, unless a low carbon grade is used. In the same environment, molybdenum-containing grades may suffer intergranular attack of the intermetallic phases such as sigma.

 

ALKALIS
As shown by the plot, austenitic stainless steels are resistant to general corrosion for all concentrations of sodium hydroxide and, for high concentrations, the usual problem is lack of solubility. However, at near boiling temperatures, austenitic stainless steels (and especially those with extensive chromium carbide precipitates) are susceptible to cracking as shown by the shaded area.

 

 

SUMMARY
If you intend to use a stainless steel with a new, relatively pure chemical, iso-corrosion curves offer an initial guide to the temperature and concentration limits against general attack. If there are contaminants or oxidants present, then the corrosion susceptibility can increase or decrease significantly and specialist advice should be obtained.

This technical article is featured in Australian Stainless magazine issue 54, Spring 2014.

200 series stainless steels - high manganese (CrMn)

Almost 7 years after former Nickel Institute Director Dr David Jenkinson's 2006 Technical Bulletin, ASSDA's technical expert, Dr Graham Sussex, revisits the CrMn grades of stainless steel.

BACKGROUND
The majority of stainless steel is drawn from the austenitic family because these grades are readily formable, weldable and tough. These chromium-nickel (CrNi) and molybdenum-containing grades were traditionally grouped under the 300 series banner.

However, driven by the increased price of nickel several years ago, there has been renewed interest in lowering the nickel content of austenitic grades while maintaining the austenitic crystal structure. This is achieved by using combinations of higher manganese and nitrogen and even by adding copper.

These high manganese grades - 200 series austenitics - were first developed in the 1930s and were expanded during World War II because of a lack of domestic nickel supplies, especially in the USA.

Many of the new 200 series alloys have proprietary compositions that can vary with manufacturers’ processing. They are not classified or standardised under the ASTM/SAE three-digit codes.

FEATURES OF 200 SERIES
The mechanical, physical and forming properties of the CrMn and CrNi grades are very similar, although the CrMn grades generally have higher tensile strength because of higher nitrogen levels and a higher work-hardening rate because of the nickel level.

The conventional CrMn grades are used in hose clamps or lamp post clamps – thin material heavily cold worked for strength. Proprietary grades are used in galling-resistant applications such as bridge pins or in marine boat shafting, although duplex grades are a strong competitor. A disadvantage of CrMn grades is that the lower nickel content means a higher risk of delayed cracking after deep drawing.

A quirk of the conventional 200 series higher manganese grades is that they do not become magnetic when they are heavily cold worked, hence their suitability for use as end rings in electrical generators.

CORROSION RESISTANCE
The corrosion resistance of the newer CrMn grades is generally inferior to similar CrNi grades. To maintain the austenitic properties, the ferrite forming elements (chromium, molybdenum and silicon) must be in the correct proportions with the austenite formers (nickel, carbon, manganese, nitrogen and copper). If the strong austenite formers such as nickel are reduced, the corrosion-resisting, ferrite-forming elements must also decrease.

SENSITISATION
This occurs when chromium combines with carbon in the steel and forms micron-sized particles of chromium carbide so the chromium is unavailable to form the protective oxide film. The original 200 series increased the carbon level to remain austenitic (see Table 1), but this encouraged sensitisation during welding and is one reason that CrMn grades are not used for fabricated items.

Table 1: Registered 200-series grades

Grade Chemical composition (wt%)
AISI UNS Cr Ni Min N
304 S30400 18.0 - 20.0 8.0 - 10.5 2.0 max 0.10 max
201 S20100 16.0 - 18.0 3.5 - 5.5 5.5 - 7.5 0.25 max
202 S20200 17.0 - 19.0 4.0 - 6.0 7.5 - 10.0 0.25 max
205 S20500 16.5 - 18.0 1.0 - 1.75 14.0 - 15.5 0.32 - 0.40


DURABILITY
The newer grades, such as the Indian-developed J1 and J4 (see Table 2), are intended for use in milder environments. The low nickel content requires a reduction in the chromium content to about 15-16% compared to the 18% industry-standard 304. This is a significant reduction in corrosion resistance, especially for the very low nickel versions, and these small differences in chromium content can have a significant effect on durability.

Table 2: Grades J1 and J4

  Chemical composition (wt%)
Grade Cr Ni Mn N Cu
J1 14.5 - 15.5 4.0 - 4.2 7.0 -8.0 0.1 max 1.5 - 2.0
J4 15.0 - 16.0 0.8 - 1.2 8.5 - 10.0 0.2 max 1.5 - 2.0


The newer, low-nickel CrMn grades are successfully used in India, mainly for components such as cookware or mixing bowls that are formed rather than welded. The use of these grades has spread across South-East Asia and especially into China where the increase in capacity for 200 series production was about 3 million tonnes last year - or about 10% of the world’s production.

CONFUSION OF GRADES
The switch in use to CrMn grades (and not just the J1 and J4 grades) has continued despite lower nickel prices because of the perceived benefit of lower price. Unfortunately, the increased use of less corrosion-resistant grades has confused the industry as the CrMn grades are not magnetic and, at least initially, appear to be stainless and are often assumed to be 304 or even 316.

The confusion arose from decades of familiarity with magnetic, lower corrosion resistance ferritic grades such as 430 in contrast to the more corrosion-resistant and non-magnetic 304 or 316. In fact, magnetism has no relationship to corrosion resistance. Grade mix-ups have caused serious corrosion failures in industry and customer dissatisfaction due to less serious corrosion defects like tea staining. This has mainly occurred in Asia but also in Australia.

The variable impurity levels, particularly of sulphur and phosphorous, was a serious issue when there was a significant volume of the new CrMn grades produced by smaller, older mills. The increase in modern production facilities will proportionately reduce this risk. However, the metallurgical necessity to increase carbon levels for austenite stability in specific CrMn alloys means that welded fabrications still require thin sections or rapid cooling to limit sensitisation and the consequent increased corrosion risk.

IDENTIFYING GRADES
It is possible to distinguish between CrMn and CrNi grades by either portable and expensive X-ray fluorescence equipment or, more simply, by drop test kits to detect Mn (CrMn vs CrNi) or Mo (304 vs 316). The kits often use a filter paper and a battery to ensure the test will work rapidly even with cold metal. See ASSDA’s Technical FAQ No. 4 for further details.

QUALITY CONTROL

Users need to ensure they have good quality control systems to avoid installation of a low-level CrMn grade rather than the expected high-level austenitic. The relatively unknown conventional 200 series has a sophisticated niche. However, for cost reasons, clients may push to use the lower CrMn grades instead of the normal CrNi austenitics or, in sheet applications, the ferritics.

SCRAP AND SORTING GRADES
The fabrication scrap and end-of-life scrap from CrMn grades are not readily distinguished from conventional CrNi grade scrap. However, the value is substantially different as the nickel is still the most costly component. This has serious implications for the scrap industry because it is likely to reduce recycling and hence the sustainable and green image of stainless steel. Fabricators will find their total costs will require rejigging as the scrap from offcuts will have lower value, probably decreasing their profitability.

Each grade of stainless steel has its merits for different applications. However, it is vital to purchase from an educated and reputable supplier of quality materials in order to achieve the desired cost and quality outcome.

This technical article is featured in Australian Stainless magazine issue 53, Autumn 2013.

12% Chromium Utility Stainless Steels

BACKGROUND

Almost all of the stainless steels in use have 16% chromium or more and have nickel or other additions to make them austenitic and hence formable, tough and readily weldable. However, the formal definition of a stainless steel is that it is an iron- and carbon-based alloy with more than 10.5% chromium. Historically, the corrosion mitigation industry regarded alloys with more than 12% chromium as stainless steels mainly because those alloys did not corrode in mild environments. Because of the perceived problem of high initial price when using stainless steels, alloys that are ‘barely’ stainless (and with low nickel to boot) are more competitive with painted or galvanised carbon steel than higher alloys.

HOW WERE THESE GRADES DEVELOPED?
More than 30 years ago, developments from the 409 grade (used for car exhausts) led to a weldable ferritic that was tough to sub-zero temperatures. Two versions were developed: a stabilised grade for corrosive environments and an unstabilised grade that matched international standards. One issue was that the titanium used for stabilisation was hard on the refractories and caused the surface finish of flat product to be less appealing. However, when end users moved to unstabilised versions, corrosion problems arose in some applications. Research lead to further alloy development and proprietary grades with outstanding resistance to weld sensitisation.

WHAT IS DIFFERENT ABOUT THESE MATERIALS?

  • They are ferritic (and attracted to a magnet), and can be bent, formed, cut and electric process welded like carbon steels.
  • The balance of their metallurgy limits grain growth when heated. So, unlike ferritics used for cladding, thick sections can be welded without excessive grain growth and embrittlement.
  • After welding, they have a duplex ferritic-martensitic microstructure that does not usually require heat treatment.
  • As ferritics, their thermal expansion is low (actually less than carbon steel) which reduces distortion risk during welding or furnace operations.
  • They have good scaling resistance in air to ~600˚C and reasonable strength at that temperature compared with more expensive austenitics with a scaling limit of ~800˚C in air.
  • Like duplex alloys, they do not suffer from chloride stress corrosion cracking.
  • They provide excellent and economic resistance in corrosive wear applications compared to hardenable carbon steels, surface-treated materials of highers alloys.

However, there are a few cautions:

  • Low chromium, low nitrogen and no molybdenum means they have low corrosion resistance (PRE~11). They will pit in marine environments and in less severe conditions they cannot be used if aesthetic appearance is critical. Painting is a useful option in aggressive environments.
  • Neither cold work nor heat treatment will increase their strength, although they are slightly stronger than 300 series stainless steels. Because they do not cold work, they should be less susceptible to galling then austenitic stainless steels.
  • While it is nothing to do with the material, supply is mostly limited to sheet or plate, i.e. bar, hot-formed sections, hollow sections and wire and generally unavailable.

WHAT ARE THE ALLOYS?
There is a plethora of proprietary and standardised grades with between 10.5% and 12% chromium. The Ferritic Solution booklet available from the ISSF [www.euro-inox.org/pdf/map/The_ferritic_solution_EN.pdf] lists about a dozen. In Australia, the major proprietary grades are 3Cr12 and 5Cr12 where the ‘3’ and ‘5’ are labels, not compositions, and may include additional letters for other grades in the family. However, these labels cover three different material design decisions – and only those in (A) below are standardised:

A. Low chromium, no molybdenum and low nickel, carbon and nitrogen. There are covered by S40977/1.4003 in ASTM A240/EN10088.2
respectively or S41003 in ASTM A240.

B. As above, but with stabilising titanium or titanium plus niobium. There are several rules for titanium content but 4 (C+N) with a limit of 0.6 is used. The Ti/Nb will lock up C and N and reduce the risk of sensitisation, i.e. it limits corrosion associated with welds.

C. As above, but with lower carbon and nitrogen limits and specific controls on ferrite and austenite stabilising elements. This gives immunity to sensitisation in corrosive environments where there is a risk of fatigue.

REPLACEMENT OF GALVANISED OR COATED CARBON STEEL BY Cr12
The cost of steel that has been galvanised is currently up to 30% less than the cost of a 12Cr utility stainless steel when transport, pickling and other costs are included. When added to the cost of better trained (and hence more expensive) staff required for fabricating stainless steel, it is apparent that on a prime cost basis, even this basic stainless steel will not be cost competitive. However, on a LCC basis, the 12Cr grades have a significant advantage primarily because of durability.

Table 1 shows the relative lifetime of zinc (as a proxy for galvanising) and aluminium vs a 12Cr stainless steel in a medium and low corrosivity environment where the atmospheric corrosion rates for carbon steel are listed averaged over a 20-year exposure. It is clear that the life cycle cost of the 12Cr stainless steel is much better than either of the alternatives listed.

WELDING OF Cr12 STAINLESS STEELS
AS/NZS 1554.6 deals with welding of structural stainless steels and compacts all three branches of the 12Cr grades under ‘1.4003’ for selection of consumables. The recommendation is to use a 309L consumable although 18-8Mn (Note 8) is also prequalified. Heat input should be between 0.5 and 1.5kJ/mm and the interpass temperature should not exceed 150˚C.

As with all stainless steels, contamination by carbon steels must be avoided and any heat tint should be removed prior to exposure to corrosive service. While owners using Cr12 alloys for corrosive abrasion service regard the in-service removal of heat-tint surface layers as sufficient, this is only true if sufficient material is removed to expose the virgin stainless steel before the first rest period with corrodents on the surface could promote pitting.

APPLICATIONS FOR 12Cr STAINLESS STEELS
Applications include piggeries, rail cars, road transport, sugar and mineral industry (especially with corrosive wear), effluent tanks, under pans for conveyors, ducting (including furnaces), BBQ plate, electrical meter boxes, floor plates, gravel screens, railway overhead support towers, etc.

ACKNOWLEDGEMENTS
This paper has been prepared with support from ASSDA colleagues and especially Acerinox, Atlas Steels and Sandvik. Their assistance is gratefully acknowledged.

This technical article is featured in Australian Stainless magazine, issue 52.