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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 30mm 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/NZS1554.6. 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 (<250ppm) all over Australia, but will rapidly corrode in seawater because seawater has very high chloride levels (20,000ppm).

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 then ~45°C and a corrosion rate of 0.13mm/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 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 0.1mm/year lines are drawn for each alloy because it is assumed that a loss of 0.1mm/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 0.1mm/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 200mg/L of chlorides are added to sulphuric acid or ten times that amount, i.e. 2,000mg/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.

Guidelines for Use of Stainless Steel in the Ground

Stainless steel can provide excellent service underground. It is stronger than polymers and copper and its resistance to chlorides and acidic acids is significantly better than carbon or galvanised steels.

The performance of stainless steel buried in soil depends on the nature of the buried environment. If the soil has a high resistivity and is well drained, performance can be excellent even in conditions where other unprotected materials suffer degradation.

BASIC RULES

The Nickel Institute guidelines for burial of bare stainless steel in soil require:

  • No stray currents (see below) or anaerobic bacteria
  • pH greater than 4.5
  • Resistivity greater than 2000 ohm.cm.

Additional recommendations include the absence of oxidising manganese or iron ions, avoidance of carbon-containing materials and ensuring a uniform, well drained fill. If the guidelines are breached, then either a higher resistivity is required, i.e. measures to lower moisture or salts and ensure resistivity exceeds 10,000 ohm.cm, or else additional protective measures may be required.
In comparison, the piling specification (AS 2159) guidelines for mild steel require a pH greater than 5 and resistivity greater than 5000 ohm.cm for soils to be non-aggressive. It is rare for bare mild steel to be buried, i.e. typical specifications include a wrap or coating possibly with a cathodic protection system.

SPECIFIC ISSUES

  • Uniform soil packing is required as variable compaction can induce differential aeration effects.
  • Avoid organic materials in the fill around buried stainless steel as they can encourage microbial attack.
  • Avoid carbon-containing ash in contact with metals in soils. Localised galvanic attack of the metal can occur.
  • Oxygen access is critical. Having good drainage and sand backfill provides this. A sand-filled trench dug through clay may become a drain and it is not appropriate. Stainless steels generally retain their passive film provided there is at least a few ppb of oxygen, i.e. 1000 times less than the concentration in water exposed to air.
  • Chlorides are the most frequent cause of problems with stainless steels. In soils, the level of chlorides vary with location, depth and, in areas with rising salinity, with time.  High surface chlorides may also occur with evaporation. This is a problem for all metals although stainless steels are not usually subject to structural failure.

The general guidelines for immersed service are that in neutral environments at ambient temperatures and without crevices, 304/304L may be used up to chloride levels of 200ppm, 316/316L up to about 1000ppm chloride and duplex (2205) up to 3600ppm chloride. The super duplex alloys (PRE>40) and the 6% molybdenum super austenitic stainless steels are resistant to seawater levels of chloride, i.e. approximately 20,000ppm. These guidelines are easy to apply in aqueous solutions.

Soil tests for chlorides may not exactly match actual exposure conditions in the soil. Actual conditions may be more (or less) severe than shown by the tests. The difference is calculable but in practice, the aqueous limits can be used as general guidelines. More specific recommendations, based on published guidelines, are provided in Table 1.

 

 

It may seem redundant to assess both chlorides and resistivity. Both are required as the resistivity is primarily affected by water content and if it is low, then quite high chlorides could be tolerated – as seen by the choice of 304/304L in high chloride/high resistivity conditions.  Despite these recommendations, most Australian practice is to use 316/316L or equivalent, primarily because of variable soils.

  • Good drainage and uniform, clean backfill are essential for bare stainless.
  • Duplex or super duplex could be replaced with appropriate austenitics and 304/304L could be replaced with a lean duplex.
  • Ferritic stainless steels of similar corrosion resistance (usually classified by Pitting Resistance Equivalent [PRE]) could also be used underground.

Potential acid sulphate soils are widespread, particularly in coastal marine areas as described in http://www.derm.qld.gov.au/land/ass/index.html. Once disturbed and drained, which also allows oxygen access, such soils typically become more acidic than pH 4 and will attack metals (although stainless steels will be less readily attacked than other metals). Detailed assessment is required if using metals in such an environment as the effect of other aggressive ions is likely to be more severe at low pH.

  1. Properly specified stainless steel can provide the longest service underground. It is strong compared to plastics and copper, and is more reliably corrosion resistant than carbon steel.
  2. Table 1 guides grade choice for soil conditions.
  3. Normal fabrication practices apply: welds must be pickled and carbon steel contamination avoided.
  4. Pipelines must be buried in clean sand or fine, uniform fill in a self-draining trench that avoids stagnant water. Organic or carbonaceous fill must be avoided.

CASE STUDIES

The Nickel Institute published a five year Japanese study in 1988 (#12005) showing 304 and 316 gave good service in buried soil, although vertically buried pipes did suffer some minor pitting and staining apparently due to differential aeration effects.

  • NI #12005 describes a five year burial exposure in Japan at 25 sites with highly varied corrosivity. After five years in marine sites, horizontal 304 pipes showed no pitting but some crevice attack under vinyl wrap. Only one 316 pipe showed any attack.
  • Vertical 304 pipe suffered attack near the base at some sites apparently due to differential aeration effects.
  • An Idaho study of a 33-year NIST burial found 12% Cr martensitics perforated. The ‘lake sand’ site had high ground water with pH 4.7 at recovery. Sensitised 304 was attacked worse than annealed but both suffered attack along the rolling direction from edges.
  • 316 was not attacked even if sensitised.

As noted, duplex stainless steel of similar corrosion resistance (PRE) to 304 and 316, respectively, would be expected to provide similar results when buried.

On a more practical level, there are several common approaches that are used when burying stainless steel:

  • Wrap the stainless steel pipe in a protective material, such as a petrolatum tape, prior to burial. If the wrapping is effective (typically an overlap no less than 55% of the wrap width is specified), then the nature of the external surface of the buried pipe is of no consequence. In this case, stainless steel is only used for its internal corrosion resistance, i.e. its resistance to corrosion by the fluid which the pipe is carrying. Some authorities prohibit this practice because of concerns that damage to the wrap could cause a perforating pit in severe environments.
  • Ensure that the soil environment surrounding the buried stainless steel is suitable for this application. In this case, the trench is dug so that it is self-draining, without there being areas where stagnant water can accumulate in contact with the buried pipe. The stainless steel pipe is then placed on a sand or crushed aggregate bed and covered by similar material. Under these circumstances,  316 grade stainless steel can be quite a suitable choice. US practice is to use 304 but Australian soils are quite variable and there have been mixed experiences with 304.
  • Above ground sections of pipework are often stainless steel as they are at risk of mechanical damage while underground pipework is polymeric - polyethylene (PE) or fibre reinforced plastic (FRP) - despite the risk of damage due to soil movement.

In all of these cases, the assumption is that the stainless steel has been fabricated to best practice. This includes pickling of welds (or mechanical removal of heat tint and chromium depleted layer followed by passivation to dissolve sulphides) and ensuring that contamination by carbon steel has been prevented. It is also assumed that the buried stainless steel does not have stickers or heavy markings that could cause crevices and lead to attack.

STRAY CURRENTS

All buried metals, including stainless steels, are at risk if there are stray currents from electrically driven transport, incorrectly installed or operated cathodic protection systems, or earthing faults in switchboards. Stray current corrosion can be identified as it causes localised general loss rather than pitting. It is also very rapid.

WHAT TEST METHODS ARE USED?

There are Australian and ASTM standards giving basic measurements of resistivity on site with 4 pin Wenner probes or in a soil box in the laboratory. More detailed checking includes water content, chlorides, organic carbon or Biological Oxygen Demand (BOD), pH and redox (or Oxidation Reduction Potential [ORP]) potential – which assess microbial attack risk but also captures the effect of oxidising ions and dissolved oxygen. Most of these test methods are covered in “Soil Chemical Methods: Australasia” written by George E Rayment and David J Lyons and published by CSIRO.

 

SOIL

Natural soils are a mixture of coarse pebbles, sand of increasing fineness through to silts and clays where the particles are less than 5 µm in diameter.  Some of the particles contain soluble salts that, if mixed with water, are likely to be corrosive. Normally, soils also contain organic material from decaying plants or ash, which can provide nutrients for microbial activity or galvanic effects, respectively.

If water is present in the soil, corrosion can take place. Metals below the water table can corrode (following the rules for immersed service). However if the soil is well compacted so oxygen cannot gain access or corrosion products cannot diffuse away, then corrosion would be stifled - even for carbon steel. Above the water table, moisture comes from percolating rain, which will, over time, leach away soluble corrosives and make the soil less aggressive. This also means that in dry climates, salts may accumulate and when there is rain, the run-off or percolating water is very aggressive.  Deposited salts can also be a problem in marine zones almost regardless of rainfall.

Most of the moisture above the water table is bound to particles but if there is sufficient water content, typically more than about 20%, enough water is free to wet buried metals.

Image pictured is the Appin Sewerage Treatment Plant, NSW. Fabricated and installed by ASSDA member and Accredited Fabricator Roladuct Spiral Tubing Pty Ltd using 316 grade stainless steel. Image courtesy of Roladuct Spiral Tubing Pty Ltd.

This technical article is featured in Australian Stainless magazine, issue 51.

The Sustainable Score Card for Stainless Steel

The greatest challenge we face is the control of our own success. With 7 billion people on earth, all with an insatiable appetite for a high standard of living, the newest dimension of materials competition is sustainability.

Sustainability is development that meets the needs of the present without compromising the ability of future generations to meet their own needs (UN World Commission on Environment and Development, 1987). In real terms, that means making choices that do minimum damage to our environment, but support a high level of human development.

The built environment is an excellent place to start. Buildings last for a long time, locking up the energy used in making their materials, requiring maintenance and consuming the energy used for heating and air-conditioning. They consume a large proportion of our resources. The choice of materials affects all 3 aspects of consumption, and, a number of building evaluation systems have been created around the world to assist in rating buildings for sustainability. Materials are scored for their energy content reuse during major refurbishment, waste management, recycled content and contribution to the overall design and running costs.

The Green Building Council of Australia rates green buildings for sustainability. The pace of registration and certification is increasing. Of the 368 certified projects, 96 were certified in the last 12 months. The push towards sustainable development in the building sector is strong and accelerating. City of Melbourne’s Council House 2 (CH2) is Australia’s first Green Star rated building to be awarded 6 Stars, which carries an international leadership status. Stainless steel was used to support screening walls of living green plants that shade the building and, required no maintenance or painting, working with the environment to keep good working conditions. Such membranes, containing plants or actively or passively screening the sun, allow the use of a smaller capacity air-conditioning plant, with lower capital costs and ongoing running costs and energy demand.

The only Gold LEED® (Leadership in Energy and Environmental Design) certified meeting venue in the world is the Pittsburgh Convention Centre in the United States. Its grade 316 stainless steel roof is used to harvest rainwater, reducing water demand on the city system - another example of the special properties of stainless steel.

Stainless steel roofing and rainwater goods give extremely low levels of run-off. See Table 1. But this is not the only reason to use stainless steel in the built environment. It contributes to sustainability because of its long service life, excellent corrosion resistance, clean and unchanging appearance and its exceptional hygiene characteristics. Stainless steel is reusable, entirely recyclable, and probably the most recycled product in the world. On top of that, it needs very little cleaning or short or long term maintenance, and makes no contribution to indoor pollution as materials emitting volatile organic compounds (VOCs) do.

There is considerable history and experience of stainless steel service life in the built environment. The Chrysler Building (1930) and Empire State Building (1931) in New York demonstrate the material’s durability, excellent appearance and resistance to corrosion. This extraordinary functionality has been played out many times with a number of examples here in Australia, including the Fujitsu Building in Brisbane, which is clad with 445M2 ferritic stainless steel. Located in a marine industrial environment, this building looks as good as it did on completion in 2002. The long life of stainless steel in these atmospheric applications shows its very high corrosion resistance. The corrosion rate of grade 316 for instance in most atmospheres is is more than 5000 times slower than the rate of carbon steel. See Figure 1 (below).

There is a considerable industry devoted to the collection and recycling of stainless steel products at the end of their life and, scrap is the standard feedstock for making stainless steel. In any stainless steel object, there is an average of 60% recycled content. New production would virtually all be made from recycled stainless steel if it were available, but the growth in the use of stainless steel and its long life in service limit the supply. Table 2 compares the recycled content and end of life capture rate of the industrial metals, and demonstrates that stainless steel is the most recycled industrial metal.

Sustainability is about much more than recycling. The energy used to make the material has a direct impact on sustainability, and all metals are energy intensive. Energy is a scarce resource, generates greenhouse gases and creates specific demands on land use likely to impact on future generations. Longevity and extraordinary recyclability will not be helpful if stainless steels’ energy consumption is much higher than other materials. Figure 2 describes the embodied energy in terms of CO2 equivalent for some of the industrial metals, and shows that stainless has a comparatively high level of embodied energy. In kilogram of CO2 per kilogram of metal, the austenitic grades are over double the footprint of carbon steel, although the ferritic grades are a little less. The footprint of stainless steel is caused by the production of alloying elements nickel and chromium, which are needed to give stainless steel its special properties, including extremely long life. Even so, efforts are ongoing in the stainless steel industry to reduce the energy content.

But in the real world, kilogram CO2  per kilogram metal comparisons are misleading. Take a typical application; a box gutter on a building. The metals have different strength, so are used with different thickness. Stainless steel gives a relatively light weight gutter (see Table 3), and hence the lowest footprint as installed. Coupled with its extended durability without maintenance, stainless comes out as the most sustainable option. Painted galvanised or Zincalume® coated carbon steel has not been included in the table as the calculation of the contributions of the components were too complex, but these materials are highly unlikely to beat the sustainability of stainless steel, even as-installed, and they have a much shorter life.

In summary, stainless steel has excellent recyclability, energy content as-installed (at least as good as other metals), extraordinary longevity and next to no need for maintenance, ever. Add to that the benefits of their special properties, which allow for the construction and operation of buildings at a lower cost. The contribution of stainless steel to sustainability is obvious and considerable.

This article was prepared by ASSDA Technical Committee member, Alex Gouch from Austral Wright Metals.

This technical article is featured in Australian Stainless magazine - Issue 50, Summer 2011/12.

Grade 431

A versatile, high strength martensitic stainless steel

Martensitic stainless steels are a less well-known branch of the stainless family. Their special features – high strength and hardness – point to their main application area as shafts and fasteners for motors, pumps and valves in the food and process industries.

The name “martensitic” means that these steels can be thermally hardened. They have a ferritic microstructure if cooled very slowly, but a quenching heat treatment converts the structure to very hard martensite, the same as it would for a low alloy steel such as 4140. Neither the familiar austenitic grades (304, 316 etc) nor the duplex grades (2205 etc) can be hardened in this way.

Grade 431 (UNS 43100) is the most common and versatile of these martensitic stainless steels. It combines good strength and toughness with very useful corrosion resistance and in its usual supply condition can be readily machined.

Chemical Composition

The composition of 431 specified in ASTM A276 is given in Table 1 below.
Grade 431_Table 1

 

 

 

The inclusion of a small amount of nickel in grade 431 is different from most other martensitic grades. This small but important addition makes the steel microstructure austenitic at heat treatment temperatures, even with such a high (for a martensitic grade) chromium content. This high temperature austenite enables formation of hard martensite by quenching.

Corrosion Resistance

The relatively high chromium content gives grade 431 pitting, crevice and general corrosion resistance approaching that of grade 304, which is very useful in a wide range of environments including fresh water and many foods.

Grade 431 has the highest corrosion resistance of any of the martensitic grades. Corrosion resistance is best with a smooth surface finish in the hardened and tempered condition.

Grade 431 is sometimes used for boat shafting and works well in fresh water but is usually not adequate for sea water.

Heat Resistance

Grade 431 has good scaling resistance to about 700°C but, as martensitic steels are hardened by thermal treatment, any exposure at a temperature above their tempering temperature will permanently soften them. 600°C is a common limit.

Mechanical Properties

The application of grade 431 is all about strength and hardness. Table 2 below lists mechanical properties of the grade annealed and in hardened and tempered “Condition T”.

Grade 431_Table 2

 

 

 

 

 

 

 

 

 

 

 

Heat Treatment

A feature of grade 431 is that it can, like other martensitic steels, be hardened and then tempered at various temperatures to generate properties within a wide spectrum, depending on whether the requirement is for highest possible hardness, or best ductility, or some balance between these. Hardening is by air or oil quenching, usually from 950-1000°C.

The tempering diagram in Figure 1 shows properties typically achieved when the hardened steel is tempered at the indicated temperature. A tempering temperature within the range 580 – 680°C is usual. Tempering between 370 and 570°C should be avoided because of resulting low impact toughness.

Tempering should follow quenching as quickly as possible to avoid cracking. Softening is usually by sub-critical annealing, by heating to 620 – 660°C and then air cooling.

Grade 431_Figure 1

Physical Properties

Density

7700kg/m3

Elastic Modulus

200GPa

Thermal Expansion (0-100°C)

10.2µm/m/°

Fabrication

Machining is readily carried out in the annealed condition, and also in the common Condition T. Modern machining equipment enables high speed machining at this hardness of about 30HRC.

Welding of 431 is rarely carried out — its high hardenability means that cracking is likely unless very careful pre-heat and post-weld heat treatments are carried out. If welding must be done this can be with 410 fillers to achieve high strength but austenitic 308L, 309L or 310 fillers give softer and more ductile welds.

Cold bending and forming of hardened 431 is very difficult because of the high strength and relatively low ductility.

Forms Available

Grade 431 is available in a wide range of bar sizes — virtually exclusively round but some hexagonal. Most other martensitic grades are only available in round bar, although the higher carbon 12% chromium “420” series of grades may also be available as hollow bar and as blocks and plates intended for tooling applications.

Alternatives

Another approach to high strength stainless steel bar is a precipitation hardening grade, such as 17-4PH. These grades have similar corrosion resistance and offer some advantages in producing long, straight, higher strength shafts.

Shafts to be used in more corrosive environments are likely to be a duplex or super duplex or nitrogen-strengthened austenitic grade. These, however, have lower achievable strengths than martensitic or precipitation hardening grades.

Specifications

Grade 431 is usually specified by ASTM A276, with composition as in Table 1. In the Australian market, however, there are usually two deviations from A276:

  1. It is most common to find this grade supplied in the hardened and tempered “Condition T” to AS 1444 or BS 970, with specified tensile strength of 850-1000MPa. Yield and elongation are typically in conformance with the limits listed above. ASTM A276 only lists a Condition A version of grade 431 — this is the annealed condition that would normally require hardening heat treatment after machining.

  2. The second deviation is that it is usual for cold finished stainless steel bars stocked in Australia to be with the all-minus ISO h9 or h10 diameter tolerances. Hot finished “black” bars with all-plus ISO k tolerances may also be available.

 

This article was prepared by ASSDA Technical Committee member Peter Moore from Atlas Steels. Further technical advice can be obtained via ASSDA’s technical inquiry line on +617 3220 0722.

This article featured in Australian Stainless magazine - Issue 48, Autumn 2011.

Remaining competitive and profitable by James Johnson, Millatec Pty Ltd

Posted 23rd September 2009

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Now is the time as an owner of a small or medium enterprise to move back into the coalface and be involved in all facets of your business. As a business owner, no one spends less money, identifies opportunities to improve productivity more or reduces waste better than you.

In the current economic climate it seems especially pertinent to discuss tools that can help you remain competitive and achieve break-even or be profitable.

The four key areas are:

  1. Financials
  2. Human resources
  3. Marketing
  4. Systemisation

Controlling your finances

When it comes to managing your finances, structure is vital. It is imperative that you plan your cash flow on a week-to-week basis to ensure debts can be paid when due. Ensure tax liabilities are allowed for. If you can’t meet your payment dates, talk to your creditors or the ATO: most will work with you, but they will take action if you are not open and honest.

To effectively monitor spending and avoid unexpected cash flow shortfalls, your financial reporting needs to be up-to-date. An ideal target is end of month plus 10 working days. To ensure reporting and recording is useful, filing of all financial transactions – including accruals – is vital.

A network of support is fundamental to the sustainability of your business. It is important to establish and maintain an open and honest relationship with your bank – during times of profit and of loss. The bank will understand the long-term fluctuations of your business and will be your best source of information on current services that suit your needs. Remember, banks do not want to see you go out of business – they will help you stay afloat.

For example, they have developed a range of new products to help with cash flow.

The current economic climate is a great time to negotiate for better deals – from freight to materials – and it is an ideal time to negotiate new leases.

Human resources: maximising productivity

Employees are the bones of your company. Have high expectations of your staff and make them known. Just as important as setting a high standard of work is letting your staff do their job and being flexible enough to make them want to stay. At the same time it is advantageous to not have any staff member who you are afraid to lose: no one should be irreplaceable.

A large part of managing human resources is managing risk. Employee training is invested time and money. Maintaining low staff turnover means retention of knowledge within the company and makes thorough training a valuable investment.

Marketing: sending the right message

If you want to maintain and grow sales, first and foremost be a marketing company. Invest in marketing as you would a new machine: work out the investment and expected return and research what is right for your business.

It is a great time for change so try the things that you have been putting off during busy periods.

The key is remembering that sales must lead production, and production must support the promise. This is a constant battle: they both need – and work just as hard as- the other. This needs to be reinforced daily.

Systemisation

Linking systems together means you maintain control of the business. Report and record weekly, monthly and quarterly. This not only helps in tracking financial movements but also ensures that in the instance of staff absence, the system will remain functional.

Linking the following systems is a good place to start:

  • Quoting (capture all costs)
  • Processing orders (no job starts without a written PO)
  • Producing job cards
  • Purchase orders (nothing gets in without one)
  • Time capture (measure productivity)
  • Stock
  • Invoicing (nothing gets out without one)
  • Financial accounting

If you have had a crippling 12 months, it is not too late to recover and come out stronger, wiser and more profitable.

This article featured in Australian Stainless magazine - Issue 46, Winter 2009.

For stainless apprentices

Posted 23rd September 2009

IMG_5599

At the beginning of 2008, ASSDA was successful in its application for funding from the Federal Government for a project focused on the integration of e-learning into industry. The funding has seen ASSDA create a Workforce Development Strategy and a Flexible Learning Delivery Pathway incorporating e-learning, with plans to develop an additional e-learning module titled Practical Skills of Surface Treatment to complement the existing Gas Tungsten Arc Welding Module.

The Workforce Development Strategy provides an industry-wide framework in which to address the workforce challenges for the stainless steel industry: skills shortages, staff retention, knowledge retention. This document assisted ASSDA in defining what the industry requires in training, skills development and the retention of employees.

The body of the project sees ASSDA working in conjunction with SkillsTech Australia and multiple industry partners to develop e-learning as a form of theory training for apprentices aiming to acquire their qualification in stainless steel fabrication.

ASSDA created a Flexible Learning Delivery Pathway that gives apprentices and employers the choice of conducting training both online and within the workplace. This form of training is beneficial to the apprentices as they are able to work at their own pace, in a location of their choice and in a nonthreatening learning environment. For the employer the pathway is economical as the apprentice can conduct their study in the workplace, therefore reducing time spent away from the workplace.

Using ASSDA’s Stainless Steel Specialist Course and existing resources within the TAFE system, SkillsTech Australia has developed an e-learning system based on the required competencies for a qualification in fabrication, with a particular focus on the unique requirements of working with stainless steel. These training modules offer learning through video, audio, text, images and interactives that are interesting to the apprentice whilst teaching them the underpinning knowledge they require to develop a skill.

In March 2009, 12 apprentices were inducted into the e-learning program for Stage 1a at SkillsTech. This stage is now complete and feedback from the apprentices has been extremely positive. Stage 1b has now commenced and will see the apprentices training solely within the workplace with a workplace mentor to oversee their theory training and instruct them in their practical experience.

This is an exciting development aimed at positioning e-learning as the training method of choice within the stainless steel industry and will help meet ASSDA’s goal of building a strong workforce with a focus on quality and innovation.

If you are interested in viewing the Workforce Development Strategy or learning more about the learning options becoming available for apprentices, call ASSDA on (07) 3220 0722.

This article featured in Australian Stainless magazine - Issue 46, Winter 2009.

strength & corrosion resistance vital

Posted 21st April 2010

SeafoodInnovUnit

As wild fish stocks decline globally, the spotlight is increasingly being shone on humane stun and slaughter methods in the rapidly growing aquaculture industry. Stainless steel components fabricated by Pryde Fabrication (ASSDA Accredited) are an integral part of a Brisbane innovation that is leading the way internationally in a shift towards faster and more humane automated percussive stun methods.

Seafood Innovations International Group Pty Ltd has spent around 10 years developing fish harvest technology which enables fish to swim naturally until the second they are stunned, reducing stress on the fish and improving flesh quality.

They have collaborated extensively during this period with Pryde Fabrication (Cleveland, Queensland) to develop the system, which incorporates a base, ramp and trigger plate made from grade 316 stainless steel.

Up to 400 of the units are being produced each year, of which around 98 per cent are for export.

Pryde Fabrication General Manager Darren Newbegin said Grade 316 stainless steel was chosen for the components primarily due to its corrosion resistance and strength. He said other design and fabrication requirements included:

  • no bacterial traps
  • robust enough to withstand the harsh environment and repetitive shock loading
  • light enough to enable easy handling of the modules for cleaning
  • configured to enable easy dismantling for cleaning

“We never considered any grade other than 316 because of the harsh environment – the majority of the units are exported overseas, where they are being used in minus temperatures, fully immersed in sea water,” he said.

There is about 15kg of stainless steel in each machine, which is laser cut, enabling a high level of accuracy for both cutting and fold marks. The rest of the procedure is performed manually, including welding, polishing and glass bead blasting to provide a pleasing surface appearance.

“Stainless steel is the perfect material to laser because it’s so clean to cut,” Mr Newbegin said.

Seafood Innovations’ Business Manager Noel Carruthers said the development of the system had benefited from choosing a fabricator in the company’s local area, as it enabled a close collaboration.

Mr Newbegin agreed with this sentiment, suggesting it was this relationship between the two companies which had contributed to making the product fit for purpose and tailored to cost and operational efficiency.

“This relationship has allowed Pryde Fabrication to be involved in a solution to world fish farming and we are excited about further growth in this Australian initiative,” he said.

Mr Carruthers said the patented system represented an enormous change to the industry, with a single unit processing 15-20 fish per minute automatically, compared with other processes such as electrocution, carbon dioxide gas, and the use of wooden clubs.

The system works by pumping a current of water, which the fish are naturally inclined to swim towards. They then reach a point where their nose hits a trigger, which releases and immediately retracts a small, blunt-nosed piston at high speed, making the fish irreversibly unconscious. The fish are then turned upside down and enter a bleed machine where they are automatically bled.

In addition to improved flesh quality, the automated system means fewer operator injuries and immediate bleeding, resulting in improved appearance of fillets when fish are processed. The ability to slaughter at the point of capture means fish potentially carrying diseases will not contaminate other waters in transit.

Although originally developed for Atlantic salmon, the system has also been refined to cater for different varieties of fish, including tilapia, pangasius, barramundi, yellowtail kingfish and cobia.

A recent installation on a Marine Harvest vessel in Norway (incorporating three sets of a four channel system) is slaughtering 20,000 fish an hour at 98% efficiency.

The equipment has been independently tested by laboratories in Norway and ongoing developments to the system are tested at Huon Aquaculture in Tasmania.

CONTACT

Pryde Fabrication
www.prydefab.com

Polishing

Welding

Pryde close up

guidelines to ensure long service life

Posted 27th August 2010

iStock-hydrotest72dpiRGB-thumbnail

Design engineers frequently specify stainless steel in industrial piping systems and tanks for its excellent corrosion resistance. While stainless steel’s unique characteristics make it a standout leader in the durability stakes of alloys, it is not completely immune to corrosion.

Premature failures of the stainless steel can occur due to Microbiologically Influenced Corrosion (MIC). This corrosion phenomenon usually occurs when raw water used for hydrostatic pressure tests is not fully removed from the pipework and there is an extended period before commissioning of the equipment. The result is localised pitting corrosion attack from microbacterial deposits that, in severe cases, can cause failure within a few weeks. MIC is easily prevented using proper hydrostatic testing techniques.

MIC

MIC failures occur by pitting corrosion, often at welds, where colonies of bacteria may form. A number of different bacterial species are known to cause the problem, but the detailed mechanism is not known.

Iron utilising bacteria appear to be the dominating microbial species involved with MIC occurring in stainless steel. Anaerobic sulphate-reducing bacteria pose a greater risk of instigating or accelerating corrosion often under a layer of aerobic slime or microbial deposits. However others, such as manganese utilising bacteria (generally from underground waters), have also been discovered.

MIC is extremely aggressive and difficult to eliminate once established, so it is surprising and disappointing that there is limited knowledge of MIC within the engineering community. Fortunately, MIC is easily avoided by using good practices during the initial hydrostatic testing. Education and promotion of proven hydrostatic testing practices which prevent MIC are vital to minimising its potential impact on the stainless steel industry.

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Hydrostatic testing practices to eliminate MIC

In order to eliminate MIC, it is recommended that the following practices are used.

1. Fabrication practices

Crevices should be eliminated or at least minimised during the fabrication process, as they are the preferred sites for attachment and growth of microbial colonies. They also provide traps for chemicals which could concentrate and cause pits.

The likelihood of MIC will also be reduced by:
> using full penetration welds; and
> purge welding to prevent the formation of heat tint; or
> removing heat tint by grinding or pickling.

Arc strikes and weld splatter should also be ground off and pickled.

2. Use clean water

The cleanest water available should be used in a hydrostatic test, such as demineralised, steam condensate or treated potable water. Untreated or raw water from dams or bores should be avoided when conducting a hydrostatic test but, where this is not possible, the water should be sterilised (eg by chlorination) before use. If sterilisation is not practical, the requirements for short residence time and subsequent drying of the system are extremely important. The cleaner the water, the less ‘food’ there is for MIC bacteria to live off and multiply.

It is important to ensure that there is no trace of sediment in the stainless steel system during testing to avoid silting, as the water is normally not circulated during a hydrostatic test. This may require the test water to be filtered to ensure it is free of all undissolved solids. Sediments can provide the conditions for crevice attack.

3. Draining and drying

Thoroughly draining and drying the stainless steel system immediately following a hydrostatic test (preferably within 24 hours, certainly within 5 days) will almost certainly prevent the occurrence of MIC.

Horizontal pipelines should be installed in a sloping direction to make them self-draining.

Drying can be achieved by pigging (cleaning with foam or rubber scrapers), followed by blowing dry air through the system. Beware of blowing higher temperature moist air through cold pipework unless the air is dried before being introduced to the system. If warm air is used, it should not be from a gas burner as condensation may occur.

Draining and drying of systems following a hydrostatic test should only be disregarded when the system is placed into service immediately following the test. Partial draining is potentially very serious as subsequent slow evaporation of even clean residual water can produce very concentrated and aggressive solutions.

Figure-1

4. Chloride content and temperature

During hydrostatic testing of stainless steel equipment, the chloride content of the test water must be within the range to which the stainless steel grade is resistant. Figure 1 shows the maximum temperatures and chloride contents to which stainless steels are resistant in water with residual chlorine of about 1 ppm.

The limits shown in Figure 1 may be exceeded provided the contact time of the water is brief, ie 24-48 hours.

If the chloride content of the test water is uncertain, the water should be analysed.

5. Standards

NACE and API standards for a number of products and installations provide guidelines for hydrostatic testing, including limits for water quality and contact times. These standards should be consulted for specific details for the fabrication in hand.

Conclusion

The benefits of stainless steel’s corrosion resistance are well proven in many industrial applications involving piping systems, but failures can occur during hydrostatic testing if care is not taken. Attention to a few simple details will prevent surprises a few months down the track, allowing the long service life available from stainless steel to be fully realised.