Logo


Preventing Coastal Corrosion (Tea Staining)

When used properly, stainless steel enjoys a strong and enduring reputation for visual appeal and structural integrity in a wide range of applications and environments.

But, like all materials, stainless steel may become stained or discoloured over time, impairing the overall look. This brown discolouration - tea staining - has been identified in coastal applications in Australia and overseas.

Factors affecting tea staining have been researched by ASSDA and the information gathered has been supported by experiences from around the world.

This article provides information on tea staining and what fabricators, specifiers and end users should do to help avoid it and enjoy the long life and clean appearance of stainless steel.

WHAT IS TEA STAINING?
Tea staining is discolouration of the surface of stainless steel by corrosion. It is a cosmetic issue that does not affect the structural integrity or the lifetime of the material. Tea staining occurs most commonly within about five kilometres of the surf and becomes progressively worse closer to the marine source.

However, wind exposure, pollution levels, local sheltering and higher temperatures can create environments where tea staining might occur 20 kilometres or more from the surf. The effect is much less severe around sheltered bays. These same factors also increase corrosion rates of alternative materials.

Other causes of staining that are not tea staining include carbon steel contamination, uncleaned welds and chemical fumes such as hydrochloric acid or bleach. The ASSDA Reference Manual has more details on this.

WHY DOES TEA STAINING OCCUR?
The relationships between the contributing factors are complex, but generally become increasingly critical closer to salty water. Tea staining occurs when local conditions (such as temperature, relative humidity and presence of corrosive substances on the surface) are too aggressive for that stainless steel grade in its installed condition.

There are important factors that promote the occurrence of tea staining that should be considered, as shown in the box and explained below.

1.    Presence of corrosive substances
The presence of sea salt on the surface of the stainless steel is one of the major factors that causes tea staining. Sea salt has the characteristic of staying wet until a very low relative humidity (RH). The result of this is that the surface stays wet (and is corroding) longer with sea salt compared with sodium chloride. However, presence of industrial pollutants could also make the conditions more aggressive.

2.    Atmospheric conditions
A combination of atmospheric conditions with high humidity (eg tropical climates) and a high temperature creates worse conditions for the occurrence of tea staining. The high humidity generates a film of moisture that dissolves the salt deposits and creates a corrosive solution on the surface. The low humidity and absence of corrosive deposits means that tea staining is rarely a problem indoors.

3.    Surface orientation and design
Poor drainage promotes corrosion whether it is because the surface is near horizontal or has a texture that traps contaminants. Conditions are very aggressive in rain sheltered areas such as the underside of sloping roofs, downpipes under eaves or in a building rain shadow. These can cause significant tea staining. Designs with corners or crevices (such as intermittent welds) can trap water and lead to more serious corrosion than tea staining.

4.    Surface roughness
Deep grooves or metal folds on a surface are more susceptible to corrosion because they can trap salts (chlorides). When the surface dries the salts become concentrated, making the conditions more aggressive. A deep groove will have more trapped water (and salts) so the bottom of the groove will be exposed to salt concentration above its resistance for longer - which will initiate corrosion. There is a critical surface roughness of approximately 0.5 μm Ra for cut or abraded surfaces. Abraded surfaces smoother than approximately 0.5µm Ra are much less susceptible to corrosion.

5.    Surface characteristics
To achieve the best corrosion performance of a stainless steel, the surface should be clean, free of contamination such as carbon steel swarf or manganese sulphide inclusions, and have a continuous passive layer. Acid pickling, acid passivation or electropolishing for sufficient time will remove these contaminants from the surface as well as restore the passive layer, leaving the stainless steel with a clean and corrosion resistant surface. If a stainless steel is welded, the heat input will locally destroy the passive layer (a dark non-protective oxide is formed around the weld). To achieve best corrosion performance and restore passivity of the weld, the heat tint and underlying chromium depleted layer must be removed. How this is done is described later.

6.    Appropriate grade
There are several hundred grades of stainless steel with different chemical composition but only about 10 in common use. All owe their corrosion resistance to the thin chromium oxide film on the surface, although other additions such as molybdenum and nitrogen can improve the corrosion resistance, especially in chloride-containing environments. A formula based on the content of these three elements is useful to rank the corrosion resistance of different grades. This Pitting Resistance Equivalent [PRE] number is calculated by %Chromium + 3.3 %Molybdenum + 16 %Nitrogen. The PRE ranges from 10.5 for the grades with the lowest corrosion resistance to more than 40. For acceptable corrosion resistance, typically a PRE of approximately 18 is adequate away from marine influences, PRE of approximately 24 is required for marine atmospheres while severe marine atmospheres may require PRE of approximately 34. The higher the PRE, the greater the corrosion resistance.

7.    Maintenance
Stainless steel is a low maintenance material but it is not generally maintenance free. A light and regular wash is best and natural rain washing may be sufficient. If not, then consider washing the stainless steel when you wash an adjacent window. Lower grades will require more regular maintenance and if the environment causes sticky deposits, a solvent and detergent mix may be required. Application of oils or waxes will temporarily restrict chloride access to the stainless steel but they need regular renewal. These temporary protectives also tend to attract debris and dull the surface.

CONDITIONS REDUCING THE RISK OF TEA STAINING

1.    Absence of corrosives - especially salt.
2.    Atmospheric conditions - lower temperatures and low relative humidity (RH) are better.
3.    Surface orientation and design - free drainage and avoidance of traps which can concentrate corrosives. This includes open exposure to allow   rain washing.
4.    Surface roughness - smoother is better.
5.    Chemical cleanliness or passivation of the surface improves the corrosion resistance.
6.    Appropriate grade for exposure conditions - increasing PRE increases corrosion resistance.
7.    Maintenance - or corrosives will accumulate.

GUIDANCE IN FABRICATION

Design, fabrication and handling
Poor design and fabrication can lead to tea staining or more serious corrosion of stainless steels. Surfaces should be free draining, boldly exposed to rain washing and avoid channelling of run-off. Horizontal surfaces or curves which cause ponding are specific problems. Abraded surfaces should not be rougher than 0.5µm Ra and the grain should be vertical to avoid ponding and collection of contaminants. For abraded surfaces, the best corrosion resistance will be achieved if a nitric acid passivation treatment is carried out as a final step.

Competent stainless steel fabricators will avoid carbon steel contamination (which can cause other corrosion problems), so choose designers and fabricators that are experienced with stainless steel to achieve the best outcome.

Appropriate grade selection
Each stainless steel has a limit to the concentration of salts that it can comfortably resist: the higher the alloying content (Cr, Mo and N), the higher the resistance to corrosion. Exposure of a particular grade of stainless steel to a more aggressive environment than it can resist will cause tea staining.

Grade 316, or a grade with equivalent corrosion resistance, should be selected as a minimum within five kilometres of the surf. For critical applications (eg splash zones, unwashed areas or rough surfaces), higher grades of stainless steel such as duplex or ‘super’ grades may be required.

The lower alloyed and less expensive grades (such as 304 or 430) will probably become tea stained or even suffer more severe corrosion in a marine environment.

Treatment of welds
Pickling after welding is one method of promoting good performance of stainless steel near the coast. This chemical treatment normally uses a mixture of nitric and hydrofluoric acid in a gel, paste or bath. It removes the welding oxide and chromium depleted layer underneath and rapidly restores the passive layer, which gives stainless steel its corrosion resistance. A darker heat tint means a thicker oxide and a longer exposure to pickling acids is required. Pickling removes material from the surface in a controlled way and may etch and dull the stainless steel surface. Excellent gas shielding, so there is no more than a pale straw colour, may avoid pickling provided the environment is mild. An alternative is to mechanically remove the scale and underlying chromium depleted layer, followed by a chemical passivation treatment using nitric acid. Any mechanical removal must not unduly roughen the surface.

Installation and inspection
After installation, the completed structure should be visually inspected for surface damage or contaminants. If contamination is suspected, several cycles of a misting and drying test with tap water is relatively simple. The sensitive ferroxyl test (described In ASTM A380) may also be used in critical applications. If discovered, imperfections should be removed and the corrosion resistance chemically restored by pickling or passivating treatments or by electropolishing.

Do not use hydrochloric acid
Hydrochloric acid, sometimes used to clean cement or mortar residues, must not be used on stainless steel — it will stain the surface and usually start more serious corrosion.  

KEY DESIGN RECOMMENDATIONS

Plan to get the desired result
Marine environments are the most aggressive for all building materials.
Stainless steel’s corrosion resistance in marine environments means that installations are likely to remain structurally sound for decades (see image on right).

It must be recognised, however, that keeping a pristine surface finish requires understanding and, usually, additional cost. Determine your expectation of the structure and plan ahead to achieve and maintain the intended result. This normally includes a maintenance program.

Environment
Tea staining is most likely to occur up to five kilometres from a surf beach and one kilometre from still marine waters. There is no hard and fast rule: wind and weather conditions play a big part and the severity of the conditions increases sharply as you approach the surf. AS 2312 suggests that in some special circumstances, 20 kilometres from the coast can still constitute a marine environment. The closer to the source of salt, the more critical it is to follow the recommendations in this Bulletin.
Areas that are sheltered or not rain washed are particularly susceptible. Tropical and high humidity areas are also more at risk of tea staining.

Specify and insist on a smooth and clean surface finish
To minimise the risk for tea staining the smoother the surface the better. A surface roughness of less than 0.5µm Ra  is strongly recomended. Surfaces smoother than 0.5µm Ra will have even better corrosion resistance. The most corrosion resistant, mechanically finished surface is a mirror polish (ASTM A480 No.8 or EN10088.2 class 2P). It is very smooth, resistant to salt accumulation and easy to clean. The surface roughness of a mirror polished surface is so low that it is not reliably measurable by mechanical (stylus) instruments.
A No.4 finish just means an abraded (linished) finish. Specifying a No.4 finish is inadequate without indicating the required roughness.  
The Euronorm standard EN10088.2 (finish 2K) recognises this and requires Ra<0.5µm but also that the abraded profile is a clean cut.

Components used near the sea can be made more resistant to tea staining if they are passivated to remove surface contaminants such as steel smears, weld spatter or sulphide inclusions. Mild levels of contamination may be removed by nitric acid passivation which should not change the surface appearance although it may slightly cloud a mirror polish. More severe contamination by particles of steel or grinding debris may require pickling which etches and usually dulls the surface. Either process may use pastes or gels (which can be applied on site) or liquids in baths in a factory. These chemical processes take longer if it is cold.

Electropolishing has been found to be extremely effective in removing surface contamination and passivating the surface. It also brightens and slightly smoothes the surface as well as rounding sharp edges and removing the peaks left from polishing operations. Electropolished surfaces have a characteristic lustre but may not be mirror smooth. A mechanically mirror polished surface will normally lose its mirror reflectance if electropolished.

Smoother mill finishes such as 2B and Bright Annealed (BA) are widely available in flat products. Provided they are not damaged during fabrication, they offer good resistance to collection of salt deposits and hence to tea staining.
Rolled embossed finishes may be suitable for some applications. These have very smooth surfaces but with a pattern that lowers reflectivity. Think carefully about the pattern and how it will be oriented — avoid pools of water sitting on the surface.

Specify and insist on the right grade
In marine environments, use grade 316 or one with equivalent corrosion resistance unless the job is aesthetically critical and regular maintenance is unlikely.

Where there are high aesthetic expectations a number of more corrosion resistant stainless steel grades can be considered. The first step up from 316 is 2205 and then the super duplex grades, although the high molybdenum austenitics and high molybdenum ferritics may also be useful. Smooth surface finish and maintenance are still important with these grades.

Treatment of welds
For general architectural applications welds should comply with AS/NZS 1554.6 Level 2, Class B. (Details of other weld finish classifications are given in the ASSDA Reference Manual). However, this specification does not guarantee the absence of structurally minor surface defects which can act as traps and corrosion initiating sites. The protruding weld can be ground flush, and good resistance to tea staining achieved (a Grade I finish) when polished to 320 grit or finer finish. The smoother the surface, the better the tea staining resistance. Passivation will occur in chloride free, moist air within a day. Chemical passivation treatment with nitric acid may be applied to:

  • Substantially reduce the time required for passivation
  • Provide a more corrosion resistant passive film
  • Remove possible iron contamination
  • Dissolve exposed manganese sulphide


Chemical passivation must be applied after abrasion if the environment is particularly aggressive.
An alternative cleaning treatment is a Grade II blast cleaned finish. This will require a post blasting passivation treatment. The blasting should remove heat tint and the chromium depleted layer but not make the surface roughness worse than 0.5 µm Ra, must not leave folds or crevices and should not embed corrodents. The Grade II stainless steel wire brushing treatment is not adequate to control tea staining.

Where a polished (or linished or ground) finish is desired, abrasives should be used with lubrication if possible. In selecting abrasives, consideration should be given to matching the surrounding finish.

A Grade II pickled finish will provide good tea staining resistance without grinding the weld flush, provided there are no significant surface crevices/defects. Where linishing or blasting is not performed, pickling of site welds (using mixed hydrofluoric plus nitric acids) should take place as a final step in the weld procedure.

Pickling will remove any fabrication contaminants and restore the passive chromium oxide layer, resulting in a corrosion resistant surface. Electrocleaning has been used instead of pickling to remove weld scale and heat tint, especially when hydrofluoric acid use is restricted. While passivation treatments do not normally affect appearance, pickling treatments are likely to dull bright surface finishes. Electropolishing is also a very effective method of passivation. ASSDA's Australian Stainless Reference Manual describes these treatments in more detail.

Specify and insist on regular maintenance
Washing removes deposits (such as salt) that can cause corrosion. It is necessary to avoid tea staining. Rain washing the surface is helpful in reducing tea staining, so design the job to take advantage of the rain, but ensure good and even drainage.

Stipulate that the stainless steel also be washed when cleaning of the surrounding area takes place. As a guide, stainless steel should be washed if a window requires washing. For best results, wash with soap or mild detergent and warm water followed by rinsing with clean cold water. The appearance of the surface can be improved further if the washed surface is wiped dry.

If routine cleaning of the surrounding area does not take place, washing frequency for the stainless steel is recommended as in Table 1 below.

It is essential that abrasive cleaners or those containing chlorides or bleach are NOT used to clean stainless steels as they will damage the surface. If some tea staining does occur, then an assessment of the 7 points is required to determine why the problem occurred. Simple mechanical polishing is unlikely to both remove current and prevent future teastaining. Reasonably simple chemical cleaning and passivation is usually the most effective treatment. ASSDA's Australian Stainless Reference Manual has more details.

Download ASSDA Technical FAQ6: Preventing Coastal Corrosion (Tea Staining) (Edition 3, Feb 2010)

Further Reading

ASSDA's Australian Stainless Reference Manual Edition 7, 2012

Australian Standard AS/NZS 1554.6 Welding Stainless Steel for Structural Purposes

ASTM Standard A380 Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems

Nickel Institute, Japan Stainless Steel Association Successful Use of Stainless Steel Building Materials publication No 12 013

Nickel Institute Guidelines for the Welded Fabrication of Nickel-containing Stainless Steels for Corrosion Resistant Services publication No 11 007.

Common Traps to Avoid


Posted 1 April 2006

Errors in stainless steel fabrication can be expensive and difficult to resolve. So a 'Get it right the first time' approach to stainless fabrication is necessary to gain the best result. Check the ASSDA website regularly for a local Stainless Steel Specialist.

ASSDA Accredited Fabricators - Ensuring the Best Result
ASSDA Accredited Fabricators
are companies and individuals that have a common understanding of successful technical practices for fabricating stainless steel.

To ensure the highest standard in quality, Accredited Fabricators belong to the ASSDA Accreditation Scheme, an ASSDA initiative that is intended to achieve self regulation of the industry, for the benefit of both industry members and end users.

The Accreditation Scheme criteria requires all fabricators to conform to stringent standards of competence, training and education, personal and professional conduct, adhering to a Code of Ethics and a Code of Practice, and committing themselves to continuing competency development.

The Scheme gives owners and specifiers of stainless steel greater certainty that fabrications using stainless steel will be performed by technically competent industry specialists.

COMMON TRAPS TO AVOID

Surface damage, defects and contamination arising during fabrication are all potentially harmful to the oxide film that protects stainless steel in service. Once damaged, corrosion may initiate. Common causes of surface damage and defects during fabrication include:

Scratches and Mechanical Damage

Scratches and gouges form crevices on the steels surface, allowing entrapment of process reactants or contaminants, providing ideal locations for corrosion. Scratches may also contain carbon steel or other contaminants embedded by the object that caused the scratch.

Scratches will also raise customer concerns in situations where appearance is important. Mechanical cleaning is the most effective way to remove them. Prevention would be better.

SURFACE CONTAMINANTS

Common contaminants likely to attack stainless steel include carbon steel and common salt. Dust and grime arising during fabrication may contain these contaminants and should be prevented from settling on stainless steels.

Oil, grease, fingerprints, crayon, paint and chalk marks may also contain products that can provide crevices for localised corrosion and also act as shields to chemical and electrochemical cleaning. They should be removed.

Residual adhesives from tape and protective plastic sometimes remain on surfaces when they are stripped. Organic solvents should remove soft adhesive particles. If left to harden, adhesives form sites for crevice corrosion and are difficult to remove.

The most frequently encountered fabrication problem is embedded iron and loose iron particles, which rapidly rust and initiate corrosion. Other common sources of contamination are abrasives previously used on carbon steel, carbon steel wire brushes, grinding dust and weld spatter from carbon steel operations, introducing iron filings by walking on stainless steel and iron embedded or smeared on surfaces during layout and handling. All should be avoided.

WELDING

The high temperature characteristics of welding can introduce surface and other defects which must be addressed.

Undercut, spatter, slag and stray arc strikes must be minimised as they are potential sites of crevice corrosion. General cleanliness and removal of potential carbon contaminants such as crayon marks, oil or grease is important in obtaining good weld quality. It is also important to remove any zinc that might be present.

Scale on a welding site. Note the crevice corrosion and corrosion from the weld heat tint at this seaside site.HEAT TINT AND SCALE

Heat tint and scale occur when stainless steel surfaces are heated to moderately high temperatures in air (3500C+) during welding.

Deleterious oxides of chromium may develop on each side and on the under surface of welds and ground areas. These oxides lower the corrosion resistance of the steel and during their formation the stainless steel is depleted of chromium. The oxidation and the portion of the underlying metal surface with reduced chromium should all be removed by mechanical, chemical or electrochemical means to achieve the best corrosion resistance.

DISTORTION

Stainless steel has a relatively high coefficient of thermal expansion coupled with low thermal conductivity, at least compared with carbon steel. So, stainless steel expands rapidly with the input of heat that occurs during welding and the heat remains close to the heating source. Distortion can result. Distortion can be minimised through using lowest amperage consistent with good weld quality, controlling interpass temperatures and using controlled tack welding, clamping jigs with copper or aluminium backing bars as heat sinks on the welds.1

Defects DiagramREMOVAL OF SURFACE CONTAMINATION

There are three methods of repairing the surface of stainless steel.

MECHANICAL CLEANING

Wire brushing should only be done with stainless steel bristles that have not been used on any other surface but stainless steel. Clean abrasive disks and clean flapper wheels are commonly used to remove heat tint and other minor surface imperfections. Also effective is blasting with stainless steel shot, cut wire or new, iron-free sand (garnet is a common choice).

This image was taken a month after installation. Corrosion resulting from rough finishing technique is evident. Note also the crevices that are likely corrosion sites.CHEMICAL AND ELECTROCHEMICAL CLEANING

Embedded iron, heat tint and some other contaminants can be removed by acid pickling, usually with a nitric-hydrofluoric acid mixture or by electropolishing. These processes remove, in a controlled manner, from the affected areas, the dark oxide film and a thin layer of metal under it, leaving a clean, defect-free surface. The protective film reforms after exposure to air.

Passivation

Passivation involves treating stainless steel surfaces with, usually, dilute nitric acid solutions or pastes. This process removes contaminants and allows for a passive film to be formed on a fresh surface, following grinding, machining etc.

Care must be taken. Nitric acid treatments will remove free iron, but not iron oxide contaminants. Passivating, unlike pickling, will not cause a marked change in the appearance of the steel surface.

Fabricated pipe showing carbon steel pickup. INSTALLATION

Stainless steel is best installed last to avoid damage during construction. Also, careful storage and handling including protective coating films are required prior to and during installation to minimise risk of damage to the stainless steel structure.

A primary goal of the stainless steel industry is to have finished products put into service in a 'passive' condition (free of corrosive reactions). Stainless steel is a robust and relatively forgiving material, but adherence to informed, good practice will ensure satisfaction for customers and suppliers alike.

Understanding stainless steel is important to its successful application. Ask your stainless steel representatives whether they have successfully completed ASSDA's Stainless Steel Specialist Course. Their commitment to product knowledge will be your key to success.

REFERENCES

1. NI & Euro Inox (1994) Design Manual for Structural Stainless Steel NiDI Ref. No. 11 013

RESOURCES

  • AS 1554.6 'Welding Stainless Steel for Structural Purposes'
  • Australian Stainless 2005 Reference Manual, ASSDA
  • Stainless Fabrication Group, New Zealand, 'Code of Practice for the Fabrication of Stainless Steel Plant and Equipment'
  • Nickel Institute (NI) 'Cleaning Stainless Steel Surfaces Prior to Sanitary Service'
  • ASSDA - www.assda.asn.au
  • BSSA - www.bssa.org.uk
  • Nickel Institute - www.nickelinstitute.org

ASSDA acknowledges the assistance and contribution of Mr Peter Moore, Technical Services Manager of Atlas Specialty Metals in the production of this article. Photographs courtesy of Peritech and Outokumpu.

This technical article featured in Australian Stainless magazine - Issue 35, Autumn 2006.

No. 4: The workhorse finish

No. 4 finish stainless steel is the workhorse of the light fabrication industry. The easiest of the finishes to maintain, No 4 finish is used for work surfaces, handrails and where appearance is important.

A 'No. 4' surface is produced by cutting the surface with abrasive belts to remove a very small amount of metal without affecting its thickness.

For architects and designers, No. 4 finish gives low gloss and best apparent flatness of panels.  For fabricators, the No 4 finish is directional, allowing easy matching of surfaces and refinishing of welds. For end users, the surface can be repaired to remove any service damage.

No. 4 finish is duller than the other common finishes, 2B and BA and is generally used where lower reflectivity or gloss is required and where welds and other fabrication marks are to be refinished to match the original surface. This is not possible with 2B and difficult with BA.

Abrasive belts have very fine grains of refractory material such as silica, alumina and zirconia embedded in an adhesive layer on a flexible cloth or paper backing. The belts are wider than the stainless steel, which is usually worked on as coil, or sometimes in individual sheets. The steel is run slowly under rolls, on which the abrasive belts run.

The polishing machines at stainless steel mills lubricate the cutting action by flooding the strip with oil. This helps to keep it cool, and gives a finer, more uniform surface.

The variability of the process means not every No 4 finish looks the same, even from the same source. Different manufacturers use belts with different combinations of grit sizes, and the finish can vary through the life of a set of belts.

Where it is important that the appearance of material matches on a job, it should all be taken from the same pack of sheets, used sequentially and in the same orientation. A reasonable match in appearance can be achieved more readily with No 4 finish than with 2B or BA mill finishes.

No-4-finish.jpg The Owner of this resource has not specified a description BA-finish.jpg
No 4 Finish 2B Finish BA Finish

Standards

Until recently, standards defined No 4 finish in terms of the coarseness of the abrasives used to produce a general purpose finish widely used for restaurant equipment, kitchen equipment, shopfronts and food processing. New editions of the American and European standards define limits of surface roughness achieved.

Finishes produced by use of abrasives may be called ground or polished or abraded or linished. These words describe a process and do not specify the end result.

ASTM A480 defines No 4 finish simply as, “General purpose polished finish, one or both sides”. It also states, “No. 4 - A linearly textured finish that may be produced by either mechanical polishing or rolling. Average surface roughness (Ra) may generally be up to 25 micro-inches (0.64 micrometres). A skilled operator can generally blend this finish.”

The practice in Australia is only to use 'No. 4' as a description of a polished finish and it is not a rolled finish. The European standard, EN10088-2, defines two finishes, '2J' and '2K'. There is no prescription of the appearance or roughness of the '2J' finish, but '2K' is defined as surface Ra below 0.5 micrometres. The notes state, ”Additional specific requirements to a 'J'-type finish, to achieve adequate corrosion resistance for marine and external architectural applications.”

Figure 1: No-4-finish-graph.jpg Figure 2: 2B-finish-graph.jpg
Figure 1: Surface trace of a typical No 4 finish
(Ra = 0.41 micrometres)
Figure 2: Surface trace of a typical 2B finish
(Ra = 0.20 micrometres)

The surface traces of Figure 1 and Figure 2 show comparisons between typical No. 4 and 2B finishes. Unlike a 2B finish which is generally rougher on thicker coil, the roughness of No. 4 does not vary with the steel thickness.

While Ra can be specified to give better control of the corrosion properties of the surface, it correlates only moderately with appearance and is also difficult to measure reproducibly.

Gloss is the amount of light reflected whether specular (mirror like) or diffuse. It is moderately correlated with appearance and with surface roughness, but can also have problems when used for specification.

Neither Ra nor gloss are suitable for specification for critical jobs in architecture. Two finishes with the same Ra can look substantially different, as can finishes with the same gloss level.

For critical jobs appearance is best specified using reference samples viewed under agreed conditions. These should be large enough that they can be viewed from a variety of angles and distances - appearance can vary with viewing angle.

Corrosion Resistance

The corrosion resistance of a No 4 finish is usually lower than that of a mill finish (BA or 2B) on the same grade.

The surface scratches or grooves produced by abrasion expose sulphide inclusions, which are always present in all steels, and can act as a catalyst for corrosion.

The passive surface layer is more likely to be disrupted somewhere on the vastly increased surface area with all its sharp peaks and deep valleys. It is difficult to keep the surface clean when there are intersecting valleys, torn metal flaps or peaks that have been folded over.

Corrosion resistance may be reduced depending on the stainless steel grade used. By using grade 316 with a No. 4 finish in aggressive environments, the corrosion resistance is negated and may be less than on 304 with a 2B or BA finish.

Figure 3: The accceleration of the corrosion of the surface at Ra above 0.5 micrometres is apparent.
Figure 3: The acceleration of the corrosion of the surface at Ra above 0.5 micrometres is apparent.

Figure 3 shows the results of electrochemical tests for corrosion of a polished surface. Corrosion resistance of a smooth surface can be better than the corrosion resistance of an abraded surface of a more highly alloyed grade.

The orientation of the No. 4 finish is also important. When the lines on the surface are vertical, drainage is easier and corrosion resistance is better than when the lines are horizontal.

The reduced corrosion resistance of the No. 4 finish is not likely to be of concern in mild applications such as food preparation and display. However, in more aggressive conditions such as marine and industrial atmospheres it is important to be aware of the reduced corrosion resistance of No. 4 finish and to take steps to improve the resistance.

Corrosion resistance of No. 4 finish can be improved by pickling the surface in a mixture of hydrofluoric and nitric acids, or passivating in a nitric acid solution.

The passivation treatment dissolves the sulphide inclusions in the surface, but doesn't change the appearance of the surface. The pickling treatment is more aggressive and removes both the sulphide inclusions and some of the rougher parts of the surface, dulling the appearance.

Unfortunately it is almost impossible to achieve a uniform finish, and it is rarely practical to pickle for better corrosion resistance. Passivation is often used. ASTM A967 “Chemical Passivation Treatments for Stainless Steel Parts” specifies a number of treatments with various acid strengths, temperature and contact time.

Electropolishing the surface can also improve the corrosion resistance and brighten the surface. The peaks on the surface are smoothed, reducing the Ra value and increasing the reflectivity or gloss. The sulphide inclusions may also be removed or reduced.

Protection of the Surface

No. 4 finish is usually supplied with a protective plastic film of white polyethylene, which often has printed lines on the plastic in the same direction as the No. 4 polish.

It is best to keep the film on the surface of the steel during fabrication, to prevent handling and transport damage. The film has limited resistance to sunlight, and should not be left on the steel in the sun for more than a week or two - an hour or two if the film isn't black underneath. The film may bake onto the surface and either become brittle or tear into strips on removal, or leave the glue on the steel surface.

Glue on the steel will trap dirt, and may cause rapid surface discolouration or tea staining. If it is suspected there is residual glue on the steel, swab the surface with a solvent such as Methyl ethyl ketone (or MEK - a solvent) available from panel beaters suppliers. You may need to test other solvents, depending on how the glue has polymerised.

The water break test tells you the surface is clean - clean water dries as a film, doesn't stand in bubbles on the surface. A final wipe with a glass or window cleaner will ensure a streak free finish.

Cleaning

No. 4 finish can usually be kept clean by wiping down with a damp soft clean cloth. For grease, moisten the soft cloth with ammonia solution, or with one of the household liquid grease removers. Very hot water is also quite effective.

Wiping should always be in the direction of the polishing lines. Some No. 4 finishes can pull threads and fluff from the cloth which are very hard to get off the steel.

Abrasive cleaners and materials such as Scotchbrite™ should never be used as these will change the appearance of the surface. If you want to change it, try an inconspicuous area, then treat the whole surface - but it's difficult to get it uniform.

There are also white powder stainless steel cleaners (Clark and Esteele), made of sulphamic acid, which can be wiped over the surface on a damp rag to brighten it - test an inconspicuous area first. Fingerprints can be made less obvious by applying a light oil to the surface. There are many proprietary products available, usually labelled 'stainless steel cleaner'. Choose an oily one, although it will tend to trap dust.

This ASSDA technical article was written by Dr Alex Gouch, Development and Technical Manager of Austral Wright Metals. ASSDA acknowledges the assistance and contribution of Mr Peter Moore, Technical Services Manager of Atlas Steels and Dr Graham Sussex, ASSDA Technical Specialist in the production of this article.

This article featured in Australian Stainless Issue 36, Winter 2006.

445M2: A New Generation Stainless Steel (Part 1)

This article is the first in a series showcasing the uses of 445M2 stainless steel. Read Part 2. Read Part 3.

Australians' love of the water has always provided challenges to the construction industry, particularly when it comes to choosing materials that can be used in aggressive environments such as near the coast or swimming pools.

Stainless steel grades 316 and 304 have long been the obvious solution in these applications, but the key factors of formability, cost and corrosion resistance are now shining the spotlight on an alternative grade.

445M2 stainless steel has been used in Australia for a number of years for roofing and walling applications, and its characteristics are now proving useful for a broader range of applications.

The material, supplied by ASSDA member Austral Wright Metals, is being used by Dunning Engineering Services Pty Ltd for a range of stainless steel pergola brackets.

Dunnings - a South Australian based manufacturer of builders and plumbers hardware, who also operate a sheet metal pressing and fabrication facility - developed the range in response to the growing demand for better corrosion-resistant products that can be used in aggressive environments.

The company experimented with punching and bending various grades of stainless steel, including 316, but it was 445M2's formability that provided the crisp, clean angles they were seeking, with the advantage of reduced tool wear.æ Dunnings was also able to fabricate with existing tooling and machinery, avoiding the prohibitive cost of new dies and tooling.

More importantly, 445M2 is a marine grade stainless steel with the corrosion resistance of 316 or better and a cost that falls between 304 and 316.

Dunning spokesperson John Gill said 445M2 resisted the salt from the surf, and gave safe performance over a long life - even when painted.

"Due to the formability of 445M2, the savings to our business have been enormous and we are now looking at other areas where 445M2 could be applied."

This article featured in Australian Stainless magazine - Issue 37, Spring 2006.

Design Software Vs. Back to Basics

New technology to assist with accurate design is always welcome, but it is important that users proceed with caution when using international design tools.

There is no doubt that designing with stainless steel offers endless opportunities for architects and engineers to be both creative and functional. At the same time, it is critical that the design is right for the application.

Thanks to the internationally-recognised research of an Australian expert, as well as some design software now available free online, getting the design right for stainless steel structures has never been easier. However, as outlined below, it is more important than ever for design engineers to use caution when using international technology.

History of Design in Australia
The Australian Standard for design of stainless steel structures, AS/NZS 4673:2001 “Cold-formed stainless steel structures” was first published in 2001 and provides methods for design calculations. Applicable to cold-formed structures, including construction with tubular hollow sections, it provides a means of designing light and innovative structural solutions.

Traditionally, design engineers have reached for ‘load tables’ – or, strictly, Member Capacity Tables. Most design offices have tables with the results of calculations for various steel sections and loading regimes, generally published by suppliers of carbon steel.

But carbon steel has different properties from stainless steel, so these tables are not right for stainless steel – they may be too conservative or not conservative enough.

Another problem is that some engineers have assumed because they can find a section in carbon steel load tables, they can source it in stainless steel – only to discover they can’t, after doing an expensive design.

Designing with Software
Load tables for stainless steel are available from the Steel Construction Institute in the UK. The SCI is an independent, technical, member-based organisation with over 850 corporate members in 40 countries around the world.

Now the SCI has made available free software for design calculations for stainless steel members, using the methods of the European Design Manual, published by Euro-Inox.

Available over the web at http://www.steel-stainless.org/software/, the software speeds structural design calculations for a range of sections and stainless steel grades.

However, a word of warning: the software uses methods in compliance with parts of Eurocode 3 “Design of steel structures”. The Australian code for design of stainless steel structures, AS/NZS 4673:2001, follows the methods of the USA code, not the Eurocode. This is logical, as the Australian codes for the design of cold-formed carbon steel structures are also aligned with the USA codes and the trend in the Australian construction industry is to employ cold-formed steel to achieve lightness, material efficiency and enhanced strength.

So the SCI software must be used with some caution – it is the best available, but not ideal. The software should not be used in conjunction with the Australian code AS/NZS 4673:2001, as mixing clauses of different specifications is not an acceptable practice. This caution applies particularly to the design of welded structural members, which is catered for by the SCI software but not within the scope of the Australian Standard.

Future Improvements
In January 2005, Professor Kim Rasmussen of Sydney University was appointed chairman of the American Society of Civil Engineers (ASCE) Standards Committee responsible for the American “Specification for the design of cold-formed stainless steel structural members”. This is the Standard that formed the basis of AS/NZS 4673:2001.

The ASCE Standards Committee will be updating the American Standard and Professor Rasmussen will present the new rules implemented in AZ/NZS4673 to the American committee, together with design recommendations derived from recent and ongoing research at Sydney University.

The ASCE Committee is expected to adopt the new rules and recommendations. Subsequently, there is likely to be an update to AZ/NZS 4673 – so there is an ongoing cycle of improvement, helped along by the world-class research in stainless steel structures undertaken by Professor Rasmussen and his students at the University of Sydney.

What Does it all Mean?
In short, international design tools such as the free software available from the SCI can provide some assistance in getting the design right for stainless steel structures, but they don’t provide all the answers and can even complicate matters. Sometimes good design means getting back to basics.

This ASSDA technical article was written by Dr Alex Gouch, Development and Technical Manager of Austral Wright Metals.

ASSDA acknowledges the assistance and contribution of Professor Kim Rasmussen from the School of Civil and Mining Engineering, University of Sydney.

This article featured in Australian Stainless magazine - Issue 37, Spring 2006.

 

445M2: A New Generation Stainless Steel (Part 2)

This article is the second in a series showcasing the uses of 445M2 stainless steel. Read Part 1. Read Part 3.

The use of stainless steel plant and equipment in the food industry continues to prove its worth as an increasing number of processors adopt its use in line with the dedication and obligation to food quality and safety.

Whilst stainless steel grades 316 and 304 offer an environment of easy maintenance and cleaning, 445M2 stainless steel goes one step further.

445M2 panels have been supplied to Bertocchi Smallgoods by ASSDA Major Sponsor Austral Wright Metals, following a four-month trial of all three grades of stainless steel.

Bertocchi, a Melbourne-based company producing hams, bacon, salamis and other specialised continental smallgoods, sought an alternative to their existing painted steel linings after they discovered the life of the linings was too short for their high cleaning standards.

The walls and ceiling of the factory are regularly cleaned in line with a guarantee of the highest quality health and safety standards, together with absolute traceability of every unit of product.

This is where stainless steel stepped in.  The hardness and smoothness of stainless steel enables it to resist the adhesion of soils and bio-films, and the excellent corrosion resistance allows it to be easily cleaned and sanitised. Indeed, laboratory tests prove stainless steel is significantly more hygienic than other materials, even when used for food contact surfaces. Moreover, the taste and colour of food products are not affected by stainless steels.

After four months of trialing panels of stainless steel grades 304, 316 and 445M2, Bertocchi Smallgoods chose the new generation ferritic grade 445M2 to line the factory – ceilings and walls. With superior corrosion resistance to grade 316, 445M2 resists the powerful cleaning agents used to keep the factory clean, as well as the hot, humid and salty atmosphere around the brine lines.

So far, Bertocchi has installed 10 tonnes of 445M2 0.7 x 1219 mm sheet with a 2B finish.  The result?  A clean, bright factory that’s easy to keep that way.  And Bertocchi intends to keep going until the entire factory is lined with 445M2.

This article featured in Australian Stainless magazine - Issue 38, Summer 2006.

Smooth and Corrosion Resistant Surfaces from the Mill

This article is the second in a series on common finishes. The first (Winter edition 2006) dealt with the abraded 'No. 4' (2K, 2J) finish. This article looks at 2D, 2B and BA: smooth and corrosion resistant surfaces produced at the steel mill. Subsequent articles in this series will cover mirror polished (No. 8 or 2P) and profiled and hot rolled (No. 1) finishes.

What are Cold Rolled Finishes?
Cold rolled finishes apply to flat products such as sheet or coil, with thickness less than about 5mm and usually less than 3mm.  They are firstly hot rolled into a strip (or cast into a slab which is hot rolled into a strip) and then cold rolled. Cold rolling reduces the thickness by at least 50%.  This smooths the surface, refines the grain structure and causes differences in the mechanical properties along and across the rolling direction.  In the case of austenitic and duplex alloys, the process hardens and strengthens the stainless steel.  Finally, the steel is softened by annealing in a furnace.  Each surface finish may undergo additional processes to improve the surface further.  The smoother the surface finish is, the higher resistance to corrosion it will be.

These mill produced finishes must be handled carefully as surface damage such as scratches, grinding marks or spatter cannot be matched by polishing with abrasives or etching with chemicals.  Of the 3 finishes, BA is most nearly able to be matched by a mirror polish.

Standards
The most common definitions of these surface finishes are provided by ASTM A480 and EN 10088. In both cases it is the cold rolled manufacturing method that is specified rather than specific, measurable characteristics about the surface. We have used ASTM A480 as an example:

ASTM A480:
No. 2D – A smooth, non-reflective cold-rolled annealed and pickled or descaled finish. This non-directional finish is favourable for the retention of lubricants in deep drawing applications.

No. 2B – A smooth, moderately reflective cold-rolled annealed and pickled or descaled finish typically produced by imparting a final light cold-rolled pass using [large diameter] polished rolls. This general-purpose finish is more readily polished than No 1 or 2D finishes. Product with 2B finish is normally supplied in the annealed plus lightly cold-rolled condition unless a tensile-rolled [harder and stronger] product is specified.

Bright Annealed [BA] Finish – A smooth, bright, reflective finish typically produced by cold rolling followed by annealing in a protective atmosphere so as to prevent oxidation and scaling during annealing.

2B
2B is the most widely used stainless steel surface finish. It is especially common in industrial, chemical and food processing applications such as process vessels and tanks. It is also used in some architectural applications that will not be closely examined for uniformity of finish such as downpipes and gutters.

When specifying this finish think about these attributes:

- 2B is the most economical finish
- it is highly corrosion resistant because it has been chemically pickled and is smooth
- over broad areas and between batches, etc it is not uniform and may not match in appearance
- it has been produced in the mill and can’t be   matched after fabrication- it is often protected by plastic films until final cleanup and commissioning

2D
2D is used around the world in applications where its low reflectivity is important. The largest application is in roofing materials. The surface is rougher than 2B and retains lubricants better making it appealing for deep drawering. 2D surfaces are not designed for appearance so the limitations on matching of weld and other surface damage is not as critical.  Railcars are a typical example where thicker sheet and ongoing abrasive damage make the rougher 2D a suitable finish.

When specifying this finish think about these attributes:

- 2D is not commonly available in Australia
- it is highly corrosion resistant because it has been chemically pickled and is relatively smooth
- over broad areas and between batches, etc it is not uniform and may not match
- it has been produced in the mill and can’t be matched after fabrication

Bright Annealed (BA)
The classic use of a BA finish is domestic: the interior of a dishwasher or clothes washing machine. In the clothes washer, it also provides a smooth, non-abrasive surface for the clothes to slide around.  The mirror like surface is also used in road mirrors where a precise image is not required.

When specifying this finish think about these attributes:

- BA is common in some grades and thicknesses, but not all
- it is highly corrosion resistant because it is very smooth
- different batches may not match
- it has been produced in the mill and can’t be exactly matched after fabrication although a mechanical mirror polish can be close.

Post Production Processing to Improve Corrosion Resistance
All the cold rolled processes include a pickling stage. Pickling is the removal of high temperature scale and the adjacent low chromium layer of metal from the surface of stainless steel by chemical means. Pickling will also remove manganese sulphide inclusions and any other contamination on the surface. Pickling results in a very clean, highly corrosion resistant surface, but will slightly roughen the surface.

Pickling any of these finishes will cause a matt or etched rougher area, most apparent on the BA surface and least on the 2D.

Differences Between Alloys
Most articles on finishes assume that standard grade austenitic stainless steel is used.  Typically a highly corrosion resistant grade has a duller appearance than the same thickness material with a lower corrosion resistance.  This illustrates the more aggressive measures required to remove oxide scale from a high alloy austenitic or duplex than a standard grade 304 or 316.

There is little data on finishes of ferritic grades but general observation shows that for comparable thickness and finishing processes, cold rolled 430 is brighter than 304. It is not known if this difference extends to comparisons between more highly alloyed grades.

This article featured in Australian Stainless magazine - Issue 38, Summer 2006.

445M2: A New Generation Stainless Steel (Part 3)

This article is the third is a series showcasing the uses of 445M2 Stainless Steel. Read Part 1. Read Part 2.

The Applied Science Building at the University of NSW is a landmark in the Eastern suburbs of Sydney. This world-class multi-storey complex of research laboratories and lecture theatres has recently been extended and upgraded, including the air-conditioning and fume extraction systems.

The new air conditioning and ventilation systems were placed on the roof. Management at the University were concerned that the upgrade would be a major disruption and not one they wished to endure more than once in a generation. They needed materials to be long-lasting and require minimal maintenance.

As contractors on the project, Croydon industries chose a new generation marine grade stainless steel. 445M2 was selected for the ducts of the roof, which were exposed to a marine atmosphere created by surf at Maroubra Beach just a few kilometres away.

Brian Clark at Croydon Industries says, "This was the first time we used 445M2, and we were a bit apprehensive - but it gave us no problems. It formed very easily, producing clean lines and well shaped panels with no flaws.

"The stream diffuser sheet of perforated metal was a dream to make.

"The punching ran cool, they sheet came out flat, with very little burr and at a lower cost than the familiar 316. We're glad we opted to use 445M2, it's been a win-win for all."

This article featured in Australian Stainless magazine - Issue 39, Autumn 2007.

Smooth and Corrosion Resistant Surfaces from the Mill

This article is the final in a series on common finishes. Previous articles in this series dealt with the workhorse No. 4 (2J/2K) (AS36, Winter 2006) and the mill finishes 2R (BA), 2B and 2D (AS38, Summer 2006). This article looks at mirror, profiled and coloured surfaces.

Mirror polished surfaces, as the name implies, have a bright reflective surface which give a mirror like image.  They are the most labour intensive mechanically finished surface with an obvious impact on cost and time of production.  As fl at product, mirror finishes are produced by post mill abrasive polishing of either hot rolled or cold rolled sheet or strip.

Although mirror finishes are only defined in standards for flat products, it is also possible to generate a mirror polish on surfaces where there is sufficient access for the grinders and polishing mops.  The exception to this is materials with large or variable microstructures such as some castings or heavily cold worked items or welds.  In these cases, it may not be possible to obtain a mirror surface with a clear image or a uniform lustre.

Mirror polished surfaces are produced by grinding with successively finer abrasives where the next grit size is not selected until all the scratches produced in the previous stage have been removed.  The surface is then buffed with “mops” (which may be soft or hard) and sticks compounded with binders and rouge of whose particle size depends on the required finish.  The buffing stage does not remove much material and, if there are scratches from earlier grinding stages, they will show up as rounded furrows.  Contamination with a larger grit particle will give isolated, but very unsightly scratches.

A mirror finish is the only one that will produce a clear image of its surroundings.  Finishes such as the cold rolled BA or an electropolished 2B or even a No 4 finish with a very low Ra that is then electropolished, will be brilliant and reflective but they will not form a sharp reflected image.

 

Specifications
Both ASTM A480 and EN10088-2 include mirror finish specifications called No 8 and 2P respectively.  Mirror finish is described as a non-directional finish which is reflective and has good image clarity.  The surface will be essentially free from grit lines due to the initial grinding stages but there will be visual differences between surfaces produced by different suppliers to these specifications.

For mirror finishes, requiring that the surface has a specific surface roughness (Ra) is not very useful.  Gloss measurements are a useful technique on fl at surfaces especially if both diffuse and specular reflection are measured.  Not surprisingly, diffuse reflectance is always higher. If a higher level of confidence is required for critical applications, then agreement on comparison with a finished sample in agreed conditions is recommended. It is the most reliable method of detecting random scratches.

Applications and Limitations
Mirror finish is most widely used for plates in presses, on the interior surface of moulds and also for small mirrors, reflectors and architectural panels.

The two primary limitations are:

• the most commonly used grades (304 and 316) are quite soft so that any cleaning process must avoid scratching the surface with residual dust or applied cleaning compounds, and

• large fl at areas of mirror polish throw scratches, grease or dirt markings into sharp contrast.

The first limitation is overcome by careful cleaning protocols using adequate water rinsing.  The effect of random marks and scratches is reduced if the surface is corrugated at the macro level described below when using surfaces that are texturised (single sided deformation) or embossed/rigidised (through thickness deformation).  However, while mirror sheet may be profi led, it is more common to apply profi ling treatments to cold rolled sheet.

Profiled or Patterned Surfaces
EN10088-2 uses categories 1M and 2M for sheet material that has been patterned on one side only.  The 1M group applies to hot rolled base materials while the much more common 2M applies to cold rolled base materials, usually with a 2B, 2D or 2R (BA) finish.  In corrosive environments it is essential to orient the pattern to allow free drainage.

Through Thickness Deformation
EN10088-2 lists 2W for cold rolled sheet that has been cold rolled into a through thickness corrugated  pattern. The cold work strengthens the sheet and may permit the use of thinner sheet.  The rigidity also helps control oil-canning, i.e. local buckling caused by thermal expansion during fixing or temperature changes during the year or day.  The surfaces are often described as embossed or rigidised.

Coloured Stainless Steel
Stainless steel can be painted provided that the surface is scrupulously clean and has sufficient profile to mechanically anchor the paint.  Using a stainless steel base metal offers the advantage that, even if the coating is damaged, any rusting is superficial and will not lead to long term structural failure as has occurred when protective coatings fail on steel or aluminium structures.

However, painted organic coatings are a clumsy means of colouring stainless steel.  If it is simply to be blackened, then the molten sodium dichromate process or a hot sulphuric acid treatment or even an electrodeposited and baked organic coating could offer greater durability.

Other colours are offered by an electrochemical deposition process that deposits and hardens an enhanced oxide film on the stainless steel although usually only on 304.  The colour depends on the thickness of the oxide and progresses from blue to gold to mauve and green with time of exposure.

The colour is similar to the interference colours in an oil slick or soap bubble with interference between the light refl ected from the top of the oxide and base metal.  In theory the colouring could be reproduced on any simple shape but in practice, it is only available on sheets.  The coated sheets may be deep drawn, formed, bent and fixed mechanically but welding destroys the coloured film.  While the oxide film is harder and more resistant to scratching than the basic 304 substrate, it is still susceptible to mechanical damage and so is not suitable in areas subject to heavy traffic.  Corrosion tests on blue coatings on 304 showed slightly improved resistance to atmospheric and acidic chloride exposure over the resistance of 304.

This article featured in Australian Stainless magazine - Issue 30, Autumn 2007.

Chlorine and chloride: Same element, very different effect

Posted 1 July 2007

Choosing the correct grade of stainless steel for a tank, pipe or process vessel requires (at the very least) information about the temperature, pH and chemical composition of the contents.  One of the most important items of the chemical composition is how much chloride (salt) is present.  Analysis reports often give the concentration as milligrams per litre (mg/L) or sometimes as parts per million (ppm) of Cl.  However, Cl is also the symbol used for the element chlorine.

So what is the difference?

Chlorine is a poisonous, yellowish green gas which readily dissolves in water to give a strong disinfectant or bleach.  The strength of a bleach solution is sometimes measured by the “available chlorine”.  Swimming pools are usually treated with dilute hypochlorite solutions which produce a few parts per million (ppm) of chlorine.  This acts as a strong, oxidising biocide.  Drinking water is normally treated to give a residual of 0.2 to 0.5 mg/L of chlorine.  (There are also other disinfection methods such as chloramine or ozone.)

Chlorine is very aggressive to stainless steels.  The Nickel Institute guidelines for continuous exposure at ambient temperatures (~20˚C) and neutral pH (~ pH7), are that 304 can cope with 2ppm chlorine and 316 ~5ppm chlorine.  In alkaline solutions (pH>7) higher concentrations are possible but this does not help much in swimming pools or drinking water.  Chlorine frequently causes corrosion problems.  Chlorine attack can occur with bleach laden washdown water if pools form in drains which are usually empty.  Chlorine concentrations in droplets or water films immediately above a still pool or water tank can be higher than the chlorine level in the bulk water. When dosing concentrated chlorine into pipes or tanks, it must be well mixed otherwise concentrated streams will eat out downstream elbows or tank walls near the chlorine inlet.

Much higher concentrations can be used for short periods as the attack on the stainless steel must initiate and form a stable pit for failure to occur.  The American Water and Wastewater Association permits 25ppm for 24 hours in cases of emergency disinfection.  The food industry can use up to 100ppm in hot water for minutes followed by rinsing and/or passivation.  It is an effective biocide because the kill rate depends on (exposure time) * (concentration of biocide) but the stainless steel is resistant to the chlorine for the relatively short, high concentration exposure.

And what about chlorides?

Chloride occurs naturally in drinking water and ranges from less than 10mg/L in Melbourne to more than 200mg/L in Adelaide. Chloride is not oxidizing and is not a biocide.  The most common form is sodium chloride.  Seawater is about 3% sodium chloride although there are other compounds.  Nickel Institute guidelines for continuous exposure at neutral pH and ambient temperatures permit chloride levels of 200ppm for 304, 1000ppm for 316 and 3600 ppm for 2205 duplex.  The guidelines allow for the presence of crevices (such as bolt heads, flanges or deposits) but assume that the surface has been passivated. In alkaline environments (pH>7) higher chloride levels can be tolerated.  Higher temperatures reduce the permissible chloride level. Temperatures over 60˚C are not recommended for 304 or 316 as they are at risk of sudden failure from chloride stress corrosion cracking.

The message

Chlorine and chloride are different forms of the same element but with vastly different effects on stainless steel.  Chlorine is bleach and stainless steels can only tolerate exposure to a few ppm continuously.  Chloride is part of the salt in natural waters and even 304 can cope with a few hundred ppm at ambient temperatures and pH~7.

This article appeared in Australian Stainless Issue 40

Life Cycle Costing and Stainless Steel

Posted 31 July 1993

Life Cycle Costing (LCC) has long been used in planning for reliability and maintenance for complex engineering systems in defence, airline, railway, offshore platform, power station, and other applications.

A basic attribute of stainless steel is the ability to provide long-term perfor-mance with a minimum of downtime and cost associated with maintenance. As a result LCC is of particular importance to the stainless industry.

Whilst the mathematics of LCC can be quite complex the International Chromium Development Association (ICDA) has developed an IBM or compati-ble PC program on floppy disk which can be easily applied to most examples.

The Australian Stainless Steel Development Association can make this program available to any interested party on request.

LCC analysis provides a more secure basis for comparing and selecting material options than the traditional method of judgements based on comparing acquisition costs alone. This particularly applies to situations where the initial cost is high and downtime for unplanned maintenance is costly.

In circumstances where stainless is being considered or introduced into new fields of applications, comparisons are often made with materials of a lower initial cost such as coated carbon steel or plastics.

Here the reasoning should progress well beyond the simple initial cost com-parison and take account of the long term cost assessments associated with mainte-nance replacement and operating stop-pages.

LCC is the tool to make this assessment and the ICDA program makes it easy.

Calculating LCC
In the LCC calculation, consideration is given only to relevant costs which are directly or indirectly affected by the material options being considered. Besides the cost of material, these include costs of installation, operation, maintenance, stop-pages, replacements and possibly the residual value at the end of the service life. The time intervals at which the various costs arise during the selected life cycle period must also be taken into account.

Before the various cost items can be put together, those that arise every year and those that occur at certain time intervals during the service life must be converted into present values.

Again the complexities of the mathematics are catered for by the PC program.

Examples are the best way of demonstrating LCC principles and application and two are offered to illustrate the point.

The first is from Swedish practice and features roofing.

The building industry is one of the most rapidly expanding markets for stainless steel and roofing is a major growth application. A method based on seam welding 0.4mrn strips of cold rolled stainless steel was invented in Sweden in the 60's and has since found favour in Europe and Japan. An LCC calculation was carried out based on these material options:

• galvanised and plastic coated carbon steel, double folded edges
• 0.4mm stainless stee I strip, seam-welded and single folded edges (type 316 for coastal areas or polluted atmospheres, otherwise type 304).

In this example the LCC period is 50 years and a real interest rate of 3% is used (comparative figures are given per sq metre):

Material Material Cost Installed Cost LCC
Carbon steel 1.1 2.1 2.1
Stainless Steel type 316 2.0 2.8 1.4
Stainless Steel type 304 1.6 2.6 1.3

 

 

 

 

The LCC result shows that stain-less steels are less costly than galvanised and plastic coated steel. Galvanised carbon steel requires replacement after about 20 years. The calculation does, however, not take into account the risk of damage to building substructures each time the covering is replaced. The stainless steel alternative is the only one which is virtually maintenance free.

The second example is a mixing tank for a water treatment plant.

The dimensions of the tank are 3 metres long, 1.5 metres wide and 1.5 metres high. The entire tank is raised off the floor by four steel channels beneath the tank; these ensure that spills do not accumulate beneath the tank.

The design brief requested evalua-ion of three materials. (i) mild steel with applied fibre-glass lining, (ii) stainless steel alternatives of Type 304 and duplex grade 2205 (UNS S31803).

As the 2205 was not readily avail-able in angle and channel products, these were substituted by type 304 for the 2205 design as these components were not to be in regular contact with the corrosive environment.

The evaluation was carried out using the LCC PC program from the International Chromium Development Association available in Australia through ASSDA.

Experience suggested that both the 304 and 2205 would probably survive without replacement for the full twenty years, whereas the mild steel was expected to last for only about eight years before replacement. In addition both the stainless steels were expected to require only minimal inspection and cleaning as regular maintenance in comparison with fairly extensive patching of the mild steel and its lining.

The "Summary of Present Value Costs" table of Figure I shows the resulting LCC analysis -the Type 304 stainless steel is lowest cost, closely followed by the 2205 and with mild steel substantially more expensive due to its higher maintenance and replacement costs.

The "Value of Lost Production" in the summary table is shown as zero -this implies all maintenance and replacement is carried out in scheduled shut downs for other plant maintenance. Shut downs causing lost production could substantially add to the Total Operating Cost of the option requiring this unscheduled maintenance.

The ICDA LCC software also gives a more detailed breakdown of the contributions to the initial costs and operating costs, and a "sensitivity analysis" on all the inputs which is shown in Figure 2. The latter gives the effect on the total LCC for each material option of an independent change (eg of 20%) in each of the inputs. This information is vital in determining which of the input items must be accurately known and which are of lesser importance. In this case the sensitivity analysis indicates that the most critical data is the time before replacement becomes necessary. The assumption was that the 304 and 2205 would both survive for the full twenty years; from the sensitivity analysis it is apparent that if the 304 fails before this time (possibly due to its lower pitting corrosion resistance compared to the 2205), the 2205 duplex stainless steel becomes by far the cheaper option. Clearly a good knowledge of the actual operating conditions to be encountered is crucial to the correct selection.

Acknowledgments:

1. This article has drawn on material contained in a publication Life Cycle Costing - Evaluation of a Method of Use For Stainless Steel Applications by Sten Von Matern of Avesta AB, Sweden.

This has been made available to ASSDA through A vesta Sheffield Pty Ltd. This contribution is gratefully acknowledged.

2. The computer diskette "Life Cycle Costing" was developed by and supplied to ASSDA by the International Chromium Development Association.

This article featured in Australian Stainless Magazine - Issue 1, July 1993.

Alternative stainless steel grades - Part 1

This article is the first of a two-part series outlining new and emerging stainless steel grades which may be considered as alternatives to the more traditional and widely known varieties. Read Part 2.

The growing demand from China and the rest of the developing world has driven up the price of alloying elements added to stainless steels.  Over the last five years nickel prices have risen to ten times what they were.

Chromium and molybdenum have also risen strongly, and the price of stainless steel scrap – which steelmakers use extensively – has soared.  Inevitably, stainless steels have also seen large price increases, with little relief in sight. Growing demand and the time required to develop new supply sources mean that nickel and other alloy prices are unlikely to drop to the levels seen a few years ago.

Higher prices are driving stainless steel users to seek more cost effective solutions:  the optimum choice of grade is a blend of engineering and economic factors, and the choice may be different in a new cost environment.  The most common stainless steel grade, 304, is used in about 60% of applications for stainless steel around the world.  Grade 304 contains about 8% of nickel, which is used to form the ductile austenite crystal structure.  Grade 316, with 10% of nickel and higher corrosion resistance given by an addition of 2% molybdenum, is also very common.  It is used in marine environments.  Users are seeking more cost effective alternatives to both these austenitic 300 series grades.

Austenitic 200 series, duplex stainless steels and ferritic grades can all be used instead of 304 and 316, if they are selected, designed, fabricated and used appropriately.  This article and the next in the series describe the alternatives to the more traditional grades, with their abilities and limitations.

The alloying elements in stainless steel contributing most to corrosion resistance are chromium and molybdenum.  Within each of the alternative groups there are grades with different corrosion resistance resulting from the chromium and molybdenum contents.

The well known austenitic 300 series grades contain the highest levels of nickel.  The austenitic 200 series grades contain less nickel, and manganese is added to make the austenite crystal structure form.  Because the 200 series grades have the austenitic crystal structure their mechanical and fabrication properties are similar to the familiar 300 series.

Ferritic grades have the same crystal structure as carbon steel, and have similar mechanical and fabrication properties and do not contain a nickel addition.

Duplex grades are not fully austenitic.  They are formulated to be a mixture of equal amounts of austenitic and ferritic grains in the microstructure, which generally means the nickel content is about half of that in an austenitic grade of the same chromium content.

AUSTENITIC 200 SERIES

These grades are austenitic despite their lower nickel because they have more manganese.  Manganese is about half as effective in forming austenite as nickel, so for every 1% of nickel left out, about 2% of manganese has to be added – at the same level of chromium, which suppresses the formation of austenite. Half the nickel in these grades has been replaced by manganese and the price of manganese is also rising strongly.

First developed in the 1930s, most of the common 200 series grades have corrosion resistance similar to the ferritic grade 430, lower than grade 304, because the chromium content is lower.  Newer Indian developments (grades J1 & J4 in the table) have centered on grades with significantly lower corrosion resistance. There are other proprietary 200 series grades with higher chromium contents used in marine and anti–galling applications.

The austenitic 200 series are the closest in behaviour to the 300 series of the alternative groups.  Hence they are the easiest to convert to.

Mechanical and Physical Properties

The tensile strength of common 200 grades exceeds 600MPa, i.e. about 20% higher than 304.  The 0.2% proof stress is more than 20% greater than that of 304 but the elongation at fracture is similar.  In contrast with carbon steel, all the austenitic stainless steel grades have tensile strengths at least double the 0.2% proof stress, a consequence of their high rate of work hardening.  Some newer grades include copper to reduce this.  Because of the austenitic microstructure of annealed 200 series grades they are ductile down to cryogenic temperatures and do not suffer brittle fracture. In comparison with the physical properties of 304, the 200 series have very similar density, elastic modulus, electrical and thermal properties.

Some 200 series grades in comparison to 304

Attributes

The ductility and formability are similar to the 300 grades although the lower nickel gives a greater risk of delayed cracking after heavy cold forming.  Welding is similar to the 300 series grades although the 200 grades may have higher carbon and may suffer sensitisation (loss of intergranular corrosion resistance) if welded in sections thicker than 5 mm.  Stress corrosion cracking resistance is similar to the 300 series.  Like 304 and 316, 200 series grades do not respond to a magnet when in the annealed condition, but become magnetic after cold work.

Limitations

The lower chromium levels mean that the 15% chromium grades have lower corrosion resistance than ferritic grade 430.  Even the 16 & 17% chromium grades are somewhat inferior to 304 in corrosion resistance, since it appears that a 200 series grade has slightly less corrosion resistance than a 300 series grade with the same chromium level.  This may be due to the high levels of sulphur present in 200 series grades from some sources.

Steelmakers do not want 200 series scrap mixed with 300 series scrap as the high manganese levels reduce the life of steelmaking refractories.  Batches of 300 series scrap suspected of being contaminated with 200 series are likely to attract only the much lower 200 series scrap price.  Hence strict segregation of off – cuts is required.
At present none of the 200 series grades are routinely stocked in Australia.

Applications

As with all grade groups, it is important to choose a grade with corrosion resistance adequate for the application.  The lower chromium 200 series greades detailed in the table are generally suitable for use with mild acids and alkalis including most foods (pH not less than 3).  They are satisfactory with 20˚C potable water and are suitable for indoor exposure – furniture, bins, etc.  They are used extensively for cookware and serving bowls – applications where the corrosion conditions are not severe since the utensils are washed and dried.  The formability and deep drawability of the 200 series are especially useful for these applications.

This article appeared in Australian Stainless Issue 40

Alternative stainless steel grades - Part 2

This article is the second of a two-part series outlining new and emerging stainless steel grades which may be considered as alternatives to the more traditional and widely known varieties. Read Part 1.

The growing demand from China and the rest of the developing world has driven up the price of alloying elements added to stainless steels.  Over the last five years nickel prices have risen to ten times what they were.

Chromium and molybdenum have also risen strongly, and the price of stainless steel scrap – which steelmakers use extensively – has soared.  Inevitably, stainless steels have also seen large price increases, with little relief in sight. Growing demand and the time required to develop new supply sources mean that nickel and other alloy prices are unlikely to drop to the levels seen a few years ago.

Higher prices are driving stainless steel users to seek more cost effective solutions:  the optimum choice of grade is a blend of engineering and economic factors, and the choice may be different in a new cost environment.  The most common stainless steel grade, 304, is used in about 60% of applications for stainless steel around the world.  Grade 304 contains about 8% of nickel, which is used to form the ductile austenite crystal structure.  Grade 316, with 10% of nickel and higher corrosion resistance given by an addition of 2% molybdenum, is also very common.  It is used in marine environments.  Users are seeking more cost effective alternatives to both these austenitic 300 series grades.

Austenitic 200 series, duplex stainless steels and ferritic grades can all be used instead of 304 and 316, if they are selected, designed, fabricated and used appropriately.  This article and the next in the series describe the alternatives to the more traditional grades, with their abilities and limitations.
The alloying elements in stainless steel contributing most to corrosion resistance are chromium and molybdenum.  Within each of the alternative groups there are grades with different corrosion resistance resulting from the chromium and molybdenum contents.

The well known austenitic 300 series grades contain the highest levels of nickel.  The austenitic 200 series grades contain less nickel, and manganese is added to make the austenite crystal structure form.  Because the 200 series grades have the austenitic crystal structure their mechanical and fabrication properties are similar to the familiar 300 series.

Ferritic grades have the same crystal structure as carbon steel, and have similar mechanical and fabrication properties and do not contain a nickel addition.
Duplex grades are not fully austenitic.  They are formulated to be a mixture of equal amounts of austenitic and ferritic grains in the microstructure, which generally means the nickel content is about half of that in an austenitic grade of the same chromium content.

Austenitic 200 Series

These grades are austenitic despite their lower nickel because they have more manganese.  Manganese is about half as effective in forming austenite as nickel, so for every 1% of nickel left out, about 2% of manganese has to be added – at the same level of chromium, which suppresses the formation of austenite. Half the nickel in these grades has been replaced by manganese and the price of manganese is also rising strongly.

First developed in the 1930s, most of the common 200 series grades have corrosion resistance similar to the ferritic grade 430, lower than grade 304, because the chromium content is lower.  Newer Indian developments (grades J1 & J4 in the table) have centered on grades with significantly lower corrosion resistance. There are other proprietary 200 series grades with higher chromium contents used in marine and anti–galling applications.

The austenitic 200 series are the closest in behaviour to the 300 series of the alternative groups.  Hence they are the easiest to convert to.

Mechanical and Physical Properties

The tensile strength of common 200 grades exceeds 600MPa, i.e. about 20% higher than 304.  The 0.2% proof stress is more than 20% greater than that of 304 but the elongation at fracture is similar.  In contrast with carbon steel, all the austenitic stainless steel grades have tensile strengths at least double the 0.2% proof stress, a consequence of their high rate of work hardening.  Some newer grades include copper to reduce this.  Because of the austenitic microstructure of annealed 200 series grades they are ductile down to cryogenic temperatures and do not suffer brittle fracture. In comparison with the physical properties of 304, the 200 series have very similar density, elastic modulus, electrical and thermal properties.

Some 200 series grades in comparison to 304

Grade    

Carbon    (max)

Manganese  Chromium  Nickel  

Copper

201 16/4 0.15 5.5-7.5 16.0-18.0 3.5-5.5 -
202 17/4 0.15 7.5-10.0 17.0-19.0 4.0-6.0 -
J1 15/4 0.08 7.0-8.0 15.0-17.0 4.0-4.5 1.5-2.0
J4 15/1 0.10 8.5-10.0 15.0-17.0 0.8-12 1.5-2.0
304 18/8 0.07 17.5-19.5 17.5-19.5 8.0-10.5 -

 

 

 

 

 

 

Attributes

The ductility and formability are similar to the 300 grades although the lower nickel gives a greater risk of delayed cracking after heavy cold forming.  Welding is similar to the 300 series grades although the 200 grades may have higher carbon and may suffer sensitisation (loss of intergranular corrosion resistance) if welded in sections thicker than 5 mm.  Stress corrosion cracking resistance is similar to the 300 series.  Like 304 and 316, 200 series grades do not respond to a magnet when in the annealed condition, but become magnetic after cold work.

Limitations

The lower chromium levels mean that the 15% chromium grades have lower corrosion resistance than ferritic grade 430.  Even the 16 & 17% chromium grades are somewhat inferior to 304 in corrosion resistance, since it appears that a 200 series grade has slightly less corrosion resistance than a 300 series grade with the same chromium level.  This may be due to the high levels of sulphur present in 200 series grades from some sources.

Steelmakers do not want 200 series scrap mixed with 300 series scrap as the high manganese levels reduce the life of steelmaking refractories.  Batches of 300 series scrap suspected of being contaminated with 200 series are likely to attract only the much lower 200 series scrap price.  Hence strict segregation of off – cuts is required.

At present none of the 200 series grades are routinely stocked in Australia.

Applications

As with all grade groups, it is important to choose a grade with corrosion resistance adequate for the application.  The lower chromium 200 series greades detailed in the table are generally suitable for use with mild acids and alkalis including most foods (pH not less than 3).  They are satisfactory with 20˚C potable water and are suitable for indoor exposure – furniture, bins, etc.  They are used extensively for cookware and serving bowls – applications where the corrosion conditions are not severe since the utensils are washed and dried.  The formability and deep drawability of the 200 series are especially useful for these applications.

The growing demand from China and the rest of the developing world has driven up the price of the alloying elements in stainless steels.  The relative cost of different grade groups of stainless steels has also changed, depending on the content of the more expensive alloying elements, particularly nickel and molybdenum.

In the last issue we described the austenitic 200 series group, one of the alternative groups to the austenitic 300 series that traditionally dominate the market.  This article describes the other two alternative groups, ferritic and duplex grades.

FERRETIC 400 SERIES

These stainless steels have the ferritic structure also found in carbon steels.  They do not contain the nickel addition used to stabilise austenite in 300 series grades.  The quality of ferritic grades has advanced with modern steelmaking equipment and, after several generations of ferritic grades, a number of technical limitations have been overcome.

Toughness is the remaining limitation that has not been overcome.  All ferritic grades show the ductile to brittle fracture transition well known from carbon steels.  Unlike the carbon steels, there is no phase transformation when heated during welding, and hence the grain size of the HAZ can be high.  This limits the toughness of the ferritic stainless steels, and with a few exceptions they are used at up to about 3mm thickness, where the toughness transition temperature after welding is adequate.

There are ferritic grades with 10.5-30% chromium, and many also contain molybdenum.  The ferritic grades have the corrosion resistance their chromium and molybdenum contents give them, and in addition they are very resistant to stress corrosion cracking.  Later generations of ferritics are not susceptible to sensitisation and intergranular corrosion.

The ease of fabrication of ferritic grades, which behave in a similar way to carbon steel, has seen them used to replace competing materials and grow the market for stainless steels.  A recent publication of the International Stainless Steel Forum “The Ferritic Solution – The Essential Guide To Ferritic Stainless Steels” (available from ASSDA) has several examples.

Grade Cr Mo N Ni C Mn Other PRE*
AUSTENITIC 300 SERIES                
304 18.1     8.1 0.04     18
316 17.2 2.1   10.2 0.04     24
FERRITIC 400 SERIES                
409 11.5       0.02   0.18Ti 12
430 16.5       0.04     17
AWM 404GP™ 21.0       0.010   0.4Cu, 0.3Ti 21
444 17.7 1.8     0.02   0.45(Ti+Nb) 24
AWM 445M2™ 22.1 1.05     0.007   0.20Ti, 0.20Nb 26
DUPLEX                
LDX 2101® 21.5 0.3 0.22 1.5 0.03 5.0   29
SAF 2304® 23.0 0.3 0.10 4.8 0.02     27
2205 22.0 3.1 0.17 5.7 0.02   0.15N 37
SAF 2507® 25.0 4.0 0.27 7.0 0.02   0.3N 46

*Pitting Resistance Equivalent (PRE) = %Cr + 3.3x%Mo + 16x%N

Mechanical and Physical Properties

Yield strength is a little higher than that of the austenitic grades, and tensile strength a little lower.  Ductility is about half that of the austenitics, and is similar to carbon steel.

Ferritic grades cannot be strengthened by heat treatment, and since their work hardening is weak they are rarely strengthened by cold work.  Ferritic stainless steel work hardens in a similar way to carbon steel, which can be an advantage, particularly in fabrication where experience and settings gained with carbon steel can be applied to ferritic stainless steels with few modifications.

Ferritic grades are ferromagnetic, and have much lower thermal expansion and higher heat conductivity than austenitic grades.

Attributes

First generation ferritic stainless steels are usually used unwelded, as they have high carbon (~0.05%), which causes the formation of brittle films of low corrosion resistance on HAZ grain boundaries.  Grade 430 is the most widely used of this group: it has enough corrosion resistance for indoor applications such as food preparation and display equipment, but is rarely fusion welded.  Grade 430 is usually used with a bright annealed (BA) finish: finishes in ferritic grades are generally brighter than their austenitic equivalent.  Large amounts of first generation ferritic grades, with molybdenum added for extra corrosion resistance, are used for automotive trim.

Second generation ferritic stainless steels have lower levels of carbon and nitrogen, and have titanium and/or niobium added to combine with what’s left.  This makes the grades more weldable, and the first second generation ferritic grade developed, 409, is now widely used in automotive muffler systems.  The current production of 409 in USA rivals the tonnage of the most popular stainless steel, 304.  Welds in second generation grades are tough at room temperature up to about 2mm thickness, and do not suffer from sensitisation or stress corrosion cracking.  There are titanium treated versions of 430, widely used in whitegoods such as welded washing machine drums.

Third generation ferritic grades have even lower carbon, nitrogen, titanium and/or niobium additions, with higher contents of the corrosion-resisting elements chromium and molybdenum.  The most common grade of the group, 444, is used for challenging applications such as heat exchangers and hot water tanks.

Fourth, or new generation grades, are further refined using vacuum equipment to achieve better toughness and weldability, and better surface quality.  They are often used in applications where austenitic grades fail by chloride stress corrosion cracking or pitting corrosion, and they are increasingly being used in many applications to replace the common austenitic grades.

Limitations

The limited toughness of ferritic grades has been noted, and they are rarely used in structural applications.

A further limitation is the tendency of ferritic stainless grades to suffer 475°C embrittlement and phase formation more quickly than austenitic grades, which limits their use to about 350°C in the higher chromium grades.  However, large tonnages of the lower chromium grades are used in automotive muffler systems at higher temperatures without problems.

Applications

The largest tonnage of ferritic grades is used in automotive muffler systems, and there are also significant uses in automotive trim, commercial catering equipment and indoor decorative applications.  The higher alloyed later generation grades give outstanding performance in heat exchanger and piping systems for chloride-containing aqueous solutions and seawater, where stress corrosion cracking of austenitic grades can be a problem.  The ferritics are also ideally suited for roll forming to roofing, walling and rainwater goods.

DUPLEX GRADES

These grades consist of an intimate mixture of about equal amounts of austenite and ferrite.  About half of the amount of nickel needed to be fully austenitic at the chromium content is added in most of the grades.  A newer grade, LDX 2101, follows the approach of the 200 series austenitics by using manganese instead of most of the nickel.

There are grades within the duplex group with a range of different corrosion resistances, depending on the chromium and molybdenum contents.  The duplex grades tend to use more chromium and less molybdenum than an austenitic grade of similar corrosion resistance - a more economical balance.

As chromium is increased in the austenitic 300 grades to improve corrosion resistance, more nickel must be added, making high chromium austenitic grades expensive.  The more corrosion resistant duplex grades, containing less nickel and a better balance of chromium and molybdenum, have penetrated the market to a greater extent than the leaner alloys, and 2205 has become the most common alloy where the corrosion resistance of grade 316 is inadequate.

The duplex grades are much more resistant to stress corrosion cracking than the austenitic grades, and they are effectively immune in potable water.  They are also less prone to sensitisation than austenitic grades, although not immune.

Mechanical and Physical Properties

Duplex grades have about twice the tensile strength and 50% higher yield strength than austenitic grades.  The ductility is about half, but is still high enough to give good formability, with work hardening behaviour similar to that of carbon steels.  Unwelded, duplex grades are tough to low temperatures (-50 to -100°C), and they can often be welded to give transition temperatures well below 0°C.

Duplex grades cannot be strengthened by heat treatment, and since their work hardening is weak they are rarely strengthened by cold work.
Duplex grades are ferromagnetic, and have lower thermal expansion and higher heat conductivity than austenitic grades.

Attributes

Their much higher strength than austenitic grades often allows duplex grades to be down-gauged to thinner material, with good savings in costs.
The higher strength can be a handicap if the opportunity of down-gauging is not taken, as forming loads are high and may be beyond the capability of the equipment.  Many of the uses of duplex grades are at thicker gauges (greater than ~1.2 mm), where the savings of down-gauging can be achieved without getting to the lighter sheet metal gauges that fabricators can find difficult to weld.

Welding duplex grades requires more control of welding parameters, particularly heat input and interpass temperature, but pre-heat, post-heat and post-weld heat treatment are not required and weldability is considered good.

Limitations

The high alloy content of most duplex grades makes them susceptible to embrittlement from the formation of intermetallic phases after extended service at high temperatures.  Corrosion resistance is also reduced.  Service temperatures are generally limited to less than about 300°C.

Applications

The higher strength of the duplex grades makes them suitable for large tanks, and savings of 40% or more in material costs can be achieved.  They are also used for heat exchangers and chemical equipment, often where chloride stress corrosion cracking has limited the life of austenitic grades.

COMPARISON OF TYPICAL TENSILE AND ELONGATION PROPERTIES OF GRADE GROUPS OF STAINLESS STEELS

tableone

 

 

 

 

 

 

 

 

 

 

 

This article appeared in Australian Stainless Issue 41.

Testing for grade confirmation

Raw material price fluctuations and increasing demand for stainless steels have driven demand for lower cost alloys as alternatives to the traditional “300” series steels. This has been met through a range of existing and new, innovative steels with different properties, performance and availability broadening the range of alloys that might be found in the market. But as with the traditional stainless steels you can’t tell what they are by looking at them.

This article describes most of the range of test methods available for grade confirmation. The method used depends on the budget, size of job and the potential consequences of having the wrong alloy.

Why test?

Contract documents may require formal test certificates.  Usually these are issued by the mill and unless there is reason to doubt them this is sufficient.  However, sometimes a positive material identification (PMI) is required for safety critical items such as LPG valves. Legal cases also tend to be very demanding about precise documentation.  Some products may also be lacking in documentation and traceability.

Unexpected poor performance often prompts calls for material testing. Such testing removes one variable in things that might have gone wrong but the cause is more frequently inadequate surface finish or errors in design or fabrication.

Finally, reverse engineering of an existing product from a competitor or overseas supplier often requires detailed materials’ information.

What level of testing is required?

General or intermediate level guidance could cover differentiation between carbon and stainless steel or between 304 and 316 or between 300 series and 200 series or ferritic grades.

Full laboratory chemical analysis will be needed for some cases (such as determining low carbon grades) or when it has become a legal rather than a technical issue.

Full mechanical and metallurgical analyses may also be required if strength or hardness are essential design elements.  If the material has undergone subsequent surface modification then the required investigation could be extensive – and expensive.

Simple physical tests

Appearance is not a reliable indicator of the grade of stainless steel as the differences are determined more by surface treatments than alloy composition.  Even the differences between mirror polished surfaces are fairly subtle.  The table below shows slight differences in density of some stainless steel alloys but density determination is not a convenient method.

Alloy

Density (g/cc)

430, 3Cr12/5Cr12

7.7

2205

7.8

304, 310

7.9

316

7.98

A widely accepted test is a magnet.  Duplex, super duplex, martensitic and ferritic stainless steels are strongly attracted to a magnet while annealed austenitic stainless steels are not.  However, cold worked austenitic stainless steels are weakly attracted to a magnet so cold formed ends to a vessel, cold rolled bolts and bent corners will be affected by a magnet.  This applies to both the conventional chromium-nickel 300 grades and the chromium-manganese 200 series austenitic grades.

The strength of the effect that a magnet has on a material can be related to the relative permeability and the graph shows the different effect of the same level of cold work (bending) on various austenitic alloys. The grades with higher nickel or austenitising elements (310 or 316) show much lower magnetic properties. In comparison, mild steel has a relative permeability somewhere between a few hundred and 2000.  Relative permeability of duplex and ferritic alloys is in the hundreds.  Precipitation hardening alloys are magnetic but the degree depends on whether the alloy is martensitic or semi-austenitic.

graph

Chemical tests

The proprietary kits are designed to test for a specific element and have a limited shelf life.  If you have a project requiring multiple tests then they are very useful. However, if you only require a couple of tests a year, then it may be cheaper and more thorough to run a full laboratory test.

Molybdenum

The most common test uses a single drop of solution to distinguish between low and high molybdenum content. The “Moly Drop” test will distinguish between 304/304L and 316/316L but the test will also give a positive result with 317/317L, 904L, the 6% Mo grades, 444, 2205 and the super duplex grades.  The test requires a clean, dry, grease free surface and it sometimes helps to lightly abrade the surface.

The yellow drop (as shown) will darken after a few minutes but the reaction speed is slower if the surface is cold.  It is a comparative test and scrap additions during production may give enough molybdenum to give a slight colouration.

The test is therefore most reliable if a known 304 and 316 are tested with the unknown. If the sample is to be used in service, then the chemicals should be washed away immediately after the test.

There is another chemical test using ethyl xanthogenate to form a red or pink complex when molybdenum ions are dissolved in solution.  The molybdenum is dissolved from the surface either by using a hydrochloric or sulphuric acid.  The strength of the colour depends on the level on molybdenum in the alloy.

Manganese

The increasing use of high manganese stainless steels has led to several manganese test kits operating on the same principal as the electrochemical test for molybdenum. The semi-quantitative results of a kit test for manganese are shown in the photographs below.

Apart from the recent low nickel, high manganese stainless grades, there have been specialist 200 series grades used in generators, higher strength (pre duplex) marine alloys and for anti-galling applications.

colouringSulphur

A practical and rapid test for a high sulphur (free machining) stainless steel (303 and 430F are the most common) is to prepare sulphur prints using photographic paper soaked in 3% sulphuric acid for several minutes.  The treated paper is pressed onto a cleaned surface for about 5 seconds. High sulphur levels are shown by a brown colour.  Once again, this is a comparative test so low and high sulphur samples should be compared to the test piece.

Instrumental techniques

There are two basic techniques each with two variants. The automated instruments are expensive and would normally be used for large projects, or by scrap metal merchants, manufacturers or specialist NDT contractors.

Spark spectroscopy requires a flat surface preferably about 20mm in diameter. An electrical spark is generated and the colour of the spark is determined by the elements present.  The elemental concentration is controlled by the intensity of the specific colours. In automated instruments, the spectrum is compared to a library of data and percentage composition is calculated for each element.  Calibration is required against materials with similar composition.  A sparking mark is left on the surface and must be removed if appearance or fatigue resistance is important.  The instrument’s accuracy tends to be lower than a laboratory instrument and exposure to air excludes measuring nitrogen.

The older “Metascopes” were also spark spectroscopes but relied on visual comparisons of line brightness so their accuracy was very operator dependant. Grinding spark identification using a hard, high speed grinding wheel is even older technology. It will cause a grinding burr and is extremely dependent on the operator skill.  Spark bursts are related to the carbon content and characteristic sparks/carrier lines are related to the alloying metals. Chromium in steel produces a spark stream that is orange-red in colour. A yellow colouration caused by nickel persists all along the spark whereas the orange specks of chromium appear only near the origin of the spark stream, in close contact with the grinding wheel. Relatively narrow and short spark streams, white-yellow in colour, are produced in type 304 stainless steels.

The second broad method is X-ray fluorescence.  Older instruments used one or more radioactive sources although more recent miniaturisation of X-ray tubes means that some instruments generate X-rays directly.  Regardless of their source, the X-rays excite electrons from the inner shells of the elements and when outer electrons fall into the newly vacant shell, a characteristic spectrum of light – generally with a number of lines – is emitted.The instrument measures the intensity of counts in each line and compares it to an internal databank.  Provided that the surface is clean and smooth and the measurement is for long enough to give good statistics (typically between 20 and 60 seconds), then the alloy can be identified.  However, because of the physics of X-ray fluorescence, it cannot analyse for light elements, especially carbon or nitrogen.  The units are light and easy to use as seen in the picture. One advantage for reporting is that results can be directly downloaded into a computer records system

Laboratory measurements

Atomic Absorption (AA) or Inductively Coupled Plasma Spectroscopy (ICP) techniques use laboratory instruments after a sample has been digested in (usually) a mixture of acids. This is slow and may be more expensive than a spark test but it will give a more complete and reliable result.  Carbon requires a separate (LECO ignition) test and detecting silicon by either method requires aggressive chemicals to get the silicon into solution.


Which test?

  • Is it 430/2205 or 304/316?
    A magnet will be strongly attracted to 430 and 2205 but only weakly to deformed parts of 304 or 316.
  • Is it 430 or 2205?
    Both are strongly magnetic but only duplex 2205 will give a positive moly drop test result.
  • Is it 304 or 316?
    A moly drop test will give a positive result with 316.
  • Is it a low carbon grade?
    Only a spark spectrometer can distinguish between low and standard carbon grades.

In all these cases a full laboratory analysis will answer the question and provide a full composition for about $100.

This article appeared in Australian Stainless Issue 42.

The assistance of ASSDA colleagues is gratefully acknowledged - in particular, Peter Moore from Atlas Steels.

Changing costs of alloying elements

Sustained economic growth in China and the rest of the developing world has seen the demand for all the metals grow faster than the minerals industry can develop new mines and smelters.  The result is soaring prices for metals, and for coal and oil.

For a country like Australia - a big supplier of metals - it’s good news, and we have all enjoyed the benefits of the minerals boom. But those of us in the stainless steel industry have seen prices increase markedly, and it has been hard to cope with.  We live in interesting times.

In broad terms there are two main factors influencing the price development of stainless steel: the cost of raw material inputs and the level of demand measured against the capacity to make the steel (the capacity utilisation).

Much of the increase in stainless steel prices has come from the increase in the price of raw material inputs, and particularly nickel, which went through a peak over US$50,000/ton in May 2007.  

It’s back to around $20,000/ton now, but that’s still four times as high as it was in October 2001 – under $5,000/ton.

But it’s not just nickel.  As we saw nickel start to get over its spike, the press filled with stories of the increases our big miners were seeking for their iron ore.  And quietly, the price of chromium has soared from under $600/ton to over $6,000/ton.

Molybdenum, the element added to improve corrosion resistance above what you can get with chromium, has outdone all the rest, from $6,200/ton to a peak over $95,000/ton!  It’s lucky a mere 2 per cent of molybdenum is so effective in improving corrosion resistance.

In response, stainless steel makers and users have sought to get the best value from the alloying elements they use, by shifting between grade families and grades.

We have seen the rise of the 200 series austenitics, which use manganese instead of some or all of the nickel to get the ductile austenite structure.  They peaked at about 10 per cent of world production – but of course the increase in demand for manganese then pushed up its price, making the 200 series less attractive economically.

Duplex grades also offer a potentially cheaper alternative, most using only half the nickel of an austenitic grade with similar corrosion resistance.  A new development, LDX 2101 from Outokumpu, combines the approaches by substituting nearly all of the nickel with manganese.

Ferritic grades have a completely different crystal structure to austenitic grades because they have no nickel added.  That can make their alloying costs much lower, but the steelmaking needed to make good quality steels is more exacting, so the overall cost savings are not as dramatic.

Nevertheless, they can offer useful cost savings.  In recent years they have grown from about 20 per cent of world stainless steel production to 25 per cent or more, and the major steelmakers are predicting they will continue to grow. A recent publication by the International Stainless Steel Forum (ISSF) details the possibilities with this family of grades.

All the talk of metals price increases makes it hard to know what relative contribution each of the elements makes to the overall cost of stainless steel. Believe it or not, many people are not even aware that all stainless steels are mostly iron, so the news about iron ore prices tends to be lost on them. What does doubling the cost of iron ore do to the cost of stainless steel? And how does that compare with the other alloying elements?  Come to that, what effect does the oil price have? It’s not so long since respected economists were predicting $200 per barrel for oil.

These graphs show the ingredient contributions for the two most common grades of stainless steel, 304 and 316.  The bars show the main alloying elements in the grades, each bar representing the average for the year, except the last bar, which is the average for the first half of 2008.

The costs are an estimate of what the steelmaker has to pay to assemble the raw materials to make stainless steel. They don’t take into account the yield achieved, or possible premiums or contract prices paid.  Nevertheless, the graphs illustrate what has happened with alloy costs.  Of course, the steelmaker then has to turn these ingredients into stainless steel, so his overall costs are much higher.  We might expect higher conversion costs for ferritics, duplex steels and the manganese austenitics over traditional stainless steels such as 304 and 316.  

It is important to recognise that raw materials costs are not the only factor in steel pricing and that many factors will influence the day to day prices offered by suppliers.  We are only looking at the costs of the main alloying elements here which is fundamental but not the whole story.

In 304 the biggest culprit for cost rises has been nickel, but in 2008 nickel cost has fallen back to the 2006 level – and chromium and iron have taken over.  Notice that the iron in stainless steel now costs more than all the alloying elements in stainless steel together did in the early 2000’s – and chromium is now costing more than nickel used to.

Astute observers will know that the price of stainless steel has actually been falling in 2008, despite the alloying costs being higher than in 2007.  The lull in the demand for stainless steel has forced the mills to reduce their prices to stimulate sales, and these are tough times for the mills.

The effect of the oil and coal price increases?  Studies suggest it takes about 12 barrels of oil to make a ton of grade 304, so the $100 rise in the price of a barrel since 2002 adds about $1,200 to the cost of a ton of 304.  To put that in perspective, the base price of stainless steel in 2002 bottomed at about that level!

So what has caused all these surges in prices, and where do we go from here – higher, stable, or a return to the earlier levels?  The cause is clear; it is economic growth in the developing world, outside the mature, stable economies that used to dominate the world economy.

This is particularly true for the BRIC group: Brazil, Russia, India and China.  These have all been growing strongly and sustainedly for a number of years, China being pre-eminent.
While their economies were still small, the net increase in demand for metals was not much affected, but their overall demand has now grown to the extent that even when their economies slow, world net demand keeps growing strongly.

The IMF continues to forecast growth rates over 10 per cent for China out to 2013.  After all, over a billion people have lived in poverty for a long time, and their government is committed to developing the country to help them out of it.  Analysts reckon that only about 15 per cent of the demand for metals in China is fuelled by demand outside China, the rest is for domestic consumption.

Stainless steel has not been singled out by these shifts in the world economy.  All the metals, except aluminium and zinc, are currently at about five times the price they were when the boom started.  So much for materials substitution as a way of getting over the price increases!  China has grown so strongly that over the current decade it will consume over half of the copper, aluminium, nickel and zinc used in the world.  Even if China does falter, the other developing countries are not far behind.

The rate of growth in demand is a real challenge for the minerals industry to keep up with it.  It’s not quite the same situation as we see in oil, but it’s not markedly different.  Does anybody think we will ever return to $30 a barrel for oil?  Unlikely, and it’s unlikely we’ll see a return to historical levels for metal prices either.

This article featured in Australian Stainless Issue 44.

Keep Contamination Out and Quality In

Posted 31 July 1993

Quality is the buzz word of the last part of the 20th Century and manufacturers ignore quality control at their peril. With proper attention to detail, stainless steel will provide satisfactory service for many years. The Chrysler building in New York has a stainless steel finish that is in excellent condition after 60 years.

Stainless steel is a quality material and for products made from it to meet the demanding quality standards required, there are some special features that those used to fabricating other materials must watch if their product is to meet their customer's expectations.

Surface contamination is one of the danger areas.

Contamination with carbon steel - embedded iron.

If stainless steel is fabricated with tools previously used for forming mild steel there is almost certain to be carbon steel pick up from these tools onto the stainless steel surface. Grinding wheels used for both carbon steel and stainless products or machining stainless steel with conventional steel cutting tools are particular dangers.

These iron particles will form a galvanic couple with the stainless steel and quickly rust. At this early stage they could be cleaned away with a mild abrasive with little or no damage to the stainless steel surface. If they are left, it could be another story!

When the carbon steel particle rusts, as it is sure to do, it will form iron oxide that will eventually turn into ferric chloride, Figure 1. There are always chloride ions available particularly in marine atmospheres. The resultant ferric chloride will rapidly attack the stainless steel and pitting corrosion will result, generally visible as rust in a pattern related to the source of contamination.

In the fabrication area mark up tables, and the platens of cutting and bending equipment should be covered with cardboard or plastic that is frequently cleaned down or replaced. Crane hooks and forklifts should be similarly protected. A protective plastic film on the surface can limit the amount of pick up that can occur during handling.

Embedded iron will not only arise from fabricating equipment. Hot rolled sheet or bar will not be treated for surface contamination when delivered. Ground finished products are safer. During the manufacture of large vessels carbon steel particles or swarf from other areas of the shop can also be transferred into the tank on workers' boots and clothing and care must be taken to avoid this.

Small particles of embedded iron can be removed by nitric acid applied either as a paste or immersion. Heavier contamination may require a nitric/hydrofluoric acid treatment. These processes are know passivation or pickling.


Other Contamination

There are other dangers during fabricating that can generate crevice corrosion conditions. Adhesive tape is the classic case. The area under the tape is low in oxygen and if moisture can seep under the tape - and it most probably can - a crevice is created where corrosion can occur.

The same sort of crevice can be formed under grease, crayon markings or residual adhesive left after the protective film is removed. Also, some films, if left exposed to the sun's UV radiation for more than a few hours harden and both the film and underlying adhesive are very difficult to remove.

In most industrial areas deposits can build up on the surface. This problem can be eliminated by washing the surface down - a good rule of thumb for architectural applications is to wash exterior stainless steel at about the same frequency as the building windows require washing. Maintenance manuals supplied with stainless steel fixtures or components should specify this cleaning requirement.

Welding also requires attention. Weld spotter and potential crevice corrosion points at the initiation and run out points of a weld run can be a problem as can contamination with grease, crayon marks or carbon steel in the weld area, including an edges, is essential.

Another area of concern is the oxide film formed during welding and the concurrent decrease in alloy content of the material under the weld. In all cases it is good practice to passivate the steel after the welding.

Written by Noel F. Herbst, Director, Nickel Development Institute of Australasia
This article featured in Australian Stainless Magazine - Issue 1, July 1993.

Stainless Steel and Plumbing Standards

After three years of development, the first stage of a Standard covering the grade and dimensions of stainless steel pipes and tubes suitable for water supply and drainage systems has been completed. This interim Standard will be converted to a full Australian Standard in 2009.

The Standards Committee included ASSDA representative Neil McPherson of OneSteel, supported by the Technical Committee.

To avoid possible confusion and protect against corrosion problems in aggressive water supply areas, grades 316 and 316L are specified for the plumbing installation Code of Practice. All materials that satisfy the requirement for water supply and drainage systems must be included in the installation Standard AS/NZS 3500 Parts 1 & 2, which covers the material, grade and approved jointing method for piping systems.

If a material is included in Part 1 Water Supply (for drinking water), it will need to be certified against a product standard to Level 1, while Part 2 Drainage & Sanitary Plumbing requires Level 2 certification. The main difference is that Level 1 products require testing under AS4020 Material in Contact with Drinking Water to confirm lack of water contamination. Stainless steel product readily passes this testing.

All fittings, including the mechanical jointed pressfit and roll grooved types used for the plumbing services, are also tested and certified. AS3688 Metallic End Connectors defines the criteria against which these fittings are certified, including the additional pressure and fatigue testing to demonstrate strength of joint assembly.

Stainless steel using mechanical jointing systems

Mild steel, copper tube and plastic pipes have dominated building water systems for many years. However, high rise developments over recent decades have changed the building industry requirements for water supply and fire protection systems. These systems now require materials with a much higher pressure rating and corrosion resistance.

Stainless steel is recognised as a material most suited to meet these requirements. However, older on-site methods for jointing and fabrication has limited the use of stainless steel.

The approval of mechanical pressfit and roll grooved systems for all water systems has provided a major market for stainless. Stainless steel pipes and fittings have been installed as a solution to specific technical issues including a corrosive environment, high pressure requirements of the hydraulic services system, high operating temperature, or where the project owners are looking for a whole-of-life sustainable product solution.

The following projects illustrate some design and installation specifications around Australia.

Casey Aged Care Facility, Heidelberg, Victoria

108mm and 76mm tube in 316L was supplied by Blucher for a low pressure system feeding rainwater from storage tanks to pumps. Stainless steel was chosen due to concern of longevity and water contamination from other materials due to water levels in storage tanks being low or empty for long periods during dry spells. The Mapress stainless steel pressfitting system was familiar to the plumbing contractor who felt it was labour saving and easy to install. Plastic pipes were used from the roof to the plastic rainwater storage tanks.

Western Corridor Recycled Water Project, SE Queensland

The Mapress 316 pressure system was chosen for rapid, simple installation. There was a lack of pipe fitters available so socket welding was not possible and other trades made the installation. Sizes ranged from 15 to 54mm with butyl rubber sealing rings containing pressures up to 1,000kPa. The stainless steel was used for potable, treated and fire water as well as compressed air. The Mapress system supplied by Blucher has been used in all three waste water treatment plants in the Western Corridor as well as in the Gold Coast Desalination Plant.

Centre Court Business Park, North Ryde, NSW

Heating and chilled/condenser water installations used 316L schedule 5 pipe in both 50 and 100mm diameter in this 30,000m2 low rise complex. Stainless steel offered reliable protection from corrosion and the Victaulic roll grooved system offered ease of assembly.

Suncorp building, Sydney CBD

Refurbishment of the combined fire and drinking water system in a 1972 building used OneSteel Building Services supplied 316L schedule 10 pipe and fittings in 3m, pre grooved lengths for assembly in restricted duct spaces. The 43 floors plus 3 basements ensured high pressure requiring strong stainless steel which also met the drinking water AS4020 requirements.

Centrepoint Tower, Sydney CBD

Stainless steel pipe and fittings were supplied by OneSteel Building Services to replace corroded carbon steel in the 305m tall tower. Systems changed were the fire and potable water and the gas lines. 300m of 316L was supplied in 2.7m lengths which were roll grooved and assembled using Victaulic couplings in a very constricted service duct. Sizes used were 100mm and 50mm in schedule 10 except for gas lines in schedule 40.

This article featured in Australian Stainless magazine - Issue 45, Summer 2009.

Choosing hot water storage tanks

Replacing your hot water system is both inconvenient and expensive, so it pays to carefully consider a hot water storage tank that will stand the test of time as well as deliver energy (and cost) efficiencies.

Storage tanks for hot water systems are made from either stainless steel or from carbon steel with a coating of vitreous enamel.

Stainless steel hot water storage tanks

A hot water storage tank made from grade 316 stainless steel. Image courtesy of Edwards Hot Water.Stainless steel tanks are made from 316 stainless steel, a material which is typically used in the marine industry because of its high corrosion resistance. The 316 stainless steel provides a natural barrier to corrosion that is virtually maintenance free.

This characteristic of stainless steel is evident by the use of 316 or its sister alloy 304 in many other areas of the home including kitchen sinks, dishwashers, washing machines, pots, pans, cutlery and fridges.

The material's strength and inherent corrosion resistance as well as its fatigue resistance from the stresses of constant temperature and pressure variation during service makes stainless steel a strong and durable option.

Vitreous enamel hot water storage tanks

Because carbon steels will corrode in contact with air or water, the surface of the tank must be protected by a layer of vitreous enamel added to line the interior of carbon steel.

However, manufacturers can't make the enamel layer perfectly impervious. The steel and enamel lining expand and contract at different rates as the temperature in the tank rises and falls. This can cause the enamel to craze or come unstuck from the steel, exposing it to water and corrosion. This is a particular problem if the water temperature is set too high.

The steel exposed at the coating defects is protected by the use of a 'sacrificial anode' that acts to prolong the life of the tank. These anodes, usually made from a magnesium alloy, require checking at regular intervals and replacement every 5-7 years on average.

Some water qualities (such as very soft or very pure water) are inappropriate for this system because characteristics of the water render the anode ineffective.

Stainless steel storage tank manufacturers

Edwards Hot Water, Aquamax and Beasley Water Systems are some of the main stainless steel storage tank manufacturers in Australia.

ASSDA member, Edwards Hot Water, has over 35 years of experience in the manufacture, supply and installation of stainless steel hot water systems. Edwards Hot Water exports tanks to 40 countries and the tanks comply with AS2712, the benchmark standard for hot water systems.

Manufacturers of stainless steel hot water tanks give long warranties because in the majority of conditions the tanks perform excellently and provide years of trouble free service without the need for regular maintenance or consumable parts.

Choosing a stainless steel hot water system means long life, low maintenance and a long warranty.

Image courtesy of Edwards Hot Water.
This article featured in Australian Stainless magazine - Issue 30, January 2005.

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.

Weathering the Financial Storm

'Remaining Competitive and Profitable' by James Johnson, Millatec Pty Ltd

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.