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The Sustainable Score Card for Stainless Steel

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

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

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

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

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

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

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

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

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

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

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

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

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

Grade 316 - the 'first step up'

If a job requires greater corrosion resistance than grade 304 can provide, grade 316 is the 'next step up'. Grade 316 has virtually the same mechanical, physical and fabrication characteristics as 304 with better corrosion resistance, particularly to pitting corrosion in chloride environments.

Grade 316 (U NS S31600) is the second most popular grade accounting for about 20% of all stainless steel produced.

This article follows on from "304 -the place to start" in Issue 10 which is also available on ASSDA's website at www.assda.asn.au

COMPOSITION
Table 1 compares three related grades - 316, 316L and 31 6H.

Grade 316L is a low carbon 316 often used to avoid possible sensitisation corrosion in welded components.

Grade 316H has a higher carbon content than 316L, which increases the strength (particularly at temperatures above about 500°C), but should not be used for applications where sensitisation corrosion could be expected.

Both 316L and 316H are available in plate and pipe, but 316H is less readily available ex-stock. 316L and 316H are sometimes stocked as standard 316 (test certificates will confirm compliance with the 'L' or 'H' specification).

CORROSION RESISTANCE
Grade 316 has excellent corrosion resistance in a wide range of media. Its main advantage over grade 304 is its increased ability to resist pitting and crevice corrosion in warm chloride environments. It resists common rusting in virtually all architectural applications, and is often chosen for more aggressive environments such as seafront buildings and fittings on wharves and piers. It is also resistant to most food processing environments, can be readily cleaned, and resists organic chemicals, dye stuffs and a wide variety of inorganic chemicals.

In hot chloride environments, grade 316 is subject to pitting and crevice corrosion and to stress corrosion cracking when subjected to tensile stresses beyond about 50°C. In these severe environments duplex grades such as 2205 (UNS S31803) or higher alloy austenitic grades including 6% molybdenum (UNS S31254) grades are more appropriate choices.

The corrosion resistances of the high and low carbon versions of 316 (316L and 316H) are the same as standard 316.

HEAT RESISTANCE
Like grade 304, 316 has good oxidation resistance in intermittent service to 870°C and in continuous service to 925°C. Continuous use of 316 in the 425-860°C range is not recommended if subsequent exposure to room temperature aqueous environments is anticipated, but it often performs well in temperatures fluctuating above and below this range.

Grade 316L is more resistant to carbide precipitation than standard 316 and 316H and can be used in the above temperature range. However, where high temperature strength is important, higher carbon values are required. For example, AS 1210 Pressure Vessels Code limits the
operating temperature of 316L to 450°C and restricts the use of 316 to carbon values of 0.04% or higher for temperatures above 550°C. 316H or the titanium-containing version 316Ti can be specified for higher temperature applications.

316 has excellent toughness down to temperatures of liquefied gases and has application at these temperatures, although lower cost grades such as 304 are more usually selected for cryogenic vessels.

PHYSICAL AND MECHANICAL PROPERTIES (see Tables 2 and 3)
Like other austenitic grades, 316 in the annealed condition is virtually nonmagnetic (ie. very low magnetic permeability). While 304 can become significantly attracted to a magnet after being cold worked, grade 316 is almost always virtually totally non-responsive. This may be a reason for selecting grade 316 in some applications.

Annealing (also referred to as solution treating) is the main heat treatment carried out on grade 316. This is done by heating to 1,010-1,120°C and rapidly cooling - usually by water quenching.

FABRICABILITY
316 can be deep drawn without intermediate heat softening enabling it to be used in the manufacture of drawn stainless parts, such as sinks and saucepans. However, for normal domestic articles the extra corrosion resistance of grade 316 is not necessary. 316 is readily brake or roll formed into a variety of other parts for application in the industrial and architectural fields.

Grade 316 has outstanding weldability and all standards welding techniques can be used. Although post-weld annealing is often not required to restore 316's corrosion resistance (making it suitable for heavy gauge fabrication) appropriate post-weld clean-up is recommended.

Machinability of 316 is lower than most carbon steels. The standard austenitic grades like 316 can be readily machined if slower speeds and heavy feeds are used, tools are rigid and sharp, and cutting fluids are involved. An 'improved machinability' version of 316 also exists.

COST COMPARISONS
The guidelines in Table 4 are approximate 'first cost' comparisons for sheet material in a standard mill finish suitable for construction projects. The appeal of stainless over its first cost competitors dramatically increases
when lifecycle costs are considered.

FORMS AVAILABLE
Grade 31 6 is available in virtually all stainless product forms including coil, sheet, plate, strip, tube, pipe, fittings, bars, angles, wire, fasteners and castings. 316L is also widely available, particularly in heavier products such as plate, pipe and bar. Most stainless steel surface finishes, from standard to special finishes, are available.

APPLICATIONS
Typical applications for 316 include boat fittings and structural members; architectural components particularly in marine, polluted or industrial
environments; food and beverage processing equipment; hot water systems; and plant for chemical, petrochemical, mineral processing, photographic and other industries.

Although 316 is often described as the 'marine grade', it is also seen as the first step up from the basic 304 grade.

Alternative grades to 316 should be considered in certain environments and applications including:

• strong reducing acids (alternatives might be 904L, 2205 or a super duplex grade),
• environments with temperatures above 50-60°C and with chlorides present (choose grades resistant to stress corrosion cracking and higher pitting resistance such as 2205 or a super duplex or super austenitic), and
• applications requiring heavy section welding (316L), substantial machining (an improved machinability version of 316), high strength or hardness (perhaps a martensitic or precipitation hardening grade).

 

This technical article featured in Australian Stainless magazine - Issue 13, May 1999.

Common specifications for flat products

Stainless steels are now cheaper than ever, but there is still room to minimise costs (see Table 1), which will improve the bottom line for individual companies, projects and the industry as a whole.

Flat productsAustralia is a relatively 'small fish' in the global stainless industry and, without the benefit of local stainless steel production, loses some flexibility on product availability. Unless you're a very large consumer of stainless steel to a single specification or Standard, ordering to common specifications will reduce costs and increase availability of products.

Flat Products - Table 1Suppliers are likely to have products to common specifications. Ordering them reduces the need for slow moving stock, increases stock turns, raises the size of single orders, and can substantially reduce costs. A similar mechanism works for mill or mill indent orders.

Flat products

Until recently, stainless steel flat products manufactured to Australian Standard 1449 were the most widely available in Australia. However, since the closure of BHP Stainless in 1997, products manufactured to this Standard are no longer commonly produced. More common international specifications will need to be recognised in Australia if economies are to be achieved (see Table 2).

Fortunately, the transition may not be difficult, because AS1449 was closely aligned with the ASTM Standards from the USA, which are also similar to the Japanese JIS Standards. Steels identical to AS1449 in nomenclature, chemical composition, mechanical properties and surface finish are readily available internationally.

Today the most commonly available stainless flat product in Australia is manufactured abroad to ASTM A2401A240M Standard specification for heat resisting chromium and chromium-nickel stainless steel plate, sheet and strip for pressure vessels, which nominates ASTM A4801A480M for additional general requirements of the steel ('M' designates the metric version, which is more appropriate in Australia).

European specifications are also emerging and EN 10088 Stainless steels has the potential to become a common specification in the Australian market. EN 10088 makes use of the established German names and numbers for stainless steel grades, Many grades in EN 10088 have close equivalents in the ASTM based Standards, but the nomenclature for grades and finishes is very different and replacements should be examined carefully. For example, in AS1449, ASTM A240M and JIS G4305, grade 304 (the most common stainless) has a minimum of European specifications are also emerging and EN 10088 Stainless steels has the potential to become a common specification in the Australian market. EN 10088 makes use of the established German names and numbers for stainless steel grades. Many grades in EN 10088 have close equivalents in the ASTM based Standards, but the nomenclature for grades and finishes is very different and replacements should be examined carefully. For example, in AS1449, ASTM A240M and JIS G4305, grade 304 (the most common stainless) has a minimum of Ordering at standard width and thickness is the best way to keep steel costs down. Each mill has equipment capable of a certain maximum width and running narrower steel is less productive.

The standard width varies from mill to mill (see Table 3), with most European mills manufacturing at 1,200mm or 1,250mm wide, with a few capable of 1,500mm and, for some thicker coil products, 2,000mm. Mills in Asia tend to standardise on the imperial widths 3', 4' and 5' (914mm, 1,219mm, 1,524mm).

Conclusions

An understanding of commonly used specifications can lead to more efficient and cheaper practices. If questions arise, your supplier or fabricator may have information on alternative Standards that are more commonly available and more suited to your requirements.

Flat Products - Table 2

 

This article featured in Australian Stainless Issue 11 - March 1998. More current information can be found in ASSDA's Australian Stainless Reference Manual.

"L" and "H" and Standard Grades of Stainless Steels

The common austenitic grades of stainless steel, 304 and 316, are also available with controlled low or high carbon contents, know as "L" and "H" variants, with particular applications.

Low carbon or "L" grades are used to prevent or delay sensitisation  of stainless steel at elevated temperatures and the resulting lower corrosion resistance. The problematic temperature zone is 450-850 °C, encountered during welding or specific application environments. "L" grades are often available in thicker selection sizes, greater than about 5mm in flat products.

High carbon or "H" grades are used for higher strength.

Substitution between standard, "L" and "H" grades is often possible allowing many specifications to be met from existing stock.

WHAT "L" GRADES ARE AND WHY THEY ARE USED
The low carbon "L" grades are used where high temperature exposure will occur, including welding of medium or heavy sections. The low carbon is one way of delaying or preventing grain boundary carbide precipitation (often referred to as sensitisation) which can result in intergranular corrosion in corrosive service environments. As shown in the time-temperature-sensitisation curve (right), there is an incubation time before the precipitation of carbides at temperatures in the range of about 450-850°C. The time for precipitation to occur is highly dependent upon the amount of carbon present in the steel, so low carbon content increases resistance to this problem. Because of their application area the "L" grades are most readily available in plate and pipe, but often also in round bar. In the absence of heavy section welding, or of high temperature exposure, the corrosion resistances of the standard and "L" grades are usually identical.

WHAT "H" GRADES ARE AND WHY THEY ARE USED
"H" grades are higher carbon versions of standard grades and have increased strength, particularly at elevated temperatures (generally above 500 °C). Long term creep strength is also higher. "H" grades are primarily available in plate and pipe. Applicable grades are most commonly 304H and 316H, but high carbon versions of 309, 310, 321, 347 and 348 are specified in ASTM A240/A240M. These grades are susceptible to sensitisation if held in the temperature range of 450-850 °C. Once sensitised, impaired aqueous corrosion resistance and some reduction in ambient temperature ductility and toughness will result (usually irrelevant in high temperature applications).

WHAT THE DIFFERENCES ARE

  1. Composition limits for 304 and 304L are identical except for carbon content (304L does permit up to 12.0%Ni, compared to 10.5% max for 304 -but given the cost of nickel it is usual for both grades to have close to the minimum of 8.5%, so there is no practical difference). Neither grade has a minimum carbon content specified. A carbon content of 0.02% for example complies with both 304 and 304L specifications.
  2. 304H has the same composition specification as 304 except for the carbon range of 0.04-0.1 0% (note the minimum limit for carbon) and that the 304H does not have the 0.10% nitrogen maximum limit which applies to both standard and "L" grades. Also, all austenitic "H" grades must have a grain size of ASTM No. 7 or coarser.
  3. The relationship between 316, 316L and 316H is the same as that between the 304 series of stainless steels. Only the carbon contents differentiate 316, 316L and 316H grades (and the nitrogen and grain size limits mentioned above). Carbon contents are listed in Table 1 (from ASTM A240/A240M). Specifications for some other products, particularly tube and pipe, have a carbon limit of 0.035% or 0.040% maximum for 304L and 316L, but are otherwise the same.

    TABLE 1:
    Grade UNS Number Specified Carbon Content (%)
    304 S30400 0.08 max
    304L S30403 0.030 max
    304H S30409 0.04 - 0.10
    316 S31600 0.08 max
    316L S31603 0.030 max
    316H S31609 0.04 - 0.10


  4. Mechanical property specification differences are illustrated in Table 2 (from ASTM A240/A240M). In practice, steel mills generally ensure that the "L" grade heats meet the strength requirements of standard grades, ie all 304L will have yield/tensile properties above 205/515 MPa, so will meet both standard and "L" grade requirements.

    TABLE 2:
    Grade UNS Strength (MPa) min Tensile Strength (MPa) min Yield (%) min Elongation Hardness (HB) max Brinell Hardness (HRB) max Rockwell
    304 S30400 515 205 40 201 92
    304L S30403 485 170 40 201 92
    304H S30409 515 205 40 201 92
    316 S31600 515 205 40 217 95
    316L S31603 485 170 40 217 95
    316H S31609 515 205 40 217 95
  5. Dimensional and other requirements are the same for standard, "L" and "H" grades.
  6. Pressure vessel codes (e.g. AS 121 O) and pressure piping codes (e.g. AS4041) give allowable working pressures for each of the grades at nominated elevated temperatures. These codes allow higher pressure ratings for standard grades than for "L" grades. The codes do not permit the use of "L" grades above 525"C (AS4041) or 425"C (AS1210). Both codes include a clause stating that for use above 550"C the standard grades must contain at least 0.04% carbon. 304 or 316 material with 0.02% carbon are therefore not permitted for these elevated  temperatures, whether called "L" or not. At temperatures from ambient up to this high temperature cut-off "L" grade heats with the standard grade pressure ratings would be permitted, so long as the material was in full compliance with the standard grade composition and mechanical property specifications. As discussed above, it is normal practice for this condition to be met.

    The pressure vessel codes give the same allowable pressure rating for "H" grades as for standard grades -this is logical as the "H" grades are simply the standard grades with their carbon contents controlled to the top half of the range, or slightly above.

ALTERNATIVE GRADE USAGE
Because of availability issues it is sometimes desirable to use a product labelled as a standard grade when an "L" or "H" grade has been specified, or vice versa. Substitution can be made under the following conditions:

  1. "L" grades can be used as standard grades so long as the mechanical properties (tensile and yield) conform to the standard grade requirements and high temperature strength is not a requirement. "L" grades usually comply with standard grade requirements, but Mills' test certificates need to be checked on a case by case basis. It is common for steel mills to supply "L" heats when standard grades have been ordered. The practice is legitimate and should  not present problems to fabricators or end users.
  2. Standard grades can be used as "L" grades as long as their carbon content meets the "L" grade maximum limits.
  3. It is increasingly common for "dual certified" products to be stocked - particularly in plate, pipe and bar. These materials fully comply with both 304 and 304L or 316/316L. Dual certified product is deliberately intended to fulfill requirements for both standard and "L" grades, but cannot be used in applications for "H" grade. If an application requires an "H" grade, this must be specified at time of order. Standard grades can often be used in place of "H" grades so long as their carbon contents meet the "H" limits (generally 0.04-0.1 0%). Grain size requirements may have to be satisfied by extra testing. The product and its test certificate may describe it as a standard 304 or 316 unless it was originally manufactured as an "H" grade. Details of the test certificate will confirm grade compliance.
  4. "H" grades can be used as standard grades so long as their carbon contents are 0.08% maximum, and nitrogen 0.10% maximum. This is likely, but would need to be checked.

REFERENCES FOR FURTHER READING
AS 1210
Pressure Vessels

AS 4041
Standard Specification for Pressure Piping

ASTM A240/A240M
Heat-resisting Chromium and Chromium-Nickel
Stainless Steel Plate, Sheet and Strip for Pressure Vessels

This technical article featured in Australian Stainless magazine - Issue 16, August 2000.

Strengths of Stainless Fasteners

Reasons for using stainless steel threaded fasteners are the same as those for selecting other stainless steel components - generally resistance to corrosive or high temperature environments. In addition to the obvious benefits in improved aesthetics and longevity however, there can be significant cost savings if the joint will require disassembly and reassembly.

GRADES AND STANDARDS
Corrosion resistant fasteners are available 'off the shelf' in a variety of materials but by far the most comprehensive range is in stainless steel with more than 6 000 items available in Australia and many thousands more able to be sourced at short notice. Generally these are produced from grade 304 (A2), grade 316 (A4) or for less demanding applications, grade 303 (A 1 ). Grade classifications A 1, A2 and A4 are in accordance with International Standard ISO 3506; head markings often show this classification. It is common practice and legitimate to manufacture items labelled as grade 304 (or A2) from grades 302HQ or 304 depending on the type of fasteners and the manufacturing process. Less commonly, fasteners are available in hardened and tempered martensitic stainless steels, such as 410 (C 1) or in a higher molybdenum version of grade 316, often designated '2343'. An outline of the range of stainless steel fasteners available in Australia can be found in the Australian Stainless Reference Manual.

Stainless steel fasteners available on the Australian market in the main are equal to or higher in tensile strength than the carbon and low alloy steel fasteners commercially used, and are higher strength than most other corrosion resistant fasteners. Table one shows the comparison between stainless steel fasteners and the various grades of carbon steel and low alloy steel fasteners and Figure one shows the strength comparison of various corrosion resistant materials.

The vast majority of stainless steel fasteners available are produced to ISO 3506 Class 70 (this designates a minimum tensile strength of 700 MPa) and are marked as such. If there is no marking it should be assumed the product is Class 50 (minimum tensile strength of 500 MPa).

If a stainless steel fastener with a higher tensile strength is required there are some products available in Class 80, these are usually produced in grade 316 stainless steel. There have recently become available some stainless steel products in Class 100, also in grade 316 material.

CORROSION CONSIDERATIONS
Where corrosion is an issue, an inexpensive olution is to specify steel fasteners with some form of plating or organic coating rather than to use products manufactured from corrosion resistant materials. Although painted, plated or galvanised fasteners are usually adequate in applications where corrosive conditions are not severe, consideration should also be given to the cost of possible failure and loss of aesthetic appearance when the protective coating becomes damaged or compromised, in comparison to the cost of stainless steel product. Damage to the coating on steel products can be easily caused by the wrench or driver used for tightening, poor plating practices or simply from the turning action of one thread against another in assembly.

TIGHTENING AND GALLING
As with all fasteners the proper installation of stainless steel products is critical to its performance; this is particularly so with respect to tightening and galling.

Galling occurs when the stainless steel oxide surface film breaks down as a result of direct metal contact. Solid-phase welding can then take place (whereby material is transferred from one surface to another). The symptoms of galling include surface damage and seizing and freezing up of equipment. Galling commonly occurs when using stainless steel nuts and bolts together, where the contact points are subjected to high tightening torques.

Fasteners made in accordance with internationally recognised standards should ensure the uniformity of threaded products. Reasonable care should be taken when handling stainless steel fasteners to avoid any thread damage and keep the threads clean and free from dirt, coarse grime or sand. If the threads are tightened on sand or dirt the possibility of galling or seizing is increased.

Ways to reduce galling include:

ROLLED THREADS
Rolled threads are less susceptible to galling than machined ones as they have a smoother surface and the grain lines follow the thread rather than cut across it, which IS the case with machined threads.

TIGHTENING TORQUE
Bolts should be tightened to the correct torque using a torque wrench as overtightening will promote galling.

LUBRICATION
It is recommended that some form of lubrication be applied to threads prior to assembly. Propriety grease-type lubricants, containing tenacious metals, oils etc are available. Some commonly used lubricants contain molybdenum disulphide or nickel powder (sometimes with graphite materials*).

HARDNESS MODIFICATION
Galling can also be reduced by using two different stainless steels, of significantly different hardnesses, on the mating surfaces. A Brinell hardness difference of 50HB may overcome galling.

A common belief that the use of grade 316 studs with grade 304 nuts (or vice versa) will avoid galling is a myth (there is a notable difference in galling).

Table two shows some suggested maximum torque values for various diameters of stainless steel fasteners. This table is a guide only based on industry tests that provide maximum clamping value with minimum risk of seizing. The values shown are based on fasteners that are dry - free of any lubricants - and wiped clean of any foreign matter. The addition of a lubricant can have a significant effect on the torque-tension relationship. A lubricated fastener requires less torque to achieve the same level of tension and also makes the torque-tension relationship more predictable. Different lubricants can also have different effects. Figure two shows that effect on the torque-tension relationship of adding a lubricant.

TABLE TWO: TORQUE GUIDE (Nm)
Bolt Size Grade 304 (A2) Grade 316 (A4)
1/4" - 20 8.5 9
1/4" - 28 11 11
5/16" - 18 15 16
5/16" - 24 16 17
3/8" - 16 27 28
3/8" - 24 29 31
7/16" - 14 42 44
7/16" - 20 45 47
1/2" - 13 58 61
1/2" - 20 61 64
9/16" - 12 77 81
9/16" - 18 85 89
5/8" - 11 125 131
5/8" - 18 141 147
3/4" - 10 173 179
3/4" - 16 168 176
7/8" - 9 263 275
7/8" - 14 262 273
1" - 8 389 406
1" - 14 351 367
1 1/8" - 7 560 586
1 1/8" - 12 529 553
1 1/4" - 7 709 740
1 1/4" - 12 651 683
1 1/2" - 6 1 204 1 261
1 1/2" - 12 953

993

Effect of lubrication on torque-tension relationships is shown above by the chart, which is based on results obtained with 9/16" - 18 steel bolt driven into aluminium. For a non-lubricated bolt, torques of 13Nm - 14Nm were required to develop tensions of 3.5kN to 6.2kN. For a lubricated bolt, torque values ranged from 7.3Nm to 8.5Nm for 4.4kN to 5.5kN tension range.

Torque values are affected in various ways by different types of lubricants. Wax on either the bolt or nut, or both, also acts to reduce the torque requirements.

Source: Skidmore-Wilhelm Mfg. Co.

NEW AUSTRALIAN STANDARD
he new Australian Standard Cold Formed Stainless Steel Structures is due to be published in early 2001. This will include sections giving specific design data for stainless steel fasteners produced to both ASTM and ISO specification systems, In addition an Appendix gives details of the grades and strength levels and their applicable markings, extracted from ISO 3506.

*Graphite is substantially more noble than stainless steel. Care in specification of graphite in contact with stainless steel is required to avoid corrosion.

This technical article featured in Australian Stainless magazine - Issue 17, January 2001.

The Workhorse of Hydrometallurgy


Posted 17 May 200

Stainless steel has earned a reputation as the material of choice for the mining and hydrometallurgical industries. This article discusses suitable grades and applications and the emerging opportunities for stainless steel in these industries.

Hydrometallurgy involves the extraction and refining of metals in aqueous solutions. It encompasses a range of processes such as leaching, solvent extraction, ion exchange, electrorefining, electrowinning, precipitation and solid/liquid separation for numerous metals including copper, zinc, nickel, cobalt, uranium, gold, silver, aluminium and rare earths. As stainless steel is the 'workhorse' material for many of these processes, especially those involving sulphuric acid solutions, hydrometallurgy is a significant market whose importance is growing as new processes are developed and applied.

HYDROMETALLURGY EXPANDING
Historically, metals extraction has been dominated by pyrometallurgical processes such as roasting and smelting, while hydrometallurgy has generally played a relatively minor role. However, since the 1950s, its role has expanded significantly, helped along by a string of new technical developments. These trends seem likely to continue as pyrometallurgical processes fall out of favour due to factors such as falling head grades, environmental pressure against gaseous emissions, the need to treat lower grades and impure ores, and the growing desire to add value by producing metals at the mine site. Significant growth areas for hydrometallurgy have been uranium ore processing in the 1950s, 1960s and 1970s, copper in the 1970s, 1980s and 1990s, and more recently nickel and cobalt.

TYPICAL APPLICATIONS
Uranium ores are almost exclusively treated by hydrometallurgy. The most common process is sulphuric acid leaching of finely ground material
at atmospheric pressure and temperatures up to about 800°C, followed by solid/liquid separation and solvent extraction or ion exchange. Stainless steels and high nickel alloys have been extensively used for tankage, pumps and piping.

Copper has traditionally been extracted from oxide ores by sulphuric acid leaching, either in agitated tanks or by spraying on heaps and dumps. Interest grew dramatically after the introduction of solvent extraction technology in the late sixties which, when coupled with electrowinning, enabled high grade copper cathode to be produced on site. More recently, this approach has been expanded to treat secondary sulphide ores such as chalcocite, and processes are now being developed for the treatment of chalcopyrite, the dominant copper mineral which is normally smelted. These new processes include pressure-oxidation, bio-oxidation and other novel leaching technology. Along with all of this has been the successful introduction of the use of stainless steel blanks for electrowinning and refining. Stainless steel is particularly suitable for copper in sulphuric acid solutions because of the inhibiting effect of copper in solution on corrosion.

Nickel and cobalt hydrometallurgy has been significantly boosted by a number of recent developments including the installation of new pressure acid leaching (PAL) operations for laterites in Western Australia, the first application of tank bio-leaching for cobalt recovery, the development of pressure-oxidation and bio-heap leaching technology for nickel sulphides. Although the PAL operations have had difficult start-ups, the PAL process is likely to become a major force in the future treatment of laterites because of its relatively low energy consumption and high nickel and cobalt recoveries.

TYPICAL USES OF STAINLESS STEEL IN A NICKEL PAL PLANT
STAGE STAINLESS STEEL USED
Ore preparation and slurrying Grade 310 or super duplex grades
Pressure leach circuit Grade 310 or super duplex grades
Counter current decantation
(ccd) circuit
Tanks - grade 316
Rakes and rabble arms - grade 316
Refinery (separating nickel, cobalt
products, making metal)
Process piping - grade 316
TOTAL APPROXIMATE STAINLESS
STEEL USAGE (PER MAJOR PLANT)
6 000 TONNES


BRIGHT FUTURE

Current trends undoubtedly point to an expanding role and bright future for hydrometallurgy in the mining and metallurgical industries. Along with this should come increased opportunity for the use of stainless steels.

Image: Nickel Heap Leaching trial at Radio Hill, WA.

This article was written by Alan Taylor, Chairman of consulting company International Project Development Services and convener of the ALTA Nickel/Cobalt 2001 (Perth, WA, May 15 - 18).

This article featured in Australian Stainless magazine - Issue 18,  May 2001.

Use of Stainless Seel in the Wine Industry

Stainless steels are widely used in the food industries, including wine production, because of their corrosion resistance and ease of cleaning which result in negligible product contamination.

Long life can be expected from stainless steel equipment provided care is taken with:

> Vessel design
> Grade selection
> Fabrication procedures
> Maintenance practices.

The precautions to be taken are not complicated but are most important - neglecting to follow them can lead to rapid failure.

VESSEL DESIGN
Much can be done in the detailed design to improve corrosion resistance. The two cardinal rules are:

1. Design for complete and free drainage
2. Eliminate or seal weld crevices.

A series of drawings, with accompanying narrative, comparing good and bad design practices, are set out in Part Ill of Nickel Development Institute publication #11 007: Guidelines for the welded fabrication of nickel-containing stainless steels for corrosion resistant services.

GRADE SELECTION
Two stainless steel grades are particularly used in the wine industry, grades 304 and 316, and both 'standard' and 'low carbon (L)' versions are available. Their compositions are shown in Table One below.

Table One: Stainless Steel Grade Selection
GRADE CHROMIUM NICKEL MOLYBDENUM CARBON
304 18.0 - 20.0 8.0 - 10.5 - 0.08 max
304L 18.0 - 20.0 8.0 - 10.5 - 0.03 max
316 16.0 - 18.0 10.0 - 14.0 2.0 - 3.0 0.08 max
316L 16.0 - 18.0 10.0 - 14.0 2.0 - 3.0 0.03 max

The 'L grades are specified w here welding is to be carried out and there is concern that time-at-temperature may be sufficient to precipitate chromium carbides and hence cause sensitisation of the metal, resulting in susceptibility to intergranular corrosion.

The molybdenum content of grade 316 significantly improves its resistance to p1tting and crevice corrosion, particularly in the presence of chlorides. It also increases the material cost by about 20 to 25 percent.

For handling waters, grade 304 is satisfactory up to about 200 parts per million (ppm) chlorides, while grade 316 can be used up to 1 000 ppm chlorides.

Tartaric, acetic, tannic, malic and citric acids are not corrosive to stainless steel at the concentrations found in juices or wines. However, the level of su lphur dioxide is an issue. Grade 304 is generally regarded as resistant to corrosion when immersed in juice or wine at free SO2 levels up to 700 ppm. Above this, grade 316 is recommended. The problem is greater in the vapour space where sulphurcontaining acids can form. Grade 304 is not recommended for use in areas where there is more than 75 ppm SO2 in the liquid. This can lead to composite tanks using grade 316 in the ullage zone and grade 304 in the submerged zone.

FABRICATION PROCEDURES
Cracks and crevices in stainless steel welds can act as initiation points for pitting and crevice corrosion. They can also result in product contamination . The aim should be for smooth weld beads without porosity, slag inclusions or undercut.

During the welding of stainless steels, 'heat tint' is formed. This is a high temperature oxide, rich in chromium which has been drawn from the stainless steel. The result is a very thin low-chromium layer on the underlying stainless steel surtace. For best corrosion resistance, both the oxide film and underlying chromiumdeficient layer must be removed. This can be done by pickling using a nitric/hydrofluoric acid mix, either in a bath or as paste, or by mechanical removal.

Since stainless steel depends for its corrosion resistance on the presence of an extremely thin, continuous chromium oxide film, any contaminants which disrupt this film will reduce its corrosion performance and can initiate pitting. A common contaminant is embedded iron particles from nearby grinding of carbon steel fabrications or the use of iron-contaminated tools. Such contamination can be removed by passivating the stainless steel with nitric acid, followed by a thorough water rinse.

After hydrotesting and before being put into service, tanks should be drained and thoroughly dried. There are too many instances of new stainless steel systems failing due to pitting or crevice corrosion because either:

> Contaminated test water has been left stagnant in the system, or
> The system has been drained but not dried, leaving pools of water to evaporate, concentrating dissolved salts and resulting in corrosive attack.

MAINTENANCE
Stainless steel is inherently a low maintenance material - but it is not zero maintenance. The main requirements are:

> Deposits which build up on stainless steel surfaces should be regularly removed. This is because crevices exist under deposits and crevice corrosion can be initiated in such areas.
> Following cleaning, regular inspection is necessary to establish the condition of the equipment. This will ensure early detection of any developing problems so that steps can be taken to prevent further deterioration.

If guidelines such as these are followed, long and trouble free service can be expected from stainless steel equipment.

Words by David Jenkinson from the Nickel Development Institute.

This article featured in Australian Stainless magazine - Issue 18, May 2001.

Aspects of Mig Welding Thin Stainless Sheet

Principles of Mig Welding

According to the AWS Welding Handbook volume 2, MIG welding is "an arc welding process that uses an arc between a continuous filler metal electrode and the weld pool. The process is used with shielding from an externally supplied gas and without the application of pressure". The wire is usually supplied in spools and fed through to the welding arc by an electric feed motor, with no manual control ofthe wire feeding process ie semiautomatic.

Most materials, except aluminium, use what is termed a ‘constant potential power source’, and this automatically regulates the arc gap by varying the burn off rate of the wire.

MODES OF METAL TRANSFER
There are four modes of metal transfer possible with the standard MIG welding system: short circuiting (or dip), globular, spray and pulsed modes.

Short circuiting transfer is the mode which uses the lowest amperage range, hence the lowest heat input of the four variants. Essentially the electrode contacts the molten weld pool, completing the electrical circuit. Resistance heating of the electrode takes place until the tip of the wire is melted off and transferred to the weld pool. The arc is then extinguished, until the tip of the wire comes back into contact with the weld pool, and the cycle begins again. This process is repeated between 20 to 200 per second.

Globular transfer uses slightly higher currents than for short circuit transfer, but lower than those used for the spray mode. The arc is continuous, but the molten metal is transferred across the arc in a characteristic globular fashion, with the globule diameter greater than that of the electrode.

Spray transfer uses the highest current ranges, having a continuous arc, and the weld metal transferred across the arc in many tiny droplets. The droplet diameter is equal to or less than the electrode diameter, and is also accompanied by an electric force propelling these droplets across the arc, hence the term spray transfer.

Pulsed transfer is achieved by using a lower welding current at which conventional spray transfer would not be possible, and then imposing background pulses of power through the system. Typical pulsing frequencies can be up to 40 kilohertz and this in turn transfers one droplet across the arc per pulse, thus achieving spray transfer at lower welding currents.

SHIELDING GASES
A variety of shielding gases are available from many suppliers, which have been refined to suit various applications. A major effect that these gases have, is their ability to influence arc stability and mode of metal transfer.

The common MIG shielding gases are Argon and CO2. At all usable welding currents, CO2 will commonly produce a drip or globular transfer, whilst the full range of transfers can be obtained with Argon.

Mixtures of these two gases as well as a one or two percent addition of oxygen are also common when MIG welding the carbon and low alloy
steels. They have been designed to get the best characteristics of both gases, and improve arc stability, metal flow, sidewall fusion, wetability
etc.

PROBLEMS ASSOCIATED WITH MIG WELDING SHEET MATERIALS
It should not come as any surprise that welding sheet metals by the MIG process must have limits, especially on the thinner sections. MIG welding is a relatively high heat input process, and hence the rate of heat transfer away from the weld pool becomes important. The surrounding material must be able to support and maintain the weld pool until it solidifies. The final weld must also have an acceptable bead profile, with visual and surface quality within the specification requirements. Heat input must be kept to a minimum, and this in turn implies a
short circuit or globular transfer mode. Unfortunately, these modes are prone to spatter.

A compromise must be reached between heat input, travel speed, weldability and bead shape, which are all influenced by specific gas compositions.

The table shows some results of MIG welding austenitic stainless steel sheet using a one millimetre diameter wire, with a variety of gases. The volts and amps recorded indicate the lowest settings which achieved the stable transfer mode indicated.

The values of interest, and which formed the basis for the experiment, are the electrical settings, namely the amps and volts. Travel speed is obviously important in reducing heat input, but the effect of gas composition on the stable mode of transfer was the primary objective.

CONCLUSIONS
The results obtained not only confirmed the expected problems when welding thin stainless sheet, but also produced some interesting points
when using the pulsed transfer mode. As expected, spray transfer is not recommended for thin sheet.

Helium, as an alternative addition to shielding gases definitely has advantages in achieving an acceptable weld profile over the more common
gas mixtures, even in semi-automatic welding, but a greater degree of competency is required by the welder to be able to handle the necessary faster travel speeds.

Pulsing helps to overcome the problems associated with bead shape and burn-through, and if the equipment is available, it negates the advantages offered by a helium containing gas. Higher travel speeds in the pulsed mode are still necessary with the helium addition. The use of a helium-containing mixture with the pulsed mode should offer a combined benefit of a lower heat input and faster production rates, and, hopefully, less distortion as a result.

Words by Jim Henderson.

Reproduced with permission from the Australasian Welding Journal Volume 45 Fourth Quarter page 21.

This article featured in Australian Stainless magazine - Issue 20, February 2002.

Coated Abrasives for Surface Finishing - Part 1

Accurate specification of a surface finish is vital for extracting maximum functionality and durability from stainless steel components. Read Part 2. Read Part 3.

Coated abrasives are important in generating the right surface finish for corrosion resistance, cleanliness, aesthetics or other requirements.

MANUFACTURING PROCESS
Primary manufacturing processes such as casting, forging or rolling produce a surface finish which may be adequate for the end function. If not, there are secondary processes such as machining, cutting, grinding, lapping or mass finishing using tumbling techniques or barrel finishing.

Surface finishing techniques may be mechanical, electrical, thermal or chemical or a combination. The finish depends on grit size, pressure and
product selection.

COATED ABRASIVES
Coated abrasives in the form of belts, rolls or discs are used during both the primary and secondary manufacture of stainless steel into coils, sheets or fabrications. Methods and machinery may vary but the broad principles are:

> do not abrade unnecessarily – start at the finest grit which will produce the desired finish quickly

> never skip more than two grit sizes in a polishing sequence or previous grit lines cannot be removed

> don’t use excessive pressure – coated abrasives cut well with minimal pressure

> don’t persist with a worn abrasive product – when a disc or belt stops cutting it should be replaced.

DEVLOPMENTS
The drive for better manufacturing has yielded improvements to grinding and polishing machinery, accompanied by developments in abrasive products. Better abrasive grains give faster stock removal and longer life which has led to increased horsepower being specified and this has necessitated improvements to the quality of backing materials.

COATED ABRASIVES AND SURFACE FINISH GENERATION
A coated abrasive has three components – backing, adhesive and grain – each of which affects the outcome.

BACKINGS are manufactured from paper, cloth, fibre or a combination, non-woven material or polyester film. The type selected should:

> have sufficient tensile strength to transmit the power loading of the machine

> be flexible enough to conform to component shape

> provide a substrate suitable for the relative strength of adhesive required

> not stretch unduly during use

> in the case of very fine grit coatings, provide a flat and true surface.

Paper backings come in a variety of weights designed for specific tasks. In the stainless steel market belts are most commonly manufactured in E (180-200gsm), F (230gsm) or G (300gsm) weights. They may also have anti-static surface treatments to minimize dust adherence.

Cloth backings come in many varieties. Most commonly, X weight (cotton) and Y weight (polyester) are used in heavier stock removal operations such as the grinding of castings and J weight, which is lighter and more flexible, is used for contour work – polishing complicated shapes. Different cloth backfills applied to the rear of the coated abrasive belt allow it to be used for dry and wet operations (oil or water) according to the type of operation.

Cloth belts are normally used on higher horsepower machines and need to be strong enough for the transmission power which can be as high as 10HP per inch width of belt (7.5kW per 25mm).

Vulcanised fibre is used extensively in the manufacture of resin fibre discs. It is less flexible than paper and cloth backings but has the strength required to withstand high operational speeds and heavy grinding pressures.

Combination backings consist of an E weight paper reinforced with an open weave cotton scrim cloth. They are usually used for very coarse grit operations.

Non-woven backings such as lofted nylon are used in the manufacture of hand polishing pads and rolls. This material also forms the basis of many convolute wheels and flap brushes (pictured) which are used to impart a special brushed finish.

BONDING ADHESIVES are used to anchor the abrasive grains in place on the backing. They can be natural animal glues but thermosetting synthetic resins are the norm. They are stronger, tougher and resist heat and fluids better. Different types and strengths have been developed according to the product’s end use. As a rule of thumb, if belts or discs are shedding (losing grain) then the bond strength needs to be increased; if the product is showing signs of glazing (metal pick up) then the bond needs to be less strong.

ABRASIVE GRAINS Most of the grains in use today are synthetically manufactured to be hard enough to penetrate the substrate being ground while still fracturing under dynamic impact to present new cutting facets. They are designed for high thermal and chemical resistance at grinding interface temperatures.

Premium grade aluminium oxide grains are an effective general purpose abrasive. Silicon carbide grain is used to impart a brighter finish, however it has a comparatively shorter product life. Although more expensive, the newly-developed zirconia alumina grain produces a more consistent finish throughout the life of the product.

Words by Charles Fenton. Charles E. Fenton is Managing Director of Klingspor Abrasive Technologies, Australia. The next article in this series will look at how backings, bonding adhesives and abrasive grains are combined.

This technical article featured in Australian Stainless magazine - Issue 22, September 2002.

Cleaning your indoor stainless steel

Quick and easy tips for keeping that shine

Retaining a sparkling finish on stainless steel surfaces is just a matter of a few simple steps. And you don't need expensive products or special equipment - ordinary household cleaners are usually all that's required. You just need to bear in mind a few easy DOs and DON'Ts...

It'll come out in the wash

Stainless steel looks best if it's cleaned regularly with plenty of water. Drying afterwards makes sure streaky marks aren't left behind.

Remember that simply wiping with a damp cloth is not as effective as it can smear dirt without removing it.

Routine cleaning prevents any stubborn stains building up.

So what will you need?

You don't need any fancy equipment. For day to day cleaning, plenty of water, some mild detergent and a cloth or soft brush will do the job. You can use a 1% ammonia solution but don't use bleach? it's just too easy to make the solution too strong and too hard to rinse it properly afterwards.

After washing, rinse in clean water and wipe the surface dry with a soft absorbent cloth. On brushed stainless steel, follow the direction of the polish for best results.

An excellent cloth to use is 3M's Scotch-Brite high performance cleaning cloth.

Watch out for scratches!

The important thing to remember is that stainless steel can be scratched by careless handling or aggressive scrubbing. Just like you would take care of a polished timber finish, avoid dragging rough items across the surface and be aware that grit trapped under other objects can be the culprit.

Avoid bad chemistry

Stainless steel may discolour if left in contact with salts or acids for extended periods. Also avoid leaving carbon steel items in contact with stainless steel, particularly if wet. But if you observe ordinary hygiene measures, like timely cleaning-up in food preparation areas, you won't have any problems.

How to handle the tough customers

Sometimes you need a tougher approach. Here's how to get rid of the most common offenders:

Fingerprints, oil & grease marks

If a mild detergent or dishwashing detergent doesn't shift unsightly fingermarks, get rid of them with a bit of glass cleaner on a soft cloth. You can also use a small amount of alcohol, methylated spirits, acetone or mineral turpentine. Then rinse with clean water and dry.

You can give longer protection to high traffic areas by lightly rubbing with olive oil or baby oil followed by a polish and shine using a soft cloth.

Tea & coffee stains

Discolouration from tea and coffee stains can be removed by soaking in a solution of boiling water and baking powder. Remember to rinse well and wipe dry.

Sticky labels

Remove sticky labels as soon as possible. Gentle heat from a hair dryer or a glue gun generally softens the glue for easy removal, or you can warm stainless steel pots and pans in the oven before peeling off the labels. Eucalyptus oil based cleaners (or eucalyptus oil on its own) often work well to remove adhesives.

Ensure you don't leave any glue on the surface ? it could trap dirt or break down and cause staining.

Rust marks

Apply cream cleanser with a soft damp cloth and rub gently.

If the mark still won't shift, it might be necessary to use a proprietary stainless steel cleaner. These are usually based on dangerous chemicals (such as phosphoric, oxalic or sulphamic acids) and must be handled with care according to the manufacturer's directions.

After cleaning it is important to neutralise the acid with a 1% ammonia or baking powder solution, rinse with clean water and wipe dry. If the rust has worn away the surface, don't despair! Bad rusting can be repaired with professional polishing but you will need to get expert advice.

Paint

Apply paint stripper, taking care to follow the safety instructions. You may need to use a nylon brush or scouring pad, but avoid metal scrapers at all costs - they will damage the surface.

Hard water scale

Heavy limescale from hard water can be loosened by soaking in a hot water and 25% vinegar solution. Rinse well with a solution of baking powder or 1% ammonia and then with clean water. Always wipe dry.

Cement and mortar

Cement and mortar splashes should be washed off before they set. Mild acids such as vinegar may be needed but not those using chloride rich chemicals. Never use brick cleaning liquids which contain hydrochloric acid. Be very careful that loosened particles don't scratch the steel surface.

Don't go against the grain

Always rub stainless steel in the same direction as the grain. Rubbing against the grain will spoil the finish and stainless will lose its shine. Worse, rubbing against the grain can damage the surface by creating microscopic crevices where dirt can collect. This can lead to corrosion spots.

Fortunately, it's usually easy to tell which is the right direction. You need to watch out for items like round handrails, which are often polished around their circumference when they're manufactured, rather than up and down the length of the tube.

If you have to scrub a stain to remove it, make sure you use a clean nylon scourer or a cloth with chalk-based cream cleaner. But test an inconspicuous area first as you could end up with a bright polished spot which doesn't match the rest of the surface.

NEVER EVER use steel wool (wire wool) to clean stainless steel.

It is usually made of carbon steel and any fragments left behind will rust onto the stainless steel surface. Using any kind of scourer which has previously been used on ordinary (carbon) steel is also a no-no for the same reason.

Stainless steel wool scouring pads are available for heavy duty work, like removing burnt food from stainless steel saucepans. These will scratch the stainless steel surface, but won't leave fragments to go rusty.

Download Technical FAQ 2

Important Disclaimer

The technical recommendations contained in this publication are necessarily of a general nature and should not be relied on for specific applications without first securing competent advice. Whilst ASSDA has taken all reasonable steps to ensure the information contained herein is accurate and current, ASSDA does not warrant the accuracy or completeness of the information and does not accept liability for errors or omissions.

This article featured in Australian Stainless Issue 21 - June 2002.

Coated Abrasives for Surface Finishing - Part 2

The last issue of Australian Stainless contained an overview of coated abrasives and guidelines for achieving the desired surface finish. This technical series continues with a comparison of grit size and hardness. Read Part 1. Read Part 3.

Early versions of abrasive sheets and rolls were made by sprinkling naturally occurring grit, such as sand or emery, onto cloth or paper coated with animal hide glue. The resulting 'sandpaper' was used for surface finishing in woodwork or preparing a surface for paint or varnish. Because the application of the grit was random the product soon became dull and lost its cut.

Not long after the development of paper products, the flexible emery cloth roll made its appearance in metal working workshops as a standard tool for rust removal and light finishing. By contrast, solid bonded grinding wheels were developed for heavy stock removal in foundries.

3 Elements: Backing, Bond and Grain
Modern coated abrasives allow stock removal up to 30 times faster than with a bonded grinding wheel. This superior performance has been brought about by improvements to all three elements of coated abrasives: backing, bond and grain.

Backing
The type of backing used sets the basic design parameters, being: strength, safety, shape, geometry, tolerance and coolant resistance.

Paper - available in various weights up to 300 gsm (grams per square metre) in widths up to 1650mm.

Cloth - cotton, polyester or a mixture, in widths up to 1550mm.

Fibre - 0.7mm (30 thou) vulcanised fibre.

Combination - linen scrim cloth plus paper in widths up to 1000mm.

Polyester film - flexible consistent thickness.

Bond
Natural glues can be used for the matrix supporting the abrasive grain but modern abrasives generally use synthetic thermosetting resins which are stronger, tougher and more heat resistant.

Grain
Abrasive grain provides the cutting edges for surface generation. Common types are:

> Aluminium oxide AI203 available with various surface treatments
> Silicon carbide SiC
> Zirconia Zr02
> Ceramic aluminium oxide SG (seeded gel).

The important characteristics of grain are hardness, friability, toughness and shape.

The graph below shows the hardness of selected materials including abrasives.


Relative Comparison of Grit Size
The selection of the correct grit size and sequence is vitally important in achieving the desired surface finish. The most common grading system in use today is the FEPA or 'P' series (Federation of European Producers of Abrasives).

However, certain products made in the US or Japan may be graded differently. Equivalents are provided in Table 1.

Abrasive Production
The majority of abrasive manufacturers employ a reel to reel process to combine the backing, adhesive bond and grit into an efficient cutting tool.

As shown below, he grain is propelled into the wet adhesive by means of an electrostatic force. This critical part of the manufacturing process ensures a sharp, long lasting product. In order to further secure the abrasive grain, an additional coat of adhesive, known as a 'size' coat, is applied and the whole product is dried and cured. Certain products (called multi-bond or super-sized) have a further coating applied in order to minimise heat build up and subsequent welding action. This feature is particularly important in the case of stainless steel grinding because non-oxidised steels are very reactive at interface grinding temperatures and they combine readily with the aluminium oxide grain. This phenomenon is visible as a silvery sheen on the surface of an abrasive belt. Once the grain has been 'capped' with metal it can perform no further cutting action and merely increases frictional heat and subsequent degradation.


Using Abrasives Economically

There are a number of factors to be considered to obtain best value from coated abrasives. If obtaining a specified, repeatable finish is an important consideration it may be most economical to limit the abrasive belt to a set amount of polishing, for example a certain number of metres of coil. If specific finish is not a criteria the abrasive can be used to the very end of its life. Saving time or power usage and utilising the most technically advanced product can be as significant overall as the amount of abrasive consumed.

 

Table 1 - Relative Comparison of Grit Size

Particle size inches Particle size microns All product other than emery Emery
Grading system Comparable grit symbol Polishing paper Cloth
CAMI FEPA
0.00026 6.5 1200 - - 4/0 -
0.00035 9.0 - - - - -
0.00036 9.2 1000 - - 3/0 -
0.00047 12.0 - - - - -
0.00048 12.2 800 - - - -
0.00059 15.0 - - - - -
0.00060 15.3 - P1200 - - -
0.00062 16.0 600 - - 2/0 -
0.00071 18.3 - P1000 - - -
0.00077 19.7 500 - - 0 -
0.00079 20.0 - - - - -
0.00085 21.8 - P800 - - -
0.00092 23.6 400 - 10/0 - -
0.00098 25.0 - - - - -
0.00100 25.75 - P600 - - -
0.00112 28.8 360 - - - -
0.00118 30.0 - P500 - - -
0.00137 35.0 - P400 - - -
0.00140 36.0 320 - 9/0 - -
0.001575 40.0 - - - - -
0.00158 40.5 - P360 - - -
0.00172 44.0 280 - 8/0 1 -
0.00177 45.0 - - - - -
0.00180 46.2 - P320 - - -
0.00197 50.0 - - - - -
0.00204 52.5 - P280 - - -
0.00209 53.5 240 - 7/0 - -
0.00217 55.0 - - - - -
0.00228 58.5 - P240 - - -
0.00230 60.0 - - - - -
0.00254 65.0 - P220 - - -
0.00257 66.0 220 - 6/0 2 -
0.00304 78.0 180 P180 5/0 3 -
0.00363 93.0 150 - 4/0 - Fine
0.00378 97.0 - P150 - - -
0.00452 116.0 120 - 3/0 - -
0.00495 127.0 - P120 - - -
0.00550 141.0 100 - 2/0 - Medium
0.00608 156.0 - P100 - - -
0.00749 192.0 80 - 0 - Coarse
0.00768 197.0 - P80 - - -
0.01014 260.0 - P60 - - -
0.01045 268.0 60 - 1/2 - -
0.01271 326.0 - P50 - - -
0.01369 351.0 50 - 1 - Ex. Coarse
0.01601 412.0 - P40 - - -
0.01669 428.0 40 - 1-1/2 - -
0.02044 524.0 - P36 - - -
0.02087 535.0 36 - 2 - -
0.02426 622.0 - P30 - - -
0.02488 638.0 30 - 2-1/2 - -
0.02789 715.0 24 - 3 - -
0.02886 740.0 - P24 - - -
0.03530 905.0 20 - 3-1/2 - -
0.03838 984.0 - P20 - - -
0.05148 1320.0 16 - 4 - -
0.05164 1324.0 - P16 - - -
0.06880 1764.0 - P12 - - -
0.07184 1842.0 12 - 4-1/2 - -

Words by Charles Fenton. Charles E. Fenton is Managing Director of Kongspor Abrasive Technologies, Australia. The next article will look at specific finishes and their generation using coated abrasives.

This technical article featured in Australian Stainless magazine - Issue 23, December 2002.

Coated Abrasives for Surface Finishing - Part 3

Our three-part series on coated abrasives concludes with information on choosing the correct abrasive product for the desired finish. Read Part 1. Read Part 2.

THE IMPORTANCE OF SURFACE ROUGHNESS

The surface roughness of stainless steel is an important factor in determining corrosion resistance. Put simply, the smoother the finish the greater the corrosion resistance, whether in the form of sheet or coil or in welded components.

SURFACE TOPOGRAPHY

Interaction between the abrasive belt and the workpiece is affected by surface topography (micro texture). Even a surface which appears perfectly flat to the naked eye has ‘asperities’, undulations between 0.05 μm and 50μm occurring 0.5μm to 5 mm apart.

A variety of instruments are available to measure surface micro texture. They work on the principle of moving a stylus over a representative length of the surface and recording the peaks and valleys.

In Australia, surface roughness is expressed in Ra. The measurement refers to the average variations of the undulations from the average surface of the sample.

Current density & surface roughnessTYPICAL FINISHES

Three stainless steel surface finishes are typically available from the mill:

  • #1 hot rolled, annealed and pickled (Ra 3 to 6μm)
  • 2B cold rolled (Ra 0.1 to 0.2μm)
  • BA bright annealed (Ra 0.06 to 0.2μm)

From these initial surfaces, a wide variety of finishes can be achieved with coated abrasives, satinising wheels and mops, buffing wheels and polishes. The type of finish generated depends on many variables: grit sequence, lubrication, raw material quality, machine type, abrasive type, pressure applied, through feed speed, abrasive belt speed and so on.

Because of all these factors, nominally identical finishes vary slightly from one producer to another. To ensure that the desired finish is delivered, specifiers should nominate the acceptable Ra (surface roughness) range and any other factors necessary for the application (for instance viewing angle or light conditions for architectural samples).

The common ASTM designations for stainless steel surfaces such as ‘No. 4’ specify a process to achieve a finish and not attributes of the surface itself. The result can fall outside the desired
surface roughness range. The Euronorm finishes of EN 10088, provide a larger number of specifications than ASTM A480 and include some which require particular Ra values.1

Although the measurements involved are microscopic, research indicates there is significantly higher resistance to corrosion in stainless surfaces with a roughness below 0.5μm Ra.

THE ROLE OF COATED ABRASIVES

Technically advanced coated abrasives are designed to optimise production by delivering consistent, measurable surface finishes. However, the operator must select the correct abrasives and the right product sequence.

Abrasive grainPolishing is hard work and even with machine operations, it takes time and care. The absolutely essential element is to remove all the polish lines from the previous stage before moving on to a finer grit. If this isn’t done, the final and finest buffing step will be marred by a streak on the surface. Although it is often impossible to rotate the work, removal of polish lines is readily
monitored by polishing at right angles for each new step.

The first grinding step should be as fine as possible. As #80 is usually the finest practicable size, it may require some time to smooth a large weld bead. An effective grit sequence for producing a mirror finish is 80, 120, 240, 320, 400, 600 and 800 before proceeding to mops.

Steps can be missed but at the cost of longer polishing times and the risk of stray scratches. Old abrasives will give a smoother finish but the results are less predictable and are operator dependent.

Lubricants may be necessary because of the poor conductivity of stainless steel. Lubricants also remove debris, improve the quality of the finish and increase the abrasive life. When buying abrasives it is important to choose a reputable product; an unknown quality could mean stray, coarse grit with its attendant final streaks which will mar the result and be especially obvious on a ‘mirror’ finish.

When silicon carbide abrasives are used a brighter, more highly reflective finish results, albeit at the expense of belt life. Other materials, for example aluminium oxide, zirconia or ceramic
grains, will give a significantly longer belt life but will produce a different overall finish.

A quality coated abrasive belt acts as a series of single point cutting tools. Each grain has the optimum shape and angle to accomplish the cutting action and subsequent chip removal. This is partly achieved through electrostatically orienting the grains in relation to the backing during manufacture. The rest comes from choosing the correct abrasive type for the job. A cutting facet which isn’t sharp enough results in random streaks on the stainless steel surface as the grain fails to cut cleanly and drags a chip along the surface. This effect is more prevalent with aluminium oxide belts.

Roughness vs Abrasive Grit Size*

Grit
Ra(μm)
500# 0.10 - 0.25
320# 0.15 - 0.22
240# 0.30 - 0.67
180# 0.42 - 0.96
120# 0.29 - 0.81
60# 2.01

GETTING IT RIGHT

Supplying the desired surface is as much a part of filling a contract as other aspects of fabrication. There are a large number of variables which impact on the surface finish. The previous two issues of Australian Stainless presented an overview of modern coated abrasives and specific information on their composition and manufacture (issues 22 and 23). Data on the hardness of abrasive materials and a relative comparison of grit size was included.

An understanding of what makes a quality abrasive product and how coated abrasives interact with the workpiece helps ensure that the corrosion resistance and aesthetic requirements of
the client are met.

1. EN 10088-2 : 1995 specifies additional specific requirements to a ‘J’-type finish, in order to achieve adequate corrosion resistance for marine and external architectural applications. Transverse Ra < 0.5μm with clean cut surface finish.

This article featured in Australian Stainless Issue 24 - March 2003. It was written by Charles E. Fenton, Managing Director of Klingspor Abrasive Technologies, Australia and Graham Sussex, ASSDA’s technical specialist.


Posted 1 July 2003

Stainless steel combines structural strength with corrosion resistance to form a superior construction material which additionally supports a range of aesthetically pleasing finishes.

The austenitic grades, typically 304 and 316, are most common and comprise 70% to 80% of all stainless steel used. Their popularity is due to their excellent corrosion resistance and mechanical properties combined with their relatively low cost. Nevertheless, the use of stainless steel hollow sections in construction has been restricted in the past by the unavailability of product larger than 150mm x 150mm x 6mm.

Today, however, the stainless steel industry internationally has the capacity to produce hollow sections up to 300mm x 300mm OD (outside dimensions) in thicknesses up to 12.5mm, matching the size range of carbon steel.

Design Codes and Research
The new AS/NZS 4673 gives minimum design requirements for static load bearing stainless members cold formed from annealed or temper rolled materials. Eurocode 3: Design of steel structures, Part 1-4: General rules - supplementary rules for stainless steel is the draft European standard for structural stainless steel design.

According to AS/NZS 4763, pending the release of Eurocode 3 as a European standard, the National Building Code of Finland used in conjunction with the draft Eurocode 3 part 1.2: General rules, structural fire design contains the most specific guidance on fire design for stainless steel members.

Finnish supplier Stalatube Oy has a research program which has concentrated on maximising stainless steel’s advantages as a construction material – corrosion and fire resistance, mechanical strength, easy maintenance and clean aesthetic looks. Good results have been found particularly in relation to work-hardening, which can more than double yield strength, and hold the increased values at temperatures up to 800°C.

Work Hardening
Normal austenitic stainless steel grades cannot be hardened by heat treatment. Hardening is achieved through cold-forming which increases mechanical properties such as the yield and tensile strength. This is particularly desirable in situations where weight is critical, for example in vehicles, or in construction where the design is enhanced by reducing bulk. Enhanced properties result in cost savings as well. The savings potential can be roughly calculated by comparing the enhanced yield strength to the base yield strength.

According to the European standard EC3 part 1.4 the design is based on the strength values shown in the table below.

Grade 0.2% proof stress, MPa min Tensile strength MPa Elongation A80, % min
EN 1.4301 (closest to grade 304) 220 540-750 45
EN 1.4404 (closest to grade 316L) 230 530-680

40

Table 2 below compares the requirements of EC3 part 1.4 (the European standard for design) and ASTM A666 regarding proof stress (yield) and tensile strength values for austenitic stainless steel grades in cold worked state.

Strength class EN 0.2% proof stress, MPa min Tensile strength, MPa min ASTM A666 nearest temper
C700 350 700 1/8 Hard
C850 530 850 1/4 Hard

he Australian Standard AS/NZS 4673:2001
permits the mechanical properties used for designing with austenitic grades to be established by testing of the finished product, ie, instead of testing a sample of the original plate or sheet, a section of the tube can be stretched to failure in a tensile testing machine to find the proof stress and ultimate tensile strengths. This allows the benefits of increased strength due to work hardening to be included when designing structures to the Australian Standard.

 

Over a short period, austenitic stainless steel sustains its mechanical values at higher temperatures than carbon steel. The figure above shows the reduction factors for elastic-modulus for stainless and 0.2% proof stress for an austenitic stainless steel. The sustainability of mechanical values makes it possible to obtain 30 minute fire resistance in stainless steel structures without any additional fire protection. These mechanical values are accepted in Finland as the basis for fire design in structures made of austenitic stainless steel hollow sections. In Australia it is possible to take advantage of the high temperature properties of stainless steels by carrying out fire tests, or by using the results of fire tests in conjunction with appropriate calculations.

The high temperature properties of stainless steel means that in suitable locations the intumescent coatings or other fire protection materials which would need to be applied to carbon steel are not required, allowing the stainless steel framing to be exposed. This is simpler and results in a much improved appearance and could be more economical and environmentally acceptable.

Designing with Stainless Hollow Sections
The qualities of stainless steel favour lightweight, slender structures, with a modern, classy feel and futuristic overtones. The combination of higher mechanical strength at room temperature and fire resistance makes stainless suitable for glass facades and glass roofs, accessways, stairways and balcony structures. A major application area in Australia is air distribution tube in sewage treatment plants.

For those projects where structural loads are being carried and design strength is critical, structural tube with guaranteed mechanical properties can be obtained; this generally implies a minimum 0.2% proof stress of 350MPa.

Designers opting for stainless steel are discovering that there is a wide range of products on the market. For example, Stalatube’s hollow sections begin with 25mm x 25mm tubes used for decorative purposes and go up to 300mm x 300mm (or 400mm x 200mm) for heavy structures with high load-bearing requirements. Profiles above 100mm x 100mm can be manufactured to customers’ own dimensions for maximum cost effectiveness. Australian stock is generally limited to 150mm in square sections although rectangular sections to 200mm x 100mm are available. Smaller sections down to about 12.7mm are also readily available. In addition to the “direct off mill” tube external finish, which is essentially that of the 2B of HRAP strip from which the tube was manufactured, grit polished product is routinely stocked in most common sizes. The polished finish presents an attractive and cost-effective product for visually exposed building components. Grit polished surfaces not only look more attractive in appropriate applications, the finish is also such that welded joints can be blended in, giving a more finished presentation.

Image 1 A 15m high, 230m long copper wall surrounds the Nordic Embassies in Berlin. A load-bearing stainless steel frame inside the enclosure supports the copper panels. The welded frame is made of polished (grit 320) 316 austenitic stainless steel hollow sections measuring 120mm x 120mm x 5mm. Stainless steel was chosen to satisfy low maintenance requirements and to provide a surface which doesn't react with the copper.

Image 2 Nokia House, Helsinki, has a double facade with single glazing 70cm from the front wall. The double facade has many advantages. The air gap between the wall and glazing cover acts as insulation, reducing the need for heating in winter and cooling in summer. It blocks traffic noise when the internal windows are opened and allows ventilation during rainy weather and below zero temperatures.
The load-bearing structures of the double facade are made from 90mm x 45mm x 3mm austenitic hollow sections with the glass fixed on the narrow side. The dimensions were calculated to satisfy the load bearing needs whilst maintaining the deflection needed to avoid the light atmosphere required. The building is located close to both the sea and the main western suburbs of Helsinki where traffic pollution occurs. Grade 316 tubes were chosen for this harsh environment.
Architect Helin & Co Structural Design Matti Ollila & Co

Words by Pekka Yrjola. Pekka Yrjola is a Research & Development Engineer at Stalatube Oy's head office in Lahti, Finland.

This article featured in Australian Stainless magazine - Issue 25, June 2003.

Chemical Surface Treatments

Successfully using stainless steel depends on environment, grade selected, surface finish, the expectations of the customer and the maintenance specified.

Stainless steels provide robust solutions, but in harsh or borderline environments with high expectations for durability, surface finish will have a substantial impact on performance. Surface finishes can be applied mechanically (usually with abrasives) and chemically.

Understanding how chemical and mechanical treatments will affect the characteristics of the surface and will enable the best possible outcome for the client and the structure. Chemical treatment can be used to improve the corrosion performance of the steel, and hence its appearance in service.

 

Stainless steels resist corrosion best if they are clean and smooth. Clean means being free of contaminants on or in the surface that can either react with the steel (like carbon steel or salt) or that create crevices or other initiation points where corrosion can start.

Smooth means having a low surface area at the 'micro' level. Mechanically abrading the surface can roughen the steel's surface and may also embed unwanted particles.

The common feature of chemical treatments is that they all clean the surface of the steel. They may also smooth or roughen the steel surface, or leave it unaffected depending on which process is chosen. But if carried out properly, they all increase the corrosion resistance.

Corrosion resistance improves as you go to the right of this graph. The graph shows the relative importance of the smoothness of the surface and chemical treatment of the surface. They can be used together to get the best corrosion resistance.

Corrosion resistance improves as you go to the right of this graph. The graph shows the relative importance of the smoothness of the surface and chemical treatment of the surface. They can be used together to get the best corrosion resistance. The study reported by G. Coates (Materials Performance - August 1990) looked at the effect of various methods of treating an artificial welding heat tint on grade 316, 2B surface.

Stainless Steel Products
During steel making, sulphur in the steel is controlled to very low levels. But even at these levels sulphide particles are left in the steel, and can become points of corrosion attack. This 'achilles heel' can be improved greatly by chemical surface treatment.

Most bar products will be slightly higher in sulphur when produced, so chemical treatment to remove inclusions in the surface of these products becomes more important.

Generally mill finishes for flat products (sheet, plate and strip) will be smoother as their thickness decreases.

A No 1 finish on a thick plate may have dimples or other imperfections and a surface roughness of 5 to 6 micrometres Ra.

A typical 2B cold rolled finish on 1.7mm thick sheet might have a surface roughness of 0.2 micrometres Ra or better as shown in Mill Forms.

New surfaces will be created during fabrication processes, (eg cutting, bending, welding and polishing). The corrosion performance of the new surfaces will generally be lower than the mill supplied product because the surface is rougher, or sulphide inclusions sitting just under the surface have been exposed or mild steel tooling contamination may have occurred.

Chemical treatments correctly performed can clean the surface and ensure the best possible corrosion performance.

Chemical surface treatments can be grouped into four categories:

  • Pickling - acids that remove impurities (including high temperature scale from welding or heat treatment) and etch the steel surface. 'Pickling' means some of the stainless steel surface is removed.
  • Passivation - oxidising acids or chemicals which remove impurities and enhance the chromium level on the surface.
  • Chelating agents are chemicals that can remove surface contaminants.
  • Electropolishing - electrochemical treatments that remove impurities and have the added beneficial effect of smoothing and brightening the surfaces.

Pickling
Mixtures of hydrofluoric (HF) and nitric acid are the most common and are generally the most effective. Acids are available as a bath, a gel or a paste.

Commercially available mixtures contain up to about 25% nitric acid and 8% hydrofluoric acid. These chemicals etch the stainless steel which can roughen and dull the surface.

Care is required with all these chemicals because of both occupational health and safety and environmental considerations. HF is a Schedule 7 poison which has implications for sale or use in most states. See ASSDA's Technical Bulletin on this subject.

Passivation
Nitric acid is most commonly used for this purpose. Passivation treatments are available as a bath, a gel or a paste. Available formulations contain up to about 50% nitric acid and may also contain other oxidisers such as sodium dichromate. Used correctly, a nitric acid treatment should not affect the appearance of the steel although mirror polished surfaces should be tested first.

Passivation works by dissolving any carbon steel contamination from the surface of the stainless steel, and by dissolving out sulphide inclusions breaking the surface.

Nitric acid may also enrich the proportion of chromium at the surface - some chelants are also claimed to do this.

Pickling and passivation: before treatment of fuel tanks for storing helicopter fuel on ships. Pickling and passivation (L-R): after treatment of fuel tanks for storing helicopter fuel on ships.
Pickling and passivation (L-R): before and after treatment of fuel tanks for storing helicopter fuel on ships. Photos courtesy of Alloy Engineers and MME Surface Finishing.

Chelants
Chelants have chemical 'claws' designed to selectively clean the surface.

The carboxylic acid group COOH is the basis for many chelants which are used in cleaners, water softening and lubricants. The pH and temperature must be correct for the chelant to do its job. Turbulent rinsing of pipes and vessels afterwards is important.

Cleaning by chelating agents tends to be based on proprietary knowledge and systems, and is less standardised than the other methods described.

The successful use of these systems needs to be established on a case by case basis.

Electropolishing
Most commonly phosphoric and sulphuric acids are used in conjunction with a high current density to clean and smooth (by metal removal) the surface of the steel.

The process preferentially attacks peaks and rounds valleys on the surface and raises the proportion of chromium at the surface.

The technique can have substantial effect on the appearance increasing lustre and brightness while only changing the measured roughness by about 30%.

Precautions
For chemical processes that etch the stainless steel, reaction times will increase with increasing grade.

More care is required with 'free machining' grades and these will usually require substantially less aggressive chemicals. The sulphur addition in these steels makes them readily attacked by chemical treatments. Care is also required when treating martensitic or low chromium ferritic stainless steels.

Detailed recommendations for each grade of stainless steel are given below.

The four categories of treatment are detailed in a number of Standards, but the most commonly used are:

  • ASTM A380 Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems.
  • ASTM A967 Chemical Passivation Treatments for Stainless Steel Parts.
  • ASTM B912 Passivation of Stainless Steels using Electropolishing.

These very useful documents give detailed recommendations on many aspects of selection, application and evaluation of these treatments. Highly recommended reading.

Dirt and grease will mask the surface from treatments outlined above. Therefore, the steel surfaces must be free of these agents before applying chemical treatments.

Many of the chemical treatments described contain strong acids. Before disposal they will require neutralisation. Check with your local authority concerning the requirements for trade waste, neutralisation and disposal.

Many of the chemicals described above will be classified as hazardous substances under State OHS legislation, with implications for purchasing, transport, storage and handling.

Chemical treatments are useful tools in cost effectively achieving peak performance with stainless steels. With appropriate training, hazards associated with their use can be managed.

This technical article featured in Australian Stainless magazine - Issue 26, November 2003.

Threaded Fittings to ISO 4144 Standard

For many years there has not been a Standard to cover the low pressure stainless steel cast pipe fittings commonly used in Australia and other countries around the world.

These are commonly termed “150lb” or “BSP” pipe fittings. In most cases the fittings that have been supplied were a mismatch of various Standards.

The fittings were dimensionally in accordance with a number of American Standards, whilst British Standard threads were used. This led to the fittings having threads that in some cases were non-compliant - basically there was insufficient length to accommodate the thread.

ASSDA, through its Technical Committee, identified this problem in the early 90s and through the publication of ASSDA’s Technical Bulletin No 1, highlighted the problems to the Australian market. ASSDA also looked for a mechanism to have these problems rectified.

ISO 4144 "Pipework - Stainless Steel Fittings Threaded in Accordance with ISO 7-1"
After investigating the alternatives it was decided that International Standard ISO 4144 could be the conduit to rectifying the problems. ISO 4144 in its 1979 form covered most of the committee’s concerns, but it did not allow for cast fittings - only wrought stainless steel.

After correspondence with the Australian and International Standard bodies, it was ascertained that ISO 4144 was due for revision, which presented a golden opportunity to have the standard rewritten to cover all of the Technical Committee’s concerns.

ASSDA was invited to represent Australia on the committee established to review the Standard and actively took part in the full process of its revision. Not all of the Committee’s recommendations were accepted. Finally, in early 2003 the new Standard was published.

What has been achieved?
The major improvements that have been adopted in the new Standard are:

a.    The use of castings as well as wrought materials.
b.    All cast fittings are to be properly heat-treated by solution annealing.
c.    The reduction in dimension, a more economical fitting.
d.    The thread standards allowed have been clearly defined.
e.    An introduction of pressure-temperature ratings for application of the fittings.
f.    The inclusion of eight new types of fittings into the Standard.
g.    The inclusion of DN 100 (4”) fittings.

Now that ISO 4144:2003 allows for the use of castings, Australia finally has a Standard that covers the products that have been in common use for many years.

The requirement that all castings are to be fully heat-treated will alleviate some of the corrosion problems that have been encountered in the past.

The dimensions of the fittings have been revised dramatically, thus giving a lighter and more economical fitting.

The wall thickness is the major dimension that has been reduced and it can be reduced by a further 20% if the fitting is made from wrought material.

ISO 7-1 sealing pipe threads are to be used on all fittings. The external and internal threads are to be tapered, but the internal threads may be parallel. The only exception to this is the threads on the Unions and their mating nut, which are allowed to have a variety of parallel threads.

Pressure temperature ratings for application of fittings have been specified (refer to Table 1).

Table 1 - Pressure-Temperature Rating

ome fitting types supplied into Australia were not covered in the old ISO 4144 Standard.

Eight new types have been included in the new Standard: these are 90° Reducing Female Elbows, Reducing Female Tees, 45° Equal Female Elbows, 90° Male x Female Elbows, Crosses, Reducing Nipples, Male x Female Unions and Male x Male Unions.

With the inclusion of the DN 100 (4”) fittings, the Standard now has a comprehensive range of products.

Some Disappointments with the New Standard
In the new Standard, apart from some minor editorial errors, there are two points of concern to the ASSDA Technical Committee.

Firstly, the new wall thicknesses that are stated as minimum could lead to a product being supplied that may not meet the expectations of the customer.

Even though the Standard allows for thin wall product, such thin walled fittings could be subject to distortion during the threading process or during installation. Care must be taken that this does not occur.

The second concern within the Standard is the length of the minimum external thread that has been adopted. The title of ISO 7-1 is “pipe threads where pressure tight joints are made on the threads”.

The minimum length specified can accommodate a thread that seals if it is manufactured to close tolerance. Care is required in manufacture to achieve this outcome.

Although it was recommended to the International Committee that it accept external thread lengths that could accommodate a thread at both ends of the tolerance range, the Committee did not adopt these recommendations.

Table 2 highlights the external thread lengths that were adopted compared to the external thread lengths that were recommended by Australia.

Table 2 - External Thread Length Comparisons

Conclusion
ASSDA believes minimum thread length is a concern to all suppliers and users of these fittings and care should be taken in their selection.

If mating fittings do not seal on the threads and interfere with the washout they may leak.

It is recommended that fittings should only be sourced from reputable and experienced manufacturers and supplied to ISO 4144:2003.

Overall, the Standard is a considerable improvement on what was available, and with care in the selection, the end user will be in a more certain and much safer environment than in the past.

Credits
This article was written by Kim Burton, Group Supply Manager of Prochem Pipeline Products Pty Ltd and an ASSDA Technical Committee member. ASSDA also acknowledges the contribution of Technical Committee member Peter Moore, Technical Services Manager of Atlas Specialty Metals, in the development of this article.

Download Technical Bulletin (April 1997 - pdf 920k)

This technical article featured in Australian Stainless magazine - Issue 27, February 2004.

Crevices and Corrosion

A crevice is a narrow gap between a piece of metal and another piece of metal or tightly adhering material like plastic or a film of bacterial growth.

Many metals and alloys are susceptible to crevice corrosion, but in stainless steel, crevices are the first and most common place for corrosive attack to begin. With a little understanding, crevice corrosion can either be avoided or minimised.

Crevices can be:
• The space under a washer or bolt head.
• The gap between plates bolted together.
• The gap between components intermittently welded.
• The space under a sticky label.
• The space between a gasket and the metal in a flange (especially if the gasket is absorbent).
• Any other tight gap.

Crevices can be designed into the structure, they can be created during fabrication or can occur during service.

Prevention measures should therefore also aim at design, fabrication and service.

Why crevices can corrode
To work at its best, stainless needs free access to oxygen. Crevices are wide enough to permit entry of moisture, but narrow enough to prevent free circulation.

The result is that the oxygen in the moisture is used up. In addition, if chlorides are present they will concentrate in the stagnant conditions and, by a combination of reactions, the moisture can become acidic.

These are all conditions that can lead to the breakdown of the passive film on the stainless. Attack can then progress rapidly.

Crevices can create conditions much more aggressive than on adjacent surfaces. Having crevices builds in weak spots where attack can begin and begin in much less severe conditions than anticipated for the remainder of the structure.

Table one shows laboratory measurements of critical temperatures needed to cause pitting on an open surface (CPT) and crevice (CCT) attack of a metal plate beneath a PTFE washer in a 10% ferric chloride solution.

The CCT is at least 20˚C lower than the temperature to cause pitting corrosion in this aggressive liquid. (Ferric chloride solution is an aggressive corrodent and is used because it is similar to the liquid in a pit when it is actively corroding.)

Factors influencing crevices:

Crevice Shape
The geometry of the crevice will influence its susceptibility to attack and the speed of progress. The narrower and deeper (relative to its width) a crevice is the worse attack will be.

Metal to flexible plastic crevices tend to be narrower than rigid metal to metal gaps so metal to plastic joints provide more aggressive crevices.

Environment
The more aggressive the liquid outside the crevice, the more likely it is that the crevice will be attacked.

This is why crevice attack can be a problem in a salty swimming pool but not in a fresh water tank.

In the atmosphere, crevices beside the sea give more problems than in rural environments. If the liquid outside the crevice is very oxidising, eg with bleach, hydrogen peroxide or ozone, then crevice attack will tend to be more severe.

Temperature
Once the CCT is exceeded, then as with pitting corrosion, higher temperatures mean corrosion is more rapid. The rule of thumb is that a 10˚C rise in temperature will double the corrosion rate.

This means that when comparing Far North Queensland to Tasmania, not only are crevices more likely to start corroding but also that once they do, they will corrode faster because the temperature is consistently higher.

Alloy resistance
Using a more corrosion resistant alloy gives less crevice attack. For example, in seawater at ambient temperature, crevices will form on 304 if there is a 0.9mm gap, on 316 if there is a 0.4mm gap and on 904L (similar corrosion resistance to 2205) if there is a 0.15mm gap.

Minimising the risk of crevice corrosion

Good design, fabrication and operating practices will anticipate and hence minimise crevice corrosion.

Design
Design to minimise the occurrence of crevices.  If a crevice is a necessary part of a component’s design – can it be made wider?

Full penetration butt welds are best for joints.  Seal lap joints and avoid gaps between pipes and fittings.

Minimise use of bolted connections and other fasteners. Where crevices can’t be avoided use a steel grade resistant to crevice corrosion in the operating environment. It is also possible to seal the crevices to keep out corrosive liquids, but care must be taken that the seal is permanent.

Be careful that the sealant “wets” the surface. If it doesn’t it may form its own crevice. Sealants that dry and shrink can form their own crevices.

Gaskets between flanges will probably form a slight crevice, but if the gasket does not absorb the liquid and is compressed between the surfaces (and not bulging around the flange), then the crevice is usually shallow enough so that crevice corrosion is not a problem.

Fabrication
Ensure full root penetration of welded joints with smooth weld bead. Avoid under cut and cracks in welding. Use of sticky labels or markers of various kinds (such as crayons) should be avoided, as should smears of grease or oil.

“Smooth and clean” at all times. ASSDA Accredited Fabricators are assessed on their knowledge of crevice corrosion.

Operation
Sediment and scale can both result in crevices.

If the problem can’t be designed out, routine maintenance will minimise risk. Crevice corrosion under bacteria film can occur. Maintaining circulation reduces the risk that debris will collect and form crevices in dead legs or low flow areas.

Further Reading
The Nickel Institute’s free publication #11021 “High Performance Stainless Steels” contains much of the information used in this article.

This publication and a mathematical model useful for assessing crevice corrosion risk can be downloaded from the Nickel Institute website - www.nickelinstitute.org

If more detailed corrosion mechanism information is required, then “Corrosion of Stainless Steels” by A. John Sedriks is a good intermediate point.


Reinforcing pad, staggered welds - adequate strength.


Reinforced pad, seal weld - best corrosion resistance.


Staggered fillets - severe crevice


Continuous fillets both sides - crevice sealed


Figure 1: Typical crack defects around a weld (WTIA)

Credits
The Australian Stainless Steel Development Association (ASSDA) would like to acknowledge the contribution of the following Technical Committee members for their contribution to the production of this article.
•  Richard Matheson - Executive Director, ASSDA
•  Graham Sussex - Technical Specialist, ASSDA
•  Peter Moore - Technical Services Manager, Atlas Specialty Metals

This article featured in Australian Stainless magazine - Issue 29, January 2004.

Cleaning of exterior stainless steel

The visual performance of outdoor stainless steel depends on five interrelated factors:

• Surface finish - smooth and clean and free of crevices.
• Grade selection - appropriate for environment.
• Good design - rain washing and uniform draining.
• Maintenance program - regular cleaning.
• End user expectations.

This technical article provides suggestions on a maintenance program for cleaning of exterior stainless steel, together with some recommendations for remedial action if stains occur beyond regular maintenance or where such maintenance has not been performed.

Maintenance: routine removal of grime

Stainless steel holds its appearance best if it is washed regularly. When washing use soap or detergent or 1% ammonia solution in warm, low chloride water with cloths or soft brushes to avoid scratching the surface.

Smears will be reduced if the surface is dried afterwards. This treatment applies to bare stainless steel but care should be taken with coloured surfaces.

Coloured and very smooth finished (eg BA or No. 8) surfaces subjected to excessive brushing or rubbing may lose gloss or even become scratched. Bleaches are not recommended.

Simply wiping with a damp cloth is not adequate as it smears corrosive deposits without removing them.

Table 1 from the ASSDA ‘Tea Staining’ Technical Bulletin provides a guide to the recommended frequency for cleaning exterior stainless steel. This Bulletin is available for viewing or download from ASSDA’s website.

Grease, oily films and other organic contamination

Oils and grease may be removed by alkaline formulations or hot water and detergents or, if necessary, by hydrocarbon solvents such as alcohol, acetone or thinners or eucalyptus oil. In all cases the surface should be rinsed with clean water and preferably dried.

For directionally grit polished finishes, wiping along the polish direction with very hot clean water and a soft, absorbent cloth is a good final step to reduce smears.

Heat from a hair dryer or glue gun may soften adhesive remnants from labels or protective films for removal.

After exposure to UV degradation from sunlight, adhesives may require similar treatment to grease stains or even abrasion, with the probability of a bright or scratched spot.

Adherent Scales and Mortar

Adherent scales and mortar may be removed chemically but NOT using chemicals containing chlorides.

NEVER use brick cleaning liquids that contain hydrochloric acid. Hot 25% acetic acid (vinegar) or warm 10% phosphoric acid are effective in removing hard water scales and dried mortar splashes.

Following the acid wash, the surface should be neutralised with dilute ammonia or sodium bicarbonate solution, rinsed and dried.

Remedial Work

The brown surface stains that can occur on stainless steel during atmospheric exposure are simply cosmetic rust stains.

This brown ‘tea staining’ on stainless steels will not progress to potential structural damage as could occur with a carbon steel structure.

The procedures outlined below may enable you to remove the tea staining. However, if the progression of damage is beyond these recommendations it is advisable to employ an experienced contractor.

Cleaning Rust Stained Flat Surfaces

Early action after the onset of tea staining is desirable, before the appearance of the underlying surface is changed.

If the surface is pitted, then it is probable that it will require mechanical repolishing. After mechanically cleaning off tea staining, it is preferable to passivate the surface by using a nitric acid gel or, if the item is portable, by immersion in a nitric acid bath. For marine exposures, passivation is very strongly recommended.

In contrast to other acids, nitric acid is a strong, oxidising acid cleaner and has the added advantage that it is a passivating agent.

The Nickel Institute has suggested that rust may be removed by the use of a 10% phosphoric or oxalic acid followed by a 1% ammonia solution neutralisation and then a water rinse.

Alternatively a mild acid based cleaner such as sulphamic acid (used in some saucepan cleaners) can be used with some care to avoid local changes in appearance. NEVER EVER use hydrochloric or sulphuric acids.

There are also proprietary chemical cleaning treatments often based on citric acid or other chelating compounds. Although these agents passivate in the sense of removing free iron and other foreign matter, they do not augment the surface oxide film.

Use of liquid acids on site is generally unsatisfactory as contact time is short and the acid may run off and damage adjacent components.

Unlike the hydrofluoric acid pickling process used after welding, a nitric acid passivation process does not normally change the surface appearance of stainless steel, although it may cloud a mirror polished surface. Careful trials on inconspicuous areas are recommended prior to full scale cleaning.

Electropolishing is also used by some contractors to smooth rough edges and both clean and passivate the surface. It can be carried out on site or, more usually, in purpose-built tanks.
Afterwards – prevention of recurrence

If tea staining has occurred, one or more of the five factors outlined in the introduction have not been considered carefully enough when the structure was designed and/or built.
To improve the structure, the following steps may be taken to prevent recurrence:
• Increase the frequency of maintenance.
• Improve the surface finish - mechanical polishing and chemical treatment on-site.
• Alter the design of the structure - redesign and replace the affected part of the structure.
• Improve grade selection - replace the structure with a more suitable grade of stainless steel.


ABOVE: A successful stainless steel installation in an outdoor application.

If consideration of the aforementioned steps indicates an uneconomic result, the stainless steel can be painted.

Paint systems using lacquers and polyurethane top coats are available and have been used successfully, but care and understanding is required.

Painting the stainless steel is a step that should only be used as a last option as it is irreversible.

This technical article was published in Australian Stainless Issue 28, May 2004. It is an extract of a Technical FAQ on ‘Exterior Cleaning of Stainless Steel’.

For technical support and advice contact ASSDA on 07 3220 0722 or email This email address is being protected from spambots. You need JavaScript enabled to view it.

Grade 2205 for High Corrosion Resistance and High Strength

Combining many of the beneficial properties of both ferritic and austenitic steels, 2205 is the most widely used duplex stainless steel grade. Its high chromium and molybdenum content gives the stainless steel excellent corrosion resistance. The microstructure provides resistance to stress corrosion cracking and ensures high strength.

The grade is generally not suitable for use at temperatures above 300oC or below -50oC because of reduced toughness outside this range.

You are most likely to encounter 2205 stainless steel being used in industrial environments such as petrochemical, chemical, oil, gas and paper plants.

Alternative Grades
2205 has been available for several years - in general this complies with UNS grade designation S31803. More recently, product has become available complying with the higher corrosion resistant composition UNS S32205, as in table 1. Both these alternatives are known as 2205.

Composition
Grade 2205 has a micro structure of roughly equal amounts of ferrite and austenite, hence the 'duplex' description. The duplex structure of 2205 has the following properties:

  • High strength.
  • Lower thermal expansion coeffecient than austenitic steels but greater than carbon steels.
  • High resistance to corrosion, particularly stress corrosion cracking, corrosion fatigue and erosion.

The high content of chromium and molybdenum and the addition of nitrogen gives the steel further beneficial characteristics:

  • High general corrosion resistance.
  • High pitting and crevice corrosion resistance.
  • Good sulphide stress corrosion cracking resistance.

The addition of nitrogen gives a further increase in pitting and crevice corrosion resistance.

Table 1: Composition of 2205 and Alternative Grades (Single Values are Maximum)

Grade Common Name C% Mn% Si% P% S% Cr% Ni% Mo% N%
S31803 2205 0.030 2.00 1.00 0.030 0.020 21.0-23.0 4.5-6.5 2.5-3.5 0.08-0.20
S32205 2205 0.030 2.00 1.00 0.030 0.020 22.0-23.0 4.5-6.5 3.0-3.5 0.14-0.20

 

Corrosion Resistance
The grade has excellent corrosion resistance and is superior to grade 316, performing well in most environments where standard austenitic grades may fail. 2205's low carbon content gives the grade a high resistance to intergranular corrosion and has better resistance to uniform, pitting and crevice corrosion due to its high chromium and molybdenum content.

As 2205 is a duplex stainless steel, the grade is also less sensitive to stress corrosion cracking in warm chloride environments, unlike austenitic stainless steels. The grade also has good resistance to stress corrosion cracking when exposed to hydrogen sulphide in chloride solutions.

High mechanical strength combined with excellent corrosion resistance gives 2205 high corrosion fatigue resistance.

Heat Resistance
Although 2205 has good high temperature oxidation resistance, this grade, like other duplex stainless steels, suffers from embrittlement if held for even short times at temperatures above 300oC. If embrittled this can only be rectified by a full solution annealing treatment. 2205 is annealed at 1020-1100oC followed by rapid cooling. This treatment applies for both solution annealing and stress relieving.

Mechanical Properties
Mechanical properties for grade 2205 stainless steels are given in table 2.

Table 2: Mechanical Properties of 2205 (Annealed Condition)

 

Table 3: Physical Properties of Grade 2205 (Typical Values in Annealed Condition)

Tensile strength 620MPa min   Density 7,805kg/m3
Yield strength 450MPa mi   Elastic modulus 200GPa
Elongation 25% min  

Mean coefficient of thermal expansion

Brinell hardness 293 HB max   0-100oC 13.7µm/m/oC
Rockwell hardness 31 HR C max   0-315oC  

 

Physical Properties
Typical physical properties for grade 2205 stainless steels are given in table 3. There are surprisingly large variations in the values from different manufacturers for notionally identical materials.

2205 has a microstructure containing approximately 50% ferrite in the annealed condition, quenched from about 10500C. Higher annealing temperatures often result in an increase of ferrite content.

Fabricability
Due to the high yield strength of 2205, greater forces are required for the cold forming of this duplex steel, and will require larger capacity equipment than would be required for austenitic steels.

Processes such as stretch forming, deep-drawing and spinning are more difficult to perform.

Welding of 2205 is good by all standard methods, however, with the following restrictions:

  • Do not pre-heat or post-heat the material, heat input must be kept low.
  • Allow the material to cool between passes, preferably to below 150oC.
  • Use correct filler grade 2209. Autogenous welding should be avoided.

Forms Available
Grade 2205 is available in hot rolled plate and strip, cold rolled sheet, plate and coil, forgings/bar, tube and pipe and in threaded fittings and flanges.

Applications
Grade 2205 is typically used in the construction of heat exchangers, pressure vessels, tanks, tubes and pipes for the following industry areas:

  • Chemical processing, transport and storage.
  • Oil and gas exploration and processing equipment.
  • Marine and other high chloride environments.
  • Pulp and paper digesters, liquor tanks and paper machines.

Specifications
For many products grade 2205 is covered by the same specifications that include the common austenitics - ASTM A240M for flat rolled and ASTM A276 for bar. Duplex grades of tube have their own specification - ASTM A789M and A790M covers duplex grades of pipe.

Credits
ASSDA would like to thank Peter Moore of Atlas Specialty Metals and Graham Sussex of ASSDA in the development of this article.

Main image: 2205 hot water tank for beef abbatoir. Photo courtesy of ASSDA Accredited Fabricator, G&B Stainless Pty Ltd.

This article featured in Australian Stainless magazine - Issue 30, January 2005.

Low nickel austenitic stainless steels

The most common grades of stainless steel are 304 and 316, which are particularly popular because their austenitic microstructure results in an excellent combination of corrosion resistance, mechanical and physical properties and ease of fabrication.

The austenitic structure is the result of the addition of approximately 8-10% nickel. Nickel is not alone in being an austenite former; other elements that are used in this way are manganese, nitrogen, carbon and copper.

The Cost of Nickel and Its Addition to Stainless Steel

The cost of the common stainless steels is substantially determined by the cost of ingredients. The cost of the chromium that is the essential "stainless ingredient" is not high, but additions of elements that improve the corrosion resistance (especially molybdenum) or that modify the fabrication properties (especially nickel) add very much to the cost.

Costs for nickel have fluctuated from US$5,000 or US$6,000 in 2001 to US$15,000 per tonne in 2004.

Similarly, molybdenum has dramatically increased from approximately US$8,000 per tonne in 2001 to around US$50,000 per tonne in 2004.

These costs impact directly on the two most common grades: 304 (18%Cr, 8%Ni) and 316 (17%Cr, 10%Ni, 2%Mo). The impact is most keenly felt in grade 316, which has suffered an increase to its cost premium above 304.

Other grades such as the duplex 2205 (22%Cr, 5%Ni, 3%Mo) and all more highly alloyed stainless steels are also affected.

Relative costs of the ingredients are shown in Figure 1, but these do vary widely and sometimes rapidly over time. These costs were correct in late 2004.

Relative Costs for Alloy Ingredients (Late 2004) Alloying Additions - Manganese Replacing Nickel

The point of the alloying elements is that they achieve certain changes to the corrosion resistance or to the microstructure (which in turn influences the mechanical and fabrication properties).

Chromium is used to achieve corrosion resistance, and molybdenum adds to this.

A common evaluation of corrosion resistance of stainless steel grades is the Pitting Resistance Equivalent (PRE), where this is usually evaluated as PRE = %Cr + 3.3 x %Mo + 16 x %N. A neat equation, but unfortunately only a guide.

The PRE gives a guide to ranking of grades, but is not a predictor of resistance to any particular corrosive environment. What is apparent is that pitting corrosion resistance can be increased by molybdenum, but also by chromium or by nitrogen additions. These are much cheaper than molybdenum. Despite its high PRE factor, nitrogen has limited effect on corrosion resistance because of low solubility, ie. <0.2%.

The microstructure of the steel is largely determined by the balance between austenite former elements and ferrite former elements.

On the austenite former side carbon, manganese, nitrogen and copper are all possible alternatives to nickel. All these elements are lower cost than nickel.

As is the case for the PRE, the Ni-equivalence formulas are a guide but do not tell the full story; each element acts in slightly different ways, and it is not possible to fully remove nickel and replace it with, for example, copper or nitrogen.

Manganese acts as an austenite former but is not as effective as nickel, and Cr-Mn steels have higher work hardening rates than do apparently equivalent Cr-Ni steels.

Carbon is a very powerful austenite former, but has only limited solubility in austenite, so is of limited value in a steel intended to be fully austenitic.

Although not recognised by the PRE formula, nickel has positive effects on resistance to some corrosive environments that manganese does not duplicate.

There can also be synergy between the elements. Addition of nitrogen has the double effect of forming austenite and of increasing the pitting corrosion resistance. And manganese is a strong austenite former in its own right, but also has the effect of increasing the solubility of nitrogen.

The Rise of the "200 Series" Steels

Manganese is therefore a viable alternative to nickel, ranging from a minor addition to an almost complete replacement.

The development of high manganese austenitic grades first occurred about fifty years ago, during one of the (several) previous periods of high nickel cost.

At that time some Cr-Mn-Ni grades were sufficiently developed to be allocated AISI grade numbers Ü 201 (17%Cr, 4%Ni, 6%Mn) and 202 (18%Cr, 4%Ni, 8%Mn) are high Mn alternatives to the straight chromium-nickel grades 301 and 302, and are still included in ASTM specifications as standard grades.

Their consumption over the following decades has been low relative to their Cr-Ni equivalents. The reasons for the poor take-up of these lower cost grades have been:

  • Very high work hardening rate (this can be an advantage in some applications).
  • Slightly inferior surface appearance Ü considered unacceptable for certain applications.
  • Additional production costs Ü higher refractory wear in melting in particular.
  • Corrosion resistance is lower in some environments, compared to Cr-Ni grades.

An additional issue is that Cr-Ni and Cr-Mn-Ni austenitic scrap is all non-magnetic, irrespective of nickel content, but scrap merchants evaluate scrap on the basis of assumed nickel content.

This has the potential to destabilise scrap markets. The upshot is that the cost reduction (due to lower ingredient cost) has been generally insufficient to move applications from the traditional Cr-Ni grades.

Take-up has often been because of technical advantages of 200-series in niche applications, not cost-driven.

Some New Contenders

The last decade has seen the rise of some new contenders in the Cr-Mn-Ni austenitic group. The main development work has been in India and the principal application has been kitchen ware Ü cooking utensils in particular.

The very high work hardening rate of the low nickel / high manganese grades has been acceptable to a point in this application, but additions of copper have also been made to reduce this problem.

India has been a fertile development and production venue for these grades because of local economic factors.

Other Asian countries have also become strong markets and more recently also producers. The Chinese market is particularly strong, and there is substantial demand in China for the Cr-Mn-Ni grades, often referred to generically as ?200-seriesî stainless steels.

Other centres of production are Taiwan, Brazil and Japan. Alloy development has resulted in a range of austenitic grades with nickel contents ranging from 1% to 4% and up to over 9% manganese. None of these grades are included in ASTM or other internationally recognised standards as yet.

The growth rate in production of these low-nickel austenitic grades has been very rapid. The most recent data published by the International Stainless Steel Forum (ISSF) shows that in 2003 as much as 1.5 million tonnes (7.5% of the worlds stainless steel) was of this type. In China the proportion has been estimated to be 25% in 2004.

Problems still exist however, and large-scale conversion of Cr-Ni applications to Cr-Mn-Ni-(Cu) grades is not likely.

The principal issue is lack of control ‹ unscrupulous suppliers misrepresenting low-nickel product as grade 304 with some resultant service corrosion failures, and degradation of scrap due to contamination by low-nickel material.

As at the start of 2005 the future is unclear. Although it seems logical that there should be a place for the low nickel austenitic grades, the practical issues may mean that grade selectors will instead choose to either continue to use the higher cost Cr-Ni grades, or to seek lower cost alternatives amongst the ferritic or duplex grades.

Credits

ASSDA would like to thank Mr Peter Moore, Technical Services Manager of Atlas Specialty Metals, for the contribution of this article.

This technical article was featured in Australian Stainless magazine # 32 - Winter 2005 and is an extract from the Grade Selection section of the Australian Stainless 2005 Reference Manual.

To order a copy of this essential technical resource for the stainless steel industry, download an order form from the ASSDA website - www.assda.asn.au or phone the ASSDA office on 07 3220 0722.

Recycling of Stainless Steel Scrap

Today, environmental factors are at the forefront of material selection for specifiers. Stainless steel’s long service life, 100 percent recyclability and its valuable raw materials make it an excellent environmental performer.

 

 Stainless steel objects rarely become waste at the end of their useful life. Recycled stainless objects are systematically separated and recovered to go back into the production process through recycling.

As well as iron, stainless steel contains valuable raw materials like chromium and nickel which makes recycling stainless steel economically viable.

Stainless steel is actively recycled on a large scale around the world by recyclers who collect and process scrap (recycled stainless steel) for re-melting all around the world.

Scrap collection
The use of stainless steel scrap is fundamental to the steelmaking process. There are two types of scrap - reclaimed scrap (old scrap) and industrial scrap (new scrap).

Reclaimed scrap includes industrial equipment, tanks, washing machines and refrigerators that have reached the end of their service life.

Industrial scrap includes industrial returns or production offcuts from manufacturing by industrial engineering and fabrication sources.

Today, stainless steel is made up of approximately 60% recycled content including:

  • 25% reclaimed scrap
  • 35% industrial scrap
  • 40% new raw materials

The useful service life of stainless steel products is long so the availability of scrap is dependent on levels of production from decades ago.

With an average content of 25% of old scrap, stainless steel is close to the theoretical maximum content of material from end-of-life products.

Recycling the scrap
Specialised expertise and sophisticated technology is needed in recycling to separate and prepare each type of alloy for remelting.

A recycling processor feeds the scrap into a large shredder to break it into smaller pieces.

It is then chemically analysed and stored by type.

This process may include ‘blending’ the scrap into chrome steels, nickel alloys and other types of stainless steels.

After blending into piles for specific customer requirements the scrap is then loaded into containers for export to overseas mills.

The global market for scrap
Virtually, all Australian stainless steel scrap goes overseas. There’s a small market for stainless steel scrap in Australia for use in the foundries business. Foundries often use profile offcut or plate material scrap products.

At least 30,000 tonnes of stainless steel scrap in Australia will be exported a year to stainless steel mills in countries including China, South Korea, Taiwan, India and Japan.

China, for example, is using approximately 800,000 tonnes of industrial scrap. Reclaimed scrap is also on the increase in China and is expected to reach 2.5 million tonnes in 2005.

For mills, scrap is important because recycled stainless steel contains valuable raw elements including chromium, nickel and molybdenum that are gathered, processed and reused in the production process. The more scrap used in furnaces by mills, the less raw materials are required in the production process.

Stainless steel mills

Scrap along with other raw materials, ferrochromium (chrome/iron), ferro moly (molybdenum/iron) and nickel are blended into an electric furnace.

After melting, impurities are removed, the molten metal is refined and the chemistry analysed to determine what final adjustments are necessary for the specific type of stainless steel being produced.

The molten stainless steel is then cast into slabs or billets before production of plate, sheet, coil, wire and other forms in preparation for use by industrial manufacturers.

The stainless returns to you
Industrial manufacturers produce stainless steel items that you use everyday including cutlery, pots and pans, kitchen sinks and many architectural, industrial and other components.

At each stage of the production and use process, stainless steel retains its basic properties and utility value. Unlike many industrial and engineering materials, stainless steel may be returned to its original quality in the supply chain without any degradation.

You can be assured that even after its long service life, your environmentally-efficient stainless steel will always return to you bright, shining and new!

For more information about stainless steel, contact the Australian Stainless Steel Development Association on 07 3220 0722 or visit www.assda.asn.au

ASSDA acknowledges the assistance and contribution of Ignatius Brun of ELG Recycling Processors, the International Stainless Steel Forum (ISSF), the Nickel Institute and Peter Moore of Atlas Specialty Metals in the production of this article.

Consumption of stainless steel scrap - 2004

  • China — 2.8 million tonnes of production using 900,000 tonnes of scrap.
  • Japan — 2.4 million tonnes of production capacity using 900,000 tonnes of scrap.
  • South Korea — 2.3 million tonnes of production using  800,000 tonnes of scrap.
    Product mix 80% austenitic, 20% ferritic.
  • Taiwan — 2.6 million tonnes of production using 600,000 tonnes of scrap.
  • India  — 1.4 million tonnes of production using 300,000 tonnes of scrap.

Note: Australia sends a proportion of stainless steel scrap to all of the above countries.

This article featured in Australian Stainless magazine - Issue 33, Spring 2005.