Materials & Processes
l Ferritic. This type of stainless steel has from 10.5 to 27 per cent chromium by weight and little or no nickel. Other alloying elements can include molybdenum, lead, aluminium and titanium. While ferritic grades generally have better engineering properties than austenitic grades - such as good ductility - the corrosion resistance and weldability is less good. Applications for ferritic grades include cooking utensils, electrical enclosures, domestic appliances, automotive exhausts and highly polished automotive trim. Although stainless steels are generally considered to be non-magnetic, ferritic grades are an exception to this rule. Note that both ferritic and martensitic grades can be found within the 400 series.
l Martensitic. With lower corrosion resistance than austenitic or ferritic stainless steels, martensitic grades nevertheless benefit by being strong, tough, readily machinable and can be hardened by heat treatment. Chromium content is typically 12 to 14 per cent, with other alloying elements being molybdenum, nickel and carbon. Cutlery is made of martensitic stainless steel due to its hardness and the ability to produce a polished surface and an edge that stays sharp. Martensitic stainless steel is popular for blades and similar components used in food processing, as well as fasteners, shafts, valves and tools.
l Precipitation-hardening martensitic. Compared with conventional martensitic grades, precipitation-hardening martensitic stainless steels can achieve higher strengths, with corrosion resistance that is similar to that of austenitic grades. Typically precipitation-hardening martensitic grades (which are designated the 600 series) have 17 per cent chromium and 4 per cent nickel content. Applications for these grades include equipment used in the paper industry, turbine blades and aerospace components.
l Duplex. As the name suggests, duplex stainless steels have a mixed microstructure containing both austenite and ferrite in roughly equal proportions. Strength is approximately twice as high as austenitic stainless steels, and duplex grades also benefit from improved resistance to pitting, crevice corrosion and stress-corrosion cracking. Duplex grades have 19 to 28 per cent chromium, up to 5 per cent molybdenum, moderate amounts of nickel and small amounts of other alloying elements (including molybdenum, nitrogen, manganese, copper and tungsten, depending on the grade). Compared with super- austenitic grades, similar material properties can be achieved but with a lower overall alloy content, which makes duplex grades more cost-effective where designers have a choice. The most commonly used duplex type is 2205. Applications for duplex stainless steels can be found in the marine, chemical, petrochemical, and pulp and paper industries.
l New developments. Given the diversity of applications for which stainless steels are suitable, it is not surprising that producers are continually developing new grades to meet the needs of particularly demanding applications. For example, in November 2010, ArcelorMittal launched a new grade, designated K44X, for a specific automotive application (note that the stainless steel business of ArcelorMittal has subsequently been spun off as Aperam). Grade K44X is
designed for high-temperature sections of automotive exhaust systems, typically from the manifold to the catalytic converter, where 1000-1050° C can be encountered. This resistance to higher temperatures has become necessary because Euro 6 emissions targets for automotive manufactures are resulting in smaller engines being used, but these are having to work harder.
K44X (AISI 444, EN 1.4521) has a chromium content of 19percent (by weight), with 2percent molybdenum and 0.6percent niobium. It is resistant to high-temperature oxidation, creep up to 1050° C, and offers good durability and thermal fatigue strength. It also benefits from good weldability and formability that is similar to ferritic grades. For applications requiring a combination of formability,
high strength, toughness, hardenability and corrosion resistance, Sandvik has developed Sandvik Nanoflex, which is a precipitation-hardenable austenitic stainless steel that utilises nanotechnology. As delivered, Nanoflex is easily formed, then the
mechanical strength can be increased significantly by heat treatment of the final product at relatively low temperatures and without causing distortion. Normally the material is delivered in the cold rolled
condition, but the material can alternatively be supplied in the annealed condition and then heat-treated after forming to increase the strength. Another leading producer of stainless steels is
ThyssenKrupp Nirosta. This company has recently been involved in a project to build stainless steel metro trains for use in Hamburg. The DT5 multiple unit, developed by the consortium Alstom/Bombardier Transportation, is described as being low on pollution, resource-friendly and quiet, while at the same time offering high safety standards and ride comfort. The body of the DT5 is made completely from Nirosta 4318 stainless steel and is based on a new lightweight design that delivers ecological benefits. Wolfgang Gebel, from ThyssenKrupp Nirosta technical
customer support, states: “With its high yield strength and good corrosion resistance, 1.4318 is ideal for lightweight construction. Even thin panels display high strength and rigidity.” Andreas Knitter, Alstom’s chief executive, adds: “We
decided to do without external painting altogether. Stainless steel surfaces are easier to clean, make expensive painting superfluous, and the high stainless steel content translates into another benefit for the environment: the new vehicles are made of 95 per cent recyclable material.”
Versatile material
From rail vehicles to cutlery, and from surgical implants to turbine blades, stainless steel (see panel page 30) is an exceptionally versatile material. In some respects that is one of its problems, in that there are so many different grades that it is misleading to think of it as ‘one material type’. As with any material, designers investigating the use of
stainless steel should look at all aspects of the design if they are to make the most of this material; for example, careful consideration of forming and joining methods is required if the optimum design is to be produced. Nevertheless, it seems certain that we will see more use of stainless steel in the future, not less. l
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