Materials • Processes • Finishes
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 19 per cent (by weight), with 2 per cent molybdenum and 0.6 per cent 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 (Fig. 3). 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
Fig. 3. Sandvik Nanoflex, a precipitation-hardenable austenitic stainless steel, is used in this shaver’s stationary cutter head, the rotating blade and the hair lifter.
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,
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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