Materials • Processes • Finishes
Stainless steels are often classified in terms of their crystal structure, as outlined below.
l Austenitic. Known as the SAE 300 series (chromium- nickel) and 200 series (chromium-manganese) stainless steels, austenitic grades are the most commonly used. They contain a maximum of 0.15 per cent carbon and a minimum of 16 per cent chromium, plus small amounts of nickel and/ or manganese to ensure that the austenitic structure is maintained from cryogenic temperatures up to the melting point. The addition of 6 per cent or more molybdenum, plus nitrogen, results in superaustenitic stainless steels (in the 600 series), which exhibit improved resistance to pitting and crevice corrosion, and a higher nickel content gives better resistance to stress-corrosion cracking. Note that austenitic stainless steels have good formability
- and cold working can increase the strength of some grades - as well as toughness and weldability. Typical applications for austenitic stainless steels include cooking utensils, containers and pipework in the food industry, and specialised grades - such as 316LVM - are suitable for surgical implants.
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.
Fig. 2. ThyssenKrupp Nirosta Praezisionsband produces precision stainless steel strip for specialist applications.
Stainless steel statistics S
tatistics released by the International Stainless Steel
Forum (ISSF) in January 2011 indicate that total global production for the first nine months of 2010 was 23 million metric tons, which represents an increase of 29 per cent over the same period in 2009 - though this has to be seen in the context of a 5.3 per cent fall in production in 2009. Total production in 2009
was 24.5 million metric tonnes, 38
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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,
compared with 25.9 million metric tonnes in 2008. Looking at production in terms
of different grades, it can be seen that the increased market shares of chrome/manganese (200 series) and chrome (400 series) stainless steels has been at the expense of chrome/ nickel (300 series) stainless steels. The increase in demand for
ferritic stainless steels is mainly due to the excellent performance of the global car industry, while the
increased market share for chrome- manganese steels can be accounted for by the very high growth achieved by privately owned stainless steel producers in China. In the first nine months of 2010,
chrome/nickel (300 series) stainless steels accounted for 57.1 per cent of production, chrome (400 series) stainless steels accounted for 31.1 per cent and chrome/ manganese (200 series) accounted for 11.8 per cent. l
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