Stainless Steel

In 2009, 5.9 million tonnes of ferrochromium were produced with more than 90% going to the stainless steel industry.

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It is chromium that confers upon stainless steel its remarkable corrosion resistance. Other elements such as nickel and, in the top grades of stainless steel, titanium and molybdenum can also be added to improve technical performance in specific environments.

The term 'stainless steel' covers a family of steels that contain chromium. To be 'stainless', the steel must contain at least 11-12%Cr but some steels contain up to 30%Cr. For example, one of the best known stainless steels is the Series 304 steel, often referred to as 18/8 stainless where 18 is the percentage of chromium and 8 is the nickel percentage. There are standard compositions for the different grades of stainless steel and designation numbers which are now universally used by the producers and consumers. These are:

 

Series 300 steels: austenitic nickel-containing grades
Series 400 steels: chromium-containing ferritic and martensitic grades with no or very little nickel
Series 200 steels: where manganese replaces some of the nickel in the 300 Series
Series 600 steels: precipitation hardening grades

 

The Series 300 steels have excellent corrosion resistance, are easily fabricated, can handle very low temperatures (cryogenic) and also high temperatures (over 900º C). In some cases, extra low carbon grades are produced for added corrosion resistance and are designated with an L; e.g. Series 304L. The addition of small amounts of Mo,Ti and Nb also improves corrosion resistance, particularly in welded areas. The Series 300 steels are easily cleanable and thus are used in environments where hygiene is important. They have very wide applications such as in architecture, food processing, hospitals, and in the chemical, petrochemical, mineral processing and paper industries.

The Series 400 steels can be martensitic, such as Series 410, 420 and 431, with moderate corrosion resistance and are used, for example, for cutlery, knife blades and surgical instruments; or they can be ferritic, such as Series 409 and 430, with good corrosion resistance and are used in domestic equipment, architectural trims, chain conveyors, and dust and fume extractors.

Antartis_Fotosearch_k4369956Stainless steel is made by melting various amounts of carbon steel and stainless steel scrap in an electric arc furnace. To this melt, controlled additions of high-carbon ferrochromium or charge chrome are made to achieve the correct chromium levels. The proportions of ferrochromium and stainless scrap used depend partly upon scrap availability and partly upon scrap prices which themselves fluctuate with availability and with the nickel contained in the scrap.

Secondary refining is carried out in AOD or VOD vessels in order to lower the carbon content of the melt without excessive oxidation losses of the chromium.

Finally, additions of nickel, and sometimes minor amounts of elements such as Mo, Ti, and Nb, are made and also trimming additions of chromium as low-carbon ferro-chromium to achieve the required grade of stainless steel.

The liquid stainless steel is then cast into ingots, billets or slabs for hot- and cold-working to other products such as plate and coil.

Apart from the stainless steels, ferrochromium alloys are used, but in much smaller quantities, in other ferrous materials such as carbon steels, full alloy steels, bearing and high speed steels, high-strength low-alloy steels and tool steels, and in some cast irons, superalloys and welding materials. Alloy cast irons use chromium to achieve hardness and resistance to corrosion, abrasion and impact, and are used for pumps, valves, pipes, rolls and wear plates.

Contact details

ICDA Paris Office:
43 rue de la Chaussée d'Antin - 75009 Paris, France - Tel.: 33 (0)1 40 76 06 89 - Fax: 33 (0)1 40 76 06 87

ICDA Beijing Office:
Unit 3, Building No.2, Shijicheng Wu Qu, Yuanda Road, Haidian District - Beijing 100097, China - Tel: 86-1391 0720 301