Stainless steel is the name for a range of steels used primarily because of its corrosion resistance. It is made up of chromium, iron and (in some cases) nickel, molybdenum and other metals. For a metal alloy (a metal made by combining two or more metallic elements) to be considered stainless steel, it should consist of at least 10.5% of Chromium; this composition prevents the iron in stainless steel from rusting and provides heat-resistant properties.
As this is “all it takes” to be deemed stainless steel, there is a huge number of metal alloys that fall into this category. These have been split into five groups (or families) of stainless steel:
Of the five families, austenitic stainless steel is the largest. This family comprises of two sub-groups, the 200 and 300 series – with the 300 series alone accounting for 49.7% of the stainless steel market in 2019.
Austenitic stainless steels are used within an array of products primarily due to their corrosion resistance, formability, strength and temperature resistance (with the austenitic microstructure being maintained at all temperatures). Industries include pharmaceutical, petrochemical and food, among many others, which depend heavily on these thanks to the lack of product contamination, cleanability and long life. Although more expensive initially than other options, the long life of austenitic stainless steels minimises cost over the entire life cycle of their use.
Ferritic stainless steels are the next most widely used after austenitic and contain low levels of carbon, large amounts of chromium and a low amount or in some cases no amount of nickel. Ferritic stainless steels are classified in the 400 series, which depending on their composition and application are then split into further sub-groups. Ferritic steels possess a body-centred cubic grain structure, which differs from austenitic and other types and in turn, causes ferritic stainless steel to become magnetic.
Duplex stainless steels take the name “duplex” because they have a two-phase microstructure which consists of ferritic and austenitic stainless steel. They are around twice as strong as standard austenitic or ferritic steels, their corrosion resistance is in a similar range as austenitic stainless steels and duplex stainless steels have significantly better ductility and toughness than ferritic grades.
Martensitic stainless steels are known for their hardness, wear resistance, high strength and corrosion resistance. These stainless steels are also ferromagnetic and can be hardened by heat treatment and due to their relatively lower chromium content, martensitic stainless steels are not as corrosion resistant as austenitic or ferritic stainless steels.
Precipitation hardening steels are corrosion-resistant alloys which in some cases can be heat treated to provide tensile strengths up to three or four times that of austenitic stainless steels, such as 304 grade stainless steel. Its strength and hardness are developed via a variety of heat treatments, which results in this extremely high strength-to-weight ratio.
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