If the sum of the alloying elements >5%, cast steel becomes high-alloyed cast steel.
From 12% Cr, steel becomes stainless in atmospheric conditions and in contact with certain acids. Cr creats a passive layer which protects the underlying material. From 12% there is enough Cr to repair damages in the passive layer through diffusion.
- Ferritic stainless steel: Ferritic SS has the composition 13-30% Cr. E.g. GX40CrSi24
- Austenitic stainless steel: Austenitic SS has the composition 18-19% CR and 8-11% Ni. E.g. GX2CrNi19-10, GX5CrNiNb19-11
- Martensitic stainless steel: Martensitic SS has the composition 11-16% Cr and less than 5% Ni. E.g. GX20Cr14, GX4CrNi13-4
- Duplex stainless steel: Duplex SS is achieved at a 50/50 ratio between ferrite and austenite. Duplex combines the strength of ferrite and the better corrosion resistance and toughness of austenite. The composition of duplex is very important. The ballans between austenitic stabilizers (Ni, N, Cu, Mn) and ferrite stabilizers (Cr, Mo, Si, V, Nb) must be correct. Heat treatments are very important to become the right ratio. Duplex steels are heat treated to 1100-1200 °C and quenched in water.
Manganese steel, also called hadfieldsteel, has the chemical composition C= 0,6 - 1,6%, Mn = 6 - 20% and Cr tot 6%. It is a strong, tough material and has as main feature cold reinforcement: the hardness increases with deformation. For example, the hardness of 200 HB rises to +400 HB upon deformation. Mn steels are used in components subject to wear by impact, abrasion by metal-metal contact and are used in amagnetic applications. Mn steels are quenched in water to freeze the austenitic structure.
Heat resistant steel
This steel is used at temperatures higher than 650°C. The strength has a subordinate role. Heat resistant steel is fully austenitic and will therefore receive no heat treatment. The major alloying elements are C, Cr, Ni and Si. Examples are GX40CrNiSi25-12, Gx40CrNiSi25-20 and GX40NiCrSi38-19.