There are four types of stainless steel families:
Used when mechanical strength characteristics are important. The most common titrate 13% chromium with at least 0.08% carbon. Other grades have higher alloying elements, with possibly a small percentage of nickel.
Examples: X20Cr13, X46Cr13, X29CrS13.
Not for tempering. There are also refractory steels high in chromium (up 27%) in this category, particularly useful in the presence of sulfur. Ferritic steels are sometimes used as a corrosion resistance barrier (clad sheet, coated sheet, protected (called "cladding")) for steel pressure panels of equipment used in petrochemical and chemical industries. These steels are often used in place of austenitic steels for the manufacture of cheap and poor quality kitchenware (dishes and knives for example).
Examples: X6Cr17, X6CrMo17-1, X3CrTi17.
These are by far the more numerous, due to their very high chemical resistance, ductility comparable to that of copper, and their good mechanical characteristics. The contents of the alloying elements are approximately 18% chromium and 10% nickel. The carbon content is very low and their stability can be improved by titanium or niobium. Thanks to their excellent ductility, these steels can also be used at low temperatures (up to 200 ° C) and compete with light alloys and steels with 9% nickel for cryogenics equipment manufacturing.
Examples: X2CrNi18-9, X2CrNiMo17-12-2.
Steels wrongly called "austenitic-ferritic"
They possess a remarkable resistance to intergranular and sea water corrosion and during the tensile test present an elastoplastic bearing. They have a mechanical behavior similar to structural steels. Liquid / solid transformation results in solidification in the ferritic phase (delta ferrite) and a second transformation, the solid state, in austenite. They should therefore be referred to as ferritic-austenitic steels. The simple act of properly designating these steels allows for the inmediate understanding that slow cooling during welding enables for a maximum of ferrite to transform into austenite and vice versa; rapid cooling will result in a ferrite gel leaving little scope for austenitic transformation.
Knowledge of these types of steel is essential for systems consisting of mechanical or welding assembled elements, bringing together of two very different stainless steels in an electrolyte can indeed cause very destructive electrochemical corrosion.