1. The Inox families

1. Martensitic steels

They are used when mechanical strength characteristics are important. The most common ones contain 13% chromium with at least 0.08% carbon. Other shades are more loaded with additives, possibly with a low percentage of nickel.

2. Ferritic steels

They don't take the heat. In this category, refractory steels with a high chromium content (up to 27%) are found, which are particularly interesting in the presence of sulfur. Ferritic steels are sometimes used as a corrosion resistance barrier (plated sheets, coated sheets, protected (called “cladded”, “cladding”))) of the walls of steel pressure equipment used in the petrochemical and chemical industries. These steels are often used instead of austenitic steels for the production of cheap and low-quality kitchen utensils (dishes and knives for example).

3. Austenitic steels

They are by far the most numerous, because of their very high chemical resistance, their ductility comparable to that of copper, and their high mechanical characteristics. The contents of addition elements are approximately 18% chromium and 10% nickel. The carbon content is very low and their stability can be improved by elements such as titanium or niobium. Because of their excellent ductility, these steels also have a field of use at low temperatures (up to minus 200°C) and compete with light alloys and 9% nickel steel for the production of equipment intended for cryogenics.

4. Steels improperly referred to as “austeno-ferritic”

They have remarkable properties of resistance to intergranular corrosion as well as to corrosion in seawater and present, during the tensile test, an elasto-plastic bearing. They have a mechanical behavior similar to structural steels. The liquid/solid transformation results in solidification in the ferritic phase (delta ferrite) then a second transformation, in the solid state, into austenite. Consequently, they should therefore be referred to as ferrito-austenitic steels. The simple fact of correctly designating these steels makes it possible to immediately understand that slow cooling, during welding, will allow a maximum of the ferritic phase to be transformed into an austenitic phase and conversely, rapid cooling will result in ferrite freezing leaving few possibilities for austenitic transformation.

Knowledge of the types of steel is essential for systems made up of elements assembled mechanically or by welding; bringing together two stainless steels that are too different in an electrolyte can in fact cause very destructive electrochemical corrosion phenomena.