INOX – Steel processing guide
2- WHICH TYPES OF STAINLESS STEELS EXIST?
Staiinless steels can be divided into three main groups: martensitic, ferritic and austenitic.
The stainless martensitic alloys are the only chromium (11 to about 18%), containing small amounts of other elements such as nickel. They are the only ones who can take hardening steel and thereby increase their mechanical properties (tensile strength, yield strength, hardness), by heat treatment. Good is their attitude to plastic deformation operations, especially in hot and risolforate discrete versions also give guarantees of machinability.
Although the ferritic stainless steels are plain chromium (the content is variable 16-28%), but can not raise their mechanical characteristics by means of heat treatments. It is easy to work by plastic deformation, both hot and cold, and can be machined with machine tools (especially the resulphurised types). They have good weldability, especially in the case of resistance welding (spot welding and rolling).
Austenitic alloys are chromium-nickel, chromium in an amount of between 17 and 26% and nickel between 7 and 22%.
Even these steels do not take quenching but can increase the tensile properties with work hardening by cold deformation (rolling, pressing, etc.). There are also different versions with a low carbon content, stabilized, for the most varied types of employment. Great is their workability, especially the cold formability (especially the drawability) and the machining by machine tools. They may also be validly welded both to resistance to both the electric arc.
In addition to these three main categories, there are also other families less known, but worthy of mention, for more specific uses; They are to be quoted, for example, the said steels “austenitic-ferritic”, also “duplex”, that have a mixed structure of austenite and ferrite. These materials are employed when particular requirements of corrosion resistance (especially against the stress corrosion); they have weldability and mechanical characteristics usually higher than those of current ferritic.
To be mentioned are also stainless steels “precipitation hardening”; these have the ability to greatly enhance the mechanical characteristics of the particular heat treatments of aging, which allow to precipitate in the metal matrix composite elements able to increase the mechanical properties of the alloy. Furthermore, the precipitation hardening possess outstanding corrosion resistance, certainly comparable to that of the classical austenitic steels.
Currently it has come at a considerable differentiation in the type of stainless steels and we can count more than a hundred types. It is thought however to gather the most current with their chemical compositions indicative and approximate corresponding standard of different countries (table 1).
Table 1 – Indicative chemical composition and correspondence between abbreviations of certain types of most used stainless steels.