Just as there is a range of structural and engineering carbon steels (ref. Olympic Steel) meeting different requirements of strength, weldability and hardness, so is there a wide range of stainless steels with progressively higher levels of strength and corrosion resistance. These types or “grades” of stainless steel have traditionally been divided into five families: Ferritic, Martensitic, Austenitic, Duplex and Precipitation Hardening. These names are derived from the crystalline microstructure, which determines their difference in metallurgical behavior. Due to the controlled addition of the alloying elements, each offers specific attributes in respect of strength and ability to resist differing environments.
Ferritic stainless steels are alloyed with a chromium content varying between 11 and 19%. These alloys have a low carbon content with little to no addition of nickel and contain a crystal structure similar to pure iron at room temperature. Ferritic stainless steels are magnetic and not hardenable by heat treatment. Ferritic grades also have good ductility and formability but a relatively poor high temperature strength compared to that of austenitic grades.
Martensitic stainless steels are plain chromium steels containing between 12 and 18% chromium and are similar to low alloy or carbon steels. Due to the addition of carbon, they can be tempered and hardened and are thus highly useful in situations where the strength of the steel is more important than its resistance to corrosion.
Austenitic stainless steels are non-magnetic. When nickel is added to stainless steel in sufficient amounts the crystal structure changes to “austenite”. The basic composition of austenitic stainless steels is 18% chromium and 8% nickel. This enhances their corrosion resistance and modifies the structure from ferritic to austenitic. Austenitic grades are the most commonly used stainless steels accounting for more than 70% of production (type 304 is the most commonly specified grade by far). They are not hardenable by heat treatment.
Duplex stainless steels hold relatively high chromium levels (between 18 and 28%) and moderate amounts of nickel (between 4.5 and 8%). The high corrosion resistance and the excellent mechanical properties combination of duplex stainless steels can be explained by their chemical composition and balanced microstructure.