The main types of stainless steel are austenitic, ferritic and martensitic. These three types of steels are identified by their microstructure or predominant crystal phase.


Austenitic:

Austenitic steels have austenite as their primary phase (face centered cubic crystal). These are alloys containing chromium and nickel (sometimes manganese and nitrogen), structured around the Type 302 composition of iron, 18% chromium, and 8% nickel. Austenitic steels are not hardenable by normal heat treatment, yet they can be work hardened. They are also non-magnetic. The most familiar stainless steel is probably Type 304, sometimes called T304 or simply 304. Type 304 surgical stainless steel is an austenitic steel containing 18-20% chromium and 8-10% nickel.


Ferritic:

Ferritic steels have ferrite (body centered cubic crystal) as their main phase. These steels contain iron and chromium, based on the Type 430 composition of 17% chromium. Ferritic steel is less ductile than austenitic steel and is not hardenable by heat treatment.


Martensitic:

The characteristic orthorhombic martensite microstructure was first observed by German microscopist Adolf Martens around 1890. Martensite is a hard transformation product, technically considered a supersaturated solution of carbon in iron. This is characterized by an acicular (or needle-like) pattern in the microstructure in both ferrous and nonferrous alloys. Martensite can be transformed into austenite when the steel is quenched and cooled below 450 degrees Fahrenheit resulting in a hardness varying from 30 to 68 Rockwell, depending on the carbon content.  It also may be tempered and hardened. Martensite gives steel great hardness, but it also reduces its toughness and makes it brittle, so few steels are fully hardened.

There are also other grades of stainless steels, such as precipitation-hardened, duplex, and cast stainless steels.