Mechanical and physical properties

Basic mechanical strength increases with alloy additions, but the atomic structure differences of the various groups of stainless steels has a more important effect.

Only the martensitic stainless steels are hardenable by heat treatment, like other alloy steels. Precipitation hardening stainless steels are strengthened by heat treatment, but use a different mechanism to the martensitic types. Very small particles are formed by the appropriate heat treatment and act as the strengthening agent in the steel matrix.
The ferritic, austenitic and duplex types cannot be strengthened or hardened by heat treatment, but respond to varying degrees to cold working as a strengthening mechanism.

Ferritic types have useful mechanical properties at ambient temperatures, but have limited ductility, compared to the austenitics. They are not suitable for cryogenic applications due to loss of impact toughness and lose strength at elevated temperatures over about 600 °C, although have been used for applications such as automotive exhaust systems very successfully.

Austenitic types, with their characteristic face centred cube 'fcc' atomic arrangement, have quite distinct properties. Mechanically they are more ductile and impact tough at cryogenic temperatures.
The main physical property difference from the other types of stainless steel is that they are 'non-magnetic' i.e. have low relative magnetic permeability, provided they are fully softened. They also have lower thermal conductivity and higher thermal expansion rates than the other stainless steel types.

Duplex types, which have a 'mixed' structure of austenite and ferrite, share some of the properties of those types, but, fundamentally are mechanically stronger than either ferritic or austenitic types.