Stainless steel is a common term for a group of corrosion resistant steels containing a minimum of 10.5% of Chromium. All stainless steels have a high resistance to corrosion. This remarkable resistance to attack is due to the naturally occurring chromium-rich oxide film present on the surface of the steel.

Types of stainless steel
There are five basic categories of stainless steel: Austenitic, Ferritic, Duplex, Martensitic and Precipitation Hardening.

Austenitic Stainless Steel
The basic composition of this type contains Chromium normally in the range of 17% -25% and Nickel 8% -20%.

Table 1: Common austenitic stainless steel grades

Note: all values are maximum unless otherwise stated.

Ferritic Stainless Steel
These are nickel-free stainless steels. They have a varying chromium content of 12-18%, but a lower carbon content than the martensitics.

The common specifications are 430, 409 and weldable 12% Cr steels.

Duplex Stainless Steel
Duplex stainless steels have a structure of approximately equal amounts of ferrite and austenite and therefore may be referred to as Ferritic-austenitic stainless steels. The chromium content varies from 18-28%. The nickel content of 4.5-8.0% is insufficient to develop a fully austenitic crystal structure.

Martensitic Stainless Steel
Martensitic stainless steels are the first branch of the plain chromium stainless steels. They have a relatively high carbon content and a chromium content of 12-18%.

Precipitation Hardening Stainless Steel
Precipitation hardening stainless steels have a minimum of 17% chromium and contain other elements that can provide strengthening through a precipitation hardening mechanism. These grades are ideal for the production of long, straight high strength shafts for pumps and also valve spindles.