In addition to chromium, nickel, molybdenum, titanium and niobium, other elements may also be added to stainless steels in varying quantities to produce a range of stainless steel grades, each with different properties.
There are a number of grades to choose from, but all stainless steels can be divided into five basic categories:
These are named according to the microstructure inherent in each steel group (a function of the primary alloying elements). Austenitic and ferritic grades account for approximately 95% of stainless steel applications.
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. 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).
Excellent corrosion resistance
Excellent weldability (all processes)
Excellent formability, fabricability and ductility
Excellent cleanability, and hygiene characteristics
Eood high and excellent low temperature properties
Non magnetic (if annealed)
Hardenable by cold work only
Computer keyboard key springs (301)
Kitchen sinks (304D)
Food processing equipment
Chemical plant and equipment
These are plain chromium stainless steels with varying chromium content between 12 and 18%, but with low carbon content.
Moderate to good corrosion resistance increasing with chromium content
Not hardenable by heat treatment and always used in the annealed condition magnetic
Weldability is poor
Formability not as good as the austenitics
These are stainless steels containing relatively high chromium (between 18 and 28%) and moderate amounts of nickel (between 4.5 and 8%). The nickel content is insufficient to generate a fully austenitic structure and the resulting combination of ferritic and austenitic structures is called duplex. Most duplex steels contain molybdenum in a range of 2.5 - 4%.
High resistance to stress corrosion cracking
Increased resistance to chloride ion attack
Higher tensile and yield strength than austenitic or ferritic steels
Good weldability and formability
Martensitic stainless steels were the first stainless steels commercially developed (as cutlery) and have relatively high carbon content (0.1 - 1.2%) compared to other stainless steels. They are plain chromium steels containing between 12 and 18% chromium.
Precipitation hardening stainless steels have been formulated so that they can be supplied in a solution treated condition, (in which they are machineable) and can be hardened, after fabrication, in a single low temperature "ageing" process.