Stainless steels are grouped by their internal microstructure into four main types: austenitic, ferritic, martensitic and duplex. Duplex, super duplex and hyper duplex are all a roughly 50:50 combination of austenitic and ferritic microstructures.
The microstructure is the crystallographic structure of a metal at a microscopic level, and it usually defines the generic properties of the grade.
Martensitic stainless steels, also referred to as precipitation-hardened (PH) grades, offer very high strength, although formability and toughness are compromised.
The more standard grades, which make up the majority of the market, are austenitic, such as 304/316 (the 3xx series), and ferritic, such as 410/420/430 (the 4xx series).