Martensitic stainless steels are a less well-known branch of the stainless family. Their special features - high strength and hardness - point to their main application areas as shafts and fasteners for motors, pumps and valves in the food and process industries. Very high hardness grades also find application as knives in cutlery and in food processing machinery.

The name "martensitic" means that these steels can be thermally hardened. They have a ferritic microstructure if cooled very slowly, but a quenching heat treatment converts the structure to very hard martensite, the same as it would for a low alloy steel such as 4140. Neither the familiar austenitic grades (304, 316, etc.) nor the duplex grades (2205 etc.) can be hardened this way.

Grade 431 (UNS 43100) is the most versatile of these martensitic stainless steels and the one most likely to be available in the Australian market. It combines good strength and toughness with very useful corrosion resistance and in its usual supply condition can be readily machined.

Grade 410 (UNS S41000) and 420 (UNS S42000) are used in domestic cutlery and the very high hardness 440C (UNS S44004) is one of a number of specialty grades for high cooking knives. A free-machining martensitic grade designated 416 (UNS S41600) exists and in the past was often used because of its excellent machinability. It is now almost never used because the high sulphur content that improved machinability a severely compromised corrosion resistance, weldability and formability.