Martensitic stainless steels are basically ternary alloys of iron, chromium, and carbon that possess a martensitic crystal structure in the hardened condition. In the basic composition, there is no nickel. They are ferromagnetic, hard enable by heat treatments, and are generally less corrosion resistant relative to the other classes of stainless steels. Chromium in the steel is in the range of 10.5–18 wt.% with a higher level of carbon than the ferritic; though, the chromium and carbon contents are balanced to ensure a martensitic structure after a complete cycle of heat treatment. Excess carbides may be present to increase wear resistance while elements such as niobium, silicon, tungsten, and vanadium may be added to modify the tempering response after hardening. Small amounts of nickel may equally be added to improve corrosion resistance in some media and to improve toughness. Sulfur or selenium is added to some grades to improve machinability.
Characteristics Martensitic stainless steels are magnetic and have good fatigue properties once heat treated. They generally do not have particularly good low-temperature properties and would not normally be used for cryogenic applications as they soon become brittle at low temperatures. Heat treatment, while improving strength and fatigue resistance, also reduces the toughness and they can also become brittle at temperatures above 430 °C. Welding needs to be carried out carefully due to their low ductility and the high carbon content of many grades makes them unsuitable for welded applications.