Martensitic stainless steels are essentially alloys of chromium and carbon that possess a distorted body-centered cubic (bcc) crystal structure (martensitic) in the hardened condition. They are ferromagnetic, hardenable by heat treatments, and are generally resistant to corrosion only to relatively mild environments. Chromium content is generally in the range of 10.5 to 18%, and carbon content may exceed 1.2%. The chromium and carbon contents are balanced to ensure a martensitic structure after hardening. Excess carbides may be present to increase wear resistance or to maintain cutting edges, as in the case of knife blades. Elements such as niobium, silicon, tungsten, and vanadium may be added to modify the tempering response after hardening. Small amounts of nickel may be added to improve corrosion resistance in some media and to improve toughness. Sulfur or selenium is added to some grades to improve machinability.
These steels are generally termed "air hardening" because when withdrawn from a furnace as austenite, cooling in still air is sufficiently rapid to produce the allotropic transformation into martensite. A wide range of strengths is achievable, with yield strengths ranging from 40 ksi (275 MPa) in an annealed condition to 280 ksi (1900 MPa) in the quenched and tempered condition. High hardness levels are also achievable, promoting metal-to-metal wear and abrasion resistance.
In general, corrosion resistance of the martensitic grades is not as good as that of the other stainless steels, due to the relatively low chromium content and high carbon content. These alloys are generally selected for applications where a combination high strength and corrosion resistance under ambient atmospheric conditions is required. The low chromium and low alloying element content of the martensitic stainless steels also makes them less costly than the other types.
Historical applications of martensitic stainless steels include:
Low carbon (less than 0.08 wt%) supermartensitic grades are seeing increasing use in oil and gas pipelines; however their hardenability and strength is reduced as compared to higher carbon grades.
Due to the formation of untempered martensite during cooling after welding, the martensitic alloys are considered the least weldable of the stainless steels.