Tensile strengths in excess of 300 ksi (2100 MPa) are possible in the higher alloyed MSS grades. Low carbon martensitic stainless grades, such as AISI type 410, have tensile strengths in excess of 200 ksi (1400 MPa) in the fully hardened condition. Figure 3 shows the hardness range, or ultimate tensile strengths, obtainable with common martensitic alloys. Similar to low-alloy steels, maximum strength and hardness of MSS primarily depend on carbon content. MSS are considered "air-hardenable," as all but the thickest sections fully harden during an air-quench heat treatment cycle to room temperature. type 409), the higher carbon content of the martensitic grades results in a complete transformation to austenite at high temperature, followed by a subsequent change to the hard martensite phase upon rapid cooling. Although the chromium level is the same as in ferritic stainless steels (e.g. The compositions of MSS, typically between 10.5 to 18 wt% chromium, are specifically formulated to render them amenable to a quench-and-temper (Q+T) heat treatment in order to produce high levels of strength and hardness. These alloys are ferromagnetic, hardenable by heat treatments and mildly corrosion resistant. MSS, alloys primarily of chromium and carbon, possess a distorted body-centered cubic (bcc) or body-centered tetragonal (bct) martensitic crystal structure in the hardened condition. Invented by Harry Brearley in 1913 as the first ever-produced "rustless steel", and commercialized and standardized in the 1930s and 1940s, historical applications of martensitic stainless steels (MSS) include cutlery, surgical instruments, scissors, springs, valves, shafts, ball bearings, turbine equipment and petrochemical equipment. Figure 2 depicts the family relationships for common martensitic, ferritic and austenitic grades. There are over 150 grades of stainless steel, of which austenitic stainless steels (type 304, type 316, 18/8, etc.) are the most widely used. As shown in Figure 1, all stainless steels can be plotted in terms of their chromium and nickel equivalents to provide a graphic representation between composition and microstructure. These elements include manganese, silicon, molybdenum, niobium, titanium and nitrogen, among others. Various alloying elements are added to the basic iron-chromium- carbon and iron-chromium-nickel systems to control microstructure and properties.
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