Martensitic Stainless Steel

KVA research and development program is dedicated to promoting the use of martensitic stainless steels in novel applications. These and other air-hardening alloys, when processed with KVA joining and thermal treatment technologies, enable cost-effective, superior mechanical and structural solutions.

KVA martensitic stainless steel processing can benefit any market segment where component weight, strength and corrosion resistance are critical issues, providing a superior alternative to existing steels or alloys, while increasing the actual and perceived value of finished products.

Stainless, or corrosion resistant, steels are defined as iron-base alloys with a minimum 10.5% chromium content, which promotes the development of an invisible, adherent and self healing chromium-rich oxide surface film. Stainless steels are commonly divided into five groups, classified by their microstructure at room temperature:

Various alloying elements are added to the basic iron-chromium- carbon and iron-chromium-nickel systems to control microstructure and properties. These elements include manganese, silicon, molybdenum, niobium, titanium and nitrogen, among others. 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.

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. Figure 2 depicts the family relationships for common martensitic, ferritic and austenitic grades.

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.

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. These alloys are ferromagnetic, hardenable by heat treatments and mildly corrosion resistant.

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. Although the chromium level is the same as in ferritic stainless steels (e.g. 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. MSS are considered “air-hardenable,” as all but the thickest sections fully harden during an air-quench heat treatment cycle to room temperature.

Similar to low-alloy steels, maximum strength and hardness of MSS primarily depend on carbon content. Figure 3 shows the hardness range, or ultimate tensile strengths, obtainable with common martensitic alloys. 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. Tensile strengths in excess of 300 ksi (2100 MPa) are possible in the higher alloyed MSS grades.

It is this air-hardening characteristic which has limited the historical use of MSS to traditionally non-welded components. However, this same potential material flaw has been exploited by KVA technology – to arrive at high strength, tough, corrosion-resistant parts through simple air-quenched thermal processing methods. This development was the direct result of the vision of KVA founder, Mr. Ed McCrink, and firsthand experience in working with MSS alloys for more than five decades.

MSS processed with KVA technology exhibits excellent mechanical properties: specific strength, stiffness, toughness, and fatigue performance, combined with increased corrosion resistance. These alloys are ideal, cost-effective materials for numerous demanding applications – ranging from miniaturized medical devices to massive oilfield pipelines – and everything in between. KVA processed MSS can replace difficult to form ultra high strength steels, expensive austenitic stainless steels, and exotic titanium alloys.