in aerospace among other applications. Creep-resisting grades. Small additions of niobium, vanadium, boron, and cobalt increase the strength and creep resistance up to about . Martensitic stainless steels can be heat treated to provide better mechanical properties. The heat treatment typically involves three steps: Austenitizing, in which the steel is heated to a temperature in the range , depending on grade. The resulting austenite has a face-centered cubic crystal structure. Quenching. The austenite is transformed into martensite, a hard body-centered tetragonal crystal structure. The quenched martensite is very hard and too brittle for most applications. Some residual austenite may remain. Tempering. Martensite is heated to around , held at temperature, then air-cooled. Higher tempering temperatures decrease yield strength and ultimate tensile strength but increase the elongation and impact resistance. Replacing some carbon in martensitic stainless steels by nitrogen is a recent development. The limited solubility of nitrogen is increased by the pressure electroslag refining (PESR) process, in which melting is carried out under high nitrogen pressure. Steel containing up to 0.4% nitrogen has been achieved, leading to higher hardness and strength and higher corrosion resistance. As PESR is expensive, lower but significant nitrogen contents have been achieved using the standard AOD process. Duplex Duplex stainless steels have a mixed microstructure of austenite and ferrite, the ideal ratio being a 50:50 mix, though commercial alloys may have ratios of 40:60. They are characterized by higher chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. Duplex stainless steels have roughly twice the yield strength of austenitic stainless steel. Their mixed microstructure provides improved resistance to chloride stress corrosion cracking in comparison to austenitic stainless steel types 304 and 316. Duplex grades are usually divided into three sub-groups based on their corrosion resistance: lean duplex, standard duplex, and super duplex. The properties of duplex stainless steels are achieved with an overall lower alloy content than similar-performing super-austenitic grades, making their use cost-effective for many applications. The pulp and paper industry was one of the first to extensively use duplex stainless steel. Today, the oil and gas industry is the largest user and has pushed for more corrosion resistant grades, leading to the development of super duplex and hyper duplex grades. More recently, the less expensive (and slightly less corrosion-resistant) lean duplex has been developed, chiefly for structural applications in building and construction (concrete reinforcing bars, plates for