Alloy compositions and mechanical properties of 9–12% chromium steels with martensitic–austenitic microstructure

Nitrogen alloyed 9–12% chromium steels with high amounts of manganese and nickel have been investigated. The steels are of a lamellar duplex microstructure, consisting of tempered martensite and about 30 vol.% austenite. The martensite is precipitation hardened with fine and homogeneously distributed vanadium nitrides. A good combination of strength and toughness has been achieved, with a room temperature yield strength level of up to 1000 MPa. As will be shown even at temperatures up to 550°C the new steels exhibit outstanding strength and ductility properties. Two different steels have been investigated. One steel was solely nitrogen alloyed, while in the other one parts of the nitrogen have been replaced by carbon. The steel which was alloyed with nitrogen only showed marked embrittlement after long-term ageing. Small carbon additions prevented ageing embrittlement. This is explained by the different precipitation behaviours.