Modifying the structure of glow discharge nitrided layers produced on high-nickel chromium-less steel with the participation of an athermal martensitic transformation

摘要

High-nickel steels composed of metastable austenite are used in the nuclear power industry, aircraft industry, arms industry, and also in the manufacture of surgical instruments. These steels can be exploited at both low and elevated temperatures. Their important advantage lies in that, during plastic deformation, austenite transforms into martensite (TRIP effect). When subjected to a phase transformation combined with precipitation hardening these steels can achieve very high strength indices (R0.2≈2200 MPa). The material examined was Ni27Ti2AlMoNb steel. The steel was subjected to cold treatment. This treatment resulted in the formation of athermal martensite (α′a). The treatment conducted in liquid nitrogen gave a phase structure of the duplex type (martensite–austenite). After the transformation, the steel was subjected to glow discharge-assisted low-temperature nitriding (≤450 °C). The structure thus produced, composed of the phase transformed steel substrate and a nitrided surface layer, was complex in respect of its phase as well as chemical composition and showed unique physical and mechanical properties. By using the martensitic transformation we can modify the depth to which nitrogen diffuses into the steel substrate during the glow discharge nitriding. This possibility permits improving its resistance to frictional wear and corrosion.