Duplex Heat Treatment of a Precipitation Hardening Carburising Steel

Abstract This article reports the microstructural development of a molybdenum and copper alloyed steel by different thermochemical heat treatments for gears and shafts in automotive applications. The case hardening is compared to a duplex heat treatment consisting of carburising, ageing and plasma nitriding. This study evaluates the microstructure, the surface hardness and hardness profiles. For all applied heat treatment routes, the ageing conditions have a minor influence on the case and core hardness compared to the influence of the carbon concentration. Retained austenite fractions decreased to < 5 % in all duplex heat-treated samples without cryogenic treatment. Material characterisation with optical light microscopy and scanning electron microscopy indicate unwanted phases of nitrogen and carbon-rich precipitates at the grain boundaries after duplex heat treatment. Therefore, the sequence of duplex heat treatment is changed to carburising followed by solely plasma nitriding where the unwanted precipitates are detected only up to a depth of 15‒20 μm from the compound layer.