Effect of low-temperature tempering on confined precipitation and mechanical properties of carburised steels

Surface-carburised steels are widely used in gears and bearings. The mechanical properties of the surface layers are crucial to improve contact fatigue and anti-wear performance. In this study, the effect of low-temperature tempering on confined precipitation and mechanical properties across surface layers in a high-alloy carburising steel (18Cr2Ni4W) were investigated. The steel was tempered at 240 °C with different tempering durations. Microstructure changes, including confined precipitation, and local mechanical properties were analysed layer-by-layer. During tempering, the morphology and size of martensite remained unchanged while confined precipitation occurred in martensitic matrix; coherent ε phase precipitates formed after 1.5 h, they then transformed to less coherent η phase within 3 h and subsequently evolved to non-coherent θ phase after 3 h of tempering. After tempering for 1.5 h, the strength and ductility of the surface layers reach a maximum value, and elongation of the surface layers with high carbon content is comparable to that of the core with low carbon content. It is revealed that the coherent ε precipitates are mainly responsible for the increase in strength; whilst ductility of the martensitic matrix increases due to a reduced carbon concentration, avoiding tempering brittleness. The results are valuable to the optimisation of surface properties of carburised steels using lower temperature tempering.