Effect of heat treatment on microstructure and mechanical properties of low-alloy wear-resistant steel NM450

Abstract In this study, we investigated the effects of the quenching and tempering temperatures on the microstructure and mechanical properties of low-alloy wear-resistant steel NM450. The quenching temperatures were 870 °C–1200 °C, and held for 36 min. After quenching with 910 °C, the tempering temperatures were 200 °C–600 °C for 60 min. The results showed that as the quenching temperature increasing, the original austenite grain size increased and the proportion of high-angle grain boundaries gradually decreased, which was detrimental to the toughness of the steel. The inter-lamellar-retained austenite remaining in the martensite lath during quenching satisfied the Kurdjumov-Saches relationship with martensite. At the tempering temperatures lower than 250 °C, carbon atoms precipitated in the form of fine carbides, which improved the yield strength of the steel. With the increasing of the tempering temperature, the activity of the carbon atoms increased, the size of the carbides increased, the effects of solid solution strengthening and precipitation strengthening weakened, so the tensile and yield strengths of the steel decreased. The low-alloy wear-resistant steel tempering at 250 °C exhibited the satisfactory combination of properties. The quasi-cleavage fracture occurred mainly over the tempering temperature range of 300 °C–400 °C, and the fracture mechanism turned into ductile fracture when the tempering temperature was higher than 500 °C.