Microstructure and properties evolution of Nb-bearing medium Cr wear-resistant cast steel during heat treatment

The effect of quenching temperature and cooling conditions on the microstructure and mechanical properties of a 0.2%Nb medium chromium wear-resistant cast steel was investigated. The microstructure, carbides and volume fraction of retained austenite were characterized using the optical microscope, scanning electron microscope, transmission electron microscope and X-ray diffraction. The influence of heat treatment on hardness, impact toughness and tensile properties of test steel was studied. It can be observed that lath martensite can be obtained under the condition of air cooling and oil cooling upon austenitizing in the range of 900–1020 °C. Total carbide content of 0.2–1.1 wt.% under air cooling is more than that under oil cooling due to the lower cooling rate. Nb6C5, M23C6 and M7C3 were found at lower austenitizing temperature, of which niobium carbide mostly located at grain boundaries, while chromium carbides were uniformly distributed in the matrix with the size of 20–50 nm. The chromium carbides are basically dissolved into the matrix in test steel austenitized at 1020 °C. Meanwhile, the negligible growth of prior austenite grain size is achieved. Specimen austenitized at 1020 °C and cooled in air + tempered at 200 °C has a best combination of hardness, plasticity and tensile strength due to fine grain size and more amount of retained austenite. Under this condition, the hardness is 58 HRC, the impact toughness is 22.92 J/cm2, and the tensile strength is 1136.9 MPa.