The effects of quenching and partitioning on the microstructure and tensile properties of high strength suspension spring steel

Abstract

The evolution of microstructure and mechanical properties were investigated by the quenching and partitioning heat treatment for a kind of micro-alloyed 51CrMnV spring steel. The results show that the microstructure of the test steel treated by quenching and partitioning heat treatment is mainly composed of martensite, retained austenite and carbides. Based on the quenching treatment, the optimal mechanical properties of test steel were obtained by the process of partitioning treatment at 350°C for 20 minutes. The tensile strength and elongation were 1812 MPa, 11.5 %, respectively. This is largely due to the microstructure of test steel having a good combination of martensite, filmy retained austenite, and fine carbides. As the partitioning temperature increases, the fraction of retained austenite decreases from 13.1 % at 250°C to 3.11 % at 350°C. Furthermore, the retained austenite almost disappears when the partitioning temperature increases to 400°C. In addition, as the partitioning temperature increases, the tensile strength remains above 1600 MPa, and the fracture behavior changes from brittle fracture to ductile fracture. At the same time, the grain size becomes larger, the proportion of large-angle grain boundaries increases, and the geometric dislocation densities become smaller.