Microstructural evolution and mechanical properties of 60Si2CrVNb spring steel under quenching-tempering heat treatment process

Abstract

This study investigated the evolution of microstructure and mechanical properties of 60Si2CrVNb spring steel under different quenching and tempering processes. The morphology of martensite, precipitate phases, and carbides, as well as the analysis of mechanical properties, were examined. The results revealed that the highest ultimate tensile strength (2260 MPa) and yield strength (2091 MPa) of 60Si2CrVNb spring steel were achieved through austenitization at 900 °C and subsequent tempering at 350 °C, which is related to the maximum dislocation strengthening effect. The optimal combination of mechanical properties for 60Si2CrVNb spring steel was obtained by quenching at 900 °C for 30 min and tempering at 400 °C for 90 min. Under these conditions, the steel exhibited an ultimate tensile strength of 2084 MPa, yield strength of 1846 MPa, elongation of 10.8%, and area reduction of 41.5%. Increasing the quenching temperature resulted in the gradual dissolution of undissolved carbides, an increase in the average grain size of austenite, coarsening of martensite, and an initial increase and subsequent decrease in tensile strength and plasticity. Furthermore, the fracture type transformed from ductile fracture to mixed fracture of ductile and brittle. On the other hand, the increasing tempering temperature led to a decrease in dislocation density and gradual decomposition of martensite lath boundaries. Additionally, the type of carbides changed from small-sized M2C and MC carbides to large-sized M3C, M7C3, and M23C6 carbides. As a result, the tensile strength of the steel decreased significantly, while its plasticity steadily increased. Moreover, the fracture behavior transformed from brittle fracture to ductile fracture.