Isothermal Heat Treatment of Wheel Steel with High Cr and Si Contents Based on Microstructure, Mechanical Properties, and Wear Performance

In this work, a new wheel steel named 40SiCr, which containing 0.4 wt.% C, 1.33 wt.% Si, 0.84 wt.% Mn and 1.07 wt.% Cr, was designed and prepared. The steel was subjected to isothermal heat treatment process, which consisted of austenitizing at 860 °C followed by duration holding at 650, 633, 613, and 593 °C, respectively, and was called 40SiCr-650, 40SiCr-633, 40SiCr-613, and 40SiCr-593. Tempering was performed at 540 °C for 2 h. With decreasing isothermal temperature, all of the volume fraction of ferrites, the pearlite colony size and the interlamellar spacing decreased, the prior austenite grain size remained unchanged, and the cementite lamellae fragmented gradually and changed into cementite spheroids, since the synergy of the increase in nucleation rate, the decrease in carbon diffusivity and the effect of Cr element. The fragmenting of cementite lamellae led to the improvement of hardness and strength but the deterioration in ductility of the steel, thus optimizing the wear resistance. The 40SiCr steels isothermally treated at 633 °C possessed a perfect combination of strength and ductility and therefore superior wear resistance.