Effect of heat treatment on microstructure and mechanical properties of the steel used for heavy duty track shoe

Effect of heat treatment on microstructure and mechanical properties of NiCrMo medium carbon low alloy steel used in the heavy duty track shoe was investigated by optical microscopy,scanning electronic microscopy(SEM),electron probe micro-analyzer(EPMA) and the tensile and impact tests at room temperature.The results demonstrated that an homogenization for 8 h at 1000 ℃ diminished significantly the dendritic segregation of carbon in the as-cast microstructure.The tensile strength increased roughly linearly with the increase in the carbon concentration,whereas the toughness decreased dramatically.When quenching from above 910 ℃,the grain coarsening would lead to the inferior of the strength and hardness and with further increasing the quenching temperature to over 970 ℃ the abnormal grain growth occurred.Tempered sorbite was obtained at 450~550 ℃ tempering.With increasing the tempering temperature the strength of the steel with a carbon content of 0.32% reduced from 1340 MPa to 1220 MPa,but the impact toughness increased from 40 J·cm-2 to 65 J·cm-2.