Effects of soaking and tempering temperature on microstructure and mechanical properties of 65Si2MnWE spring steel

The heat treatment of soaking, oil quenching and tempering was employed on 65Si2MnWE spring steel. Various soaking and tempering temperatures were attempted to investigate their effects on the microstructure and mechanical properties of 65Si2MnWE steel. The microstructure such as martensite and precipitations were carefully examined, and the tensile, impact and hardness tests were carried out to evaluate the mechanical properties. The results show that the coarsening of martensitic occurs at high soaking temperature due to the facts of precipitation dissolution, dislocation reduction, and also the reduction of reserved austenite. Large amount of alloyed carbide precipitations distribute along grain boundaries after tempering, and these precipitations tried to dissolve into matrix at high tempering temperature. Moreover, with the increase of tempering temperature, the transformation and growth of carbide, and the recovery of martensite simultaneously take place. According to the analysis on mechanical properties, the optimal temperature range for soaking is determined as 850-900 °C, and the optimal tempering temperature should be in the range of 350-400 °C.