Influence of vacuum graded austempering treatment on microstructure and mechanical properties of M50 steel

Abstract

By means of microstructure observation and mechanical property tests, the effects of vacuum graded austempering treatment on carbide precipitation, bainite transformation, hardness and impact toughness of M50 steel were studied. Results show that after the steel was vacuum grading treated at 560 °C for 10 min, 20 min, 30 min and 60 min and austempered at 200 °C for 2 h, the microstructure is mainly composed of martensite, acicular lower bainite, retained austenite, and carbides (C) in different shapes. With the extension of the grading time from 10 min to 60 min, the content of precipitated carbides after austempering treatment increases from 2.38% to 3.79%, that of the lower bainite increases from 13.14% to 16.03%, and the hardness decreases from 65.06 HRC to 64.26 HRC. After tempering treatment at 550 °C × 2 h for 3 times, the content of precipitated secondary carbides increases from 3.86% to 5.37%, and the hardness of the steel decreases compared to the corresponding austempered steel. Thereinto, the hardness of the tempered steel with the grading time of 20 min is the highest to be 62.92 HRC. After vacuum grading treatment at 560 °C × 10 min, austempering and tempering treatments, the impact toughness of the M50 steel is 16.63 J/cm2, which is enhanced by 50.6% compared to the steel only with vacuum quenching and tempering. This is related to the formation of bainite after graded austempering treatment and the uniform precipitation of fine carbides after tempering.