Effect of double quenching process and tempering temperature on the microstructure and mechanical properties of a High Strength Low Alloy Steel

Abstract

Effect of double quenching process and tempering temperature on the microstructure and mechanical properties of a High Strength Low Alloy Steel was investigated by OM, SEM, TEM methods and so on. The results demonstrated that double quenching process could refine prior austenite grains (PAG) effectively and the average diameter of PAG was refined by 5.4μm compared with single quenching process. Yield strength and tensile strength of double quenching and tempered specimens (QQT) and single quenching and tempering specimens (QT) exhibited similar variation trend with tempering temperature. Yield strength increased monotonously with increasing tempering temperatures. However, tensile strength descended at first and then recovered with rising tempering temperature. QQT specimens displayed greatly improved impact absorbing energy at -80°C but a little improved yield strength compared with QT specimens owing to the refined microstructure caused by double quenching process. Considerable austenite formed during holding process of tempering (670°C) in which elements of nickel, manganese and copper aggregated. And some of the aforementioned austenite transformed to hard phases (secondary Martensite and M-A islands) and some are retained as islands of austenite after water cooling. The presence of hard phases is the main reason for recovering tensile strength and deteriorating impact absorbing energy at -80°C.