Microstructure Transformation Behaviour of Austenite Cooling for SAE9310 Steel

With thermodynamic calculation software and XRD,the equilibrium transformation for SAE9310 steel after austenizing was investigated.Through expansion and metallographic methods the continuous cooling transformation curves of the steel were determined using Formastor-F II dilatometer.The results show that the equilibrium microstructure of SAE9310 steel was composed of α-Fe,γ-Fe,M23C6 and M7C3 carbides.The products of austenite transformation were granular bainite and ferrite when the cooling rate was lower than 0.056 ℃·s-1.Granular bainite could be mainly formed with small amount of martensite as the cooling rates were from 0.056 ℃·s-1 to 1.9 ℃·s-1.The products of austenite transformation were lath martensite with small quantity retained austenite when the cooling rate was higher than 1.9 ℃·s-1 and the granular bainite disappeared gradually.The hardness of the steel increased with the increase of cooling rate.