Low Temperature Bainite Transformation Behavior And Mechanical Properties Of High Carbon Cr-Si-Mo Steel

The purpose of this article is to study isothermal transformation kinetics,microstructure and mechanical properties of high carbon Cr-Si-Mo steels A, B and C（with different Si contents）. Through the analysis on microstructure and mechanicalproperty of the steels processed by austempering, select the steel with the best mechanicalproperties.DIL402C type thermal expansion instrument has been applied to the determinationA_c1sand A_c1fof three kinds high carbon Cr-Si-Mo steels, and the determination of A, B andC three kinds of high carbon Cr-Si-Mo steels Mspoints and the kinetics of isothermaltransformation curve of different austempering temperature was held on thethermalmechanical simulator model of Gleeble–3500. According to the isothermaltransformation kinetics curves, the corresponding austempering process was set up.Mechanical properties, including hardness, tensile and impact toughness, were measuredfor different austempering process samples. In addition, phase composition andmicrostructure morphologies were examined by X ray diffractometry, transmissionelectron microscopy and optical microscopy.Experimental results show that the Msof A, B, C high carbon Cr-Si-Mo steels were195,196and203℃, respectively; A_c1spoint is754,765and772℃, respectively; A_c1fpoint is784,795and803℃, respectively.The low-temperature bainitic microstructurecomposed of lath-like bainitic ferrite and film-like retained austenite can be formed in thethree high carbon Cr-Si-Mo steels by austempering at220,230and240℃, respectively.With increasing austempering temperature, the hardness decreases, while yield strengthincreases. According to the XRD of the three kinds of steels, with austenitizingtemperature and austempering temperature increases, the retained austenite in themicrostructure increases. The optimum process of steel A with the excellentstrength-toughness combination is austenitizing at880℃for1hour, and austempering at240℃for4hours. In this way the hardness, the yield strength, the tensile strength andimpact toughness of steel A reach HRC60.9,1808MPa,2382MPa and72.3J/cm², respectively. The optimum process of steel B with the excellent strength-toughnesscombination is austenitizing at870℃for1hour, and austempering at230℃for4hours.In this way the hardness, the yield strength, the tensile strength and impact toughness ofsteel B reach HRC61.1,1534MPa,2145MPa and81.2J/cm², respectively. and theoptimum process of steel C with the excellent strength-toughness combination isaustenitizing at890℃for1hour, and austempering at230℃for4hours. In this way thehardness, the yield strength, the tensile strength and impact toughness of steel C reachHRC60.4,1443MPa,2050MPa and92.6J/cm², respectively. Tempering at280,310and350℃for two hours on the steel B samples after austenitizing at870℃for1hour, andaustempering at230℃for4hours, the hardness, yield strength and tensile strengthdecreases with tempering temperature increases, and the elongation increases gradually.