Low-temperature Bainitic Transformation Behavior, Microstructure And Mechanical Properties Of Si-Al-rich Medium-carbon Alloy Steel

The purpose of this dissertation is to investigate isothermal transformation kinetics,microstructure characteristic and mechanical properties of medium carbon Al Si richalloy steel processed by austempering and the effect of tempering on mechanicalproperties.The isothermal transformation dynamics curve of the steel was simulated using thesoftware. Using a dilatometer phase transformation points, Ac1and Ac3, were measured. MSand isothermal bainitic transformation kinetics curve were measured on a thermalmechanical simulator model of Gleeble–3500. Mechanical properties, including hardness,tensile and impact toughness, were estimated for different heat treatment samples. Inaddition, phase composition and microstructure morphologies were examined by X raydiffractometry, optical microscopy and transmission electron microscopy.Results show that transformation points, A_c1, A_c3and MS, are determined as768,830and210oC. The bainitic transformation was carried out in a temperature range of210277oC. The low-temperature bainitic microstructure composed of lath-like bainitic ferrite andfilm-like retained austenite can be formed in the steel by isothermal transformation. Thetempered microstructure was also examined.With the increase in isothermal transformation temperature the hardness and thetensile strength decrease, while yield strength increase. The hardness and the tensilestrength of the sample isothermally transformed at235oC reach598HV0.5and2031MPa,respectively. When isothermally transformed at265oC, yield strength reaches1095MPa.With the increase of tempering temperature, the hardness and the tensile strength decrease,while the yield strength increases. The hardness significantly decreases and the yieldstrength increases to1232MPa when tempering at320oC after isothermallytransformation at235oC. In addition, the optimum comprehensive properties is obtainedin the sample tempered at320oC after isothermally transformed at235oC, that is, the hardness is517HV0.5, tensile strength is1859MPa, the yield strength is1232MPa,the impact toughness is46.3J/cm².