Microstructure And Mechanical Properties Of Medium-carbon Alloyed Steels By Austempering

The aim of the present dissertation is to investigate bainitic transformation kinetics,microstructure characteristic and mechanical properties of medium-carbonSi–Mn–Cr–W–Al steels by austempering.Time-temperature-transformation (TTT) curves of the medium-carbonSi–Mn–Cr–W–Al steels were calculated by using a software for phase diagram calculationand properties simulation of metalic materials. The transformation points, Ac1, Ac3and Ms,were determined by using thermal dilatometry. The phase composition and microstructurefeature of samples obtained by different austempering processes were studied by X-raydiffractometry (XRD), optical microscopy (OM) and transmission electron microscopy(TEM). Tensile and notched impact properties were measured. In addition, the tensile andimpact fracture mechanisms were briefly analyzed by scanning electron microscopy(SEM).Results show that carbide-free bainitic microstructure composed of lathy bainiticferrite and retained austenite can be formed in the medium-carbon Si–Mn–Cr–W–Al steelby austempering. With the austempering temperature elevating from260°C to330°C, thelath thickness of bainitic ferrite can reach about80-120nm. Before approaching to aconstant, the hardness have reduced in the beginning, and then increased, and decreasedlastly.By comparing the properties of samples with different processes, we obtained threeoptimum processes which result in excellent combination of mechanical properties inmedium-carbon Si–Mn–Cr–W–Al steels: austempering at300°C for4h for4#steel, at280°C for8h for5#steel, and285°C for20h for7#steel. The mechanical properties inthe4#medium-carbon Si–Mn–Cr–W–Al steel were measured as follows: the tensilestrength1582MPa, the yield strength1392MPa, the total elongation14.4%, the yieldratio0.88, the impact toughness of Charpy-U sample87.5J/cm2.The mechanicalproperties of the5#medium-carbon Si–Mn–Cr–W–Al steel were measured as follows: thetensile strength1651MPa, the yield strength1382MPa, the total elongation15.6%, the yield ratio0.84, the impact toughness of Charpy-U sample82.9J/cm2.The mechanicalproperties of the7#medium-carbon Si–Mn–Cr–W–Al steel were measured as follows: thetensile strength1592MPa, the yield strength1325MPa, the total elongation16.4%, theyield ratio0.83, the impact toughness of Charpy-U sample82.5J/cm2.