Effects Of M/B Complex Microstructure Heat Treatments On Microstructure And Properties Of H13 Steel

H13 steel is a kind of die steel widely used in die industry and key components.However,the impact toughness of this kind of steel is greatly reduced while it has very high hardness,which limits its application in the coexistence of strong impact and severe abrasion.To solve the problem,this thesis carried out the study on the complex microstructures of Martensite(M)and Bainite(B)of H13 steel.Firstly,different heat treatments were processed,including the austempering-quenching-tempering(AQT)process with B as the leading phase before M,the quenching-austempering-tempering(QAT)process with M as the leading phase before B,and the quenching-temperingaustempering(QTA)process with B transformation after the precipitation of carbides.At the same time,the conventional heat treatment Quenching and Tempering at high temperature(QT)was also processed for comparison.What's more,the microstructures of H13 samples were studied by metallographic microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM),etc.Meanwhile,the mechanical properties of them were tested.The effects of heat treatments on the microstructure and mechanical properties were analyzed in detail.The results are the following:(1)After the AQT process,carbon-free bainite(CFB)+M complex microstructure is formed in H13 steel.Such complex structure not only insufficiently improvs the hardness and strength but also increases the brittleness of H13 steel.It expands the brittleness temperature zone as well.(2)After the QAT process,lower bainite(LB)+M complex microstructure is resulted in H13 steel,which can improve the tempering stability and maintain the hardness of 50 HRC tempered at 610 ? of H13 steel.Among them,the QAT process with pre-quenching temperature of 160 ? can make H13 steel having high toughness and strength after tempering at low temperature,and the impact absorption energy and tensile strength are much higher than those samples with full M microstructure.(3)After the QTA process,LB+ tempered martensite(M')complex microstructure is formed in H13 steel,which can not only improve the tempering stability,but also make H13 steel having both superior hardness and toughness.The QTA process with pre-quenching temperature of 210 ? can bring the peak hardness 57.5 HRC to H13 steel,which is equal to the peak hardness value of sample treated by the conventional process QT.While the impact absorption energy reaches 10.1 J,which is nearly three times than that by QT.Moreover,the tensile strength reaches 2173 MPa,which is also higher than that by QT.(4)The peak hardness and tensile strength of the appropriate proportion of complex microstructures of CFB/M,LB/M and LB/M ' are higher than those with only M' structure.(5)Secondary hardening also occurs in LB during tempering.However,the brittleness resulted from the secondary hardening was greater than that for M'.The main reason for the brittleness of LB slab is that the carbides segregating at the phase boundary after high temperature tempering.To summary,H13 steel treated by QTA process with pre-quenched temperature at 210? can maintain both superior hardness and impact toughness,which can be used in severe conditions with the coexistence of strong impact and abrasion.