Effect Of Heat Treatment Process And Hot Pressing Deformation Treatment On Microstructure And Properties Of Cr5MoVNi Steel

Special environments and components put higher requirements on steel to meet the comprehensive mechanical properties such as strength,toughness and hardness.Focusing on the aim of high carbon and high strength,this thesis studies the influence of alloying elements on the formation kinetics of carbides.A new martensite steel of Cr5MoVNi was developed by optimization on the composition and the heat treatment processes,combined with the modification treatment.The effects of different heat treatment processes and thermoplastic deformation on the microstructure and mechanical properties of Cr5MoVNi steel were investigated by Optical Microscope(OM),Scanning Electron Microscope(SEM),Transmission Electron Microscope(TEM),Differential Scanning Calorimeter(DSC),X-ray Diffraction(XRD),compression experiments and impact experiments.After 930?quenching(180??430?)tempering,Cr5MoVNi steel is composed of needle-like tempered martensite,retained large network of carbides and precipitated phases.With the increase of tempering temperature,the hardness shows a downward trend,reaching 60.9HRC when tempered at180?.The impact toughness first rises and then decreases,and reaches the peak value of 5.9J/cm~2due to tempering at 380?.The compressive fracture strength reaches the peak value of 3767 MPa and the yield strength is beyond2500 MPa due to tempering at 330?.After quenching(1000??1200?)and medium temperature tempering(380?),the microstructure of Cr5MoVNi steel is composed of austenite,tempered martensite,retained network of carbides and dispersed carbide precipitates.As the quenching temperature increases,the retained austenite content increases and the tempered martensite content decreases.At the same time,the large network of carbides and the precipitates decrease.With the increase of quenching temperature,the hardness shows a downward trend.After quenching at 1200?and tempering at 380?,the hardness is 15.2HRC.The impact toughness first rises and then decreases.After quenching at 1100?and tempering at 380?,the peak value is 20.1J/cm~2.The compressive fracture strength first rises and then decreases.It reaches the maximum value of5621MPa due to quenching at 1100?and tempering at 380?.However,the yield strength of the alloy decreases substantially,and only 500 MPa is obtained due to quenched at 1100?and tempered at 380?.However,during the compression process,secondary hardening occurs,which is due to the phase transition from austenite to martensite.As a result,a rapid increase in compressive fracture strength and the hardness occurs.After hot pressing deformation treatment(1000??1250?),Cr5MoVNi steel is quenched at 930?and tempered at 180?.The microstructure is mainly composed of tempered martensite,retained network carbides and dispersed precipitates.After hot pressing deformation treatment,the number of the dispersed carbides increases,and the retained network carbide decreases or even disappears.The hardness change slightly,the impact toughness is significantly improved,reaching the value of 5.3 J/cm~2due to hot deformation at 1100?.The compressive fracture strength and the yield strength are significantly increased by 576 MPa,and 550 MPa,respectively.