Microstructural evolution and mechanical properties of 55NiCrMoV7 hot-work die steel during quenching and tempering treatments

55NiCrMoV7 hot-work die steel is mainly used to manufacture heavy forgings in the fields of aerospace and automobile.This study aims to clarify the effects of heat treatment on the microstructural evolution and mechanical properties of the steel,in order to find out an optimal heat treatment scheme to obtain an excellent balance of strength,ductility and toughness.The steel was quenched at temperature from 790℃ to 910℃ followed by tempering treatments of 100–650℃ for 5 h.The mechanical property tests were carried out by tensile,impact toughness and hardness.Optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM)were used to observe the austenite grains,lath martensite,carbides and fracture morphology.The results show that the quenching temperature mainly influences the austenite grain size and the volume fraction of undissolved carbides(UCs),while the tempering temperature mainly influences the size and morphology of the martensite with a body centered cubic(BCC)and the carbides with a face centered cubic(FCC).The mechanical properties of the steel,including yield and tensile strength,ductility,impact toughness and hardness,get an excellent balance at a quenching range of 850–870C.As the tempering temperature increases,the yield and tensile strength and hardness decrease,while the ductility and impact toughness increase.These variation trends can be further verified by fracture SEM observation and analysis.Combined with a macro-micro coupled finite element(MMFE)modeling technique,the cooling rate,microstructural evolution and yield strength of the steel were predicted and compared with the tested data.