Research On Microstructure And Mechanical Properties Of A Cr-Mo-V-Ni High Strength Steel

Steel with comprehensive mechanical properties of high strength and toughness become increasingly desirable in engineering applications under complex environments.Here,a new kind of Cr-Mo-V-Ni high strength steel?Fe-1.30Cr-0.62Mo-0.46V-1.60Ni?was designed and prepared to obtain high strength and toughness.The effects of different heat treatment processes on the microstructure and properties were studied by differential scanning calorimetry?DSC?,optical microscope?OM?,scanning electron microscope?SEM?,transmission electron microscope?TEM?,tensile test and impact test.The temperatures of Ac₁?709??and Ac₃?868??of the studied steel were determined by DSC,based on which the heat treatment processes were designed.Namely,the annealing temperature is 950?and the quenching temperature is870?950?.The as-cast microstructure of the studied steel is composed of ferrite and pearlite.After quenching,the microstructure of the steel consists of lath martensite,retained austenite and undissolved carbides.The finest structure of martensite was obtained after due to quenching at 930?and the hardness shows the maximum value of 56 HRC.The main phase of the studied steel is lath tempered martensite due to tempering at 180?and 230?after quenching at 930?.When the tempering temperature is increased to 480?,the matrix is tempered troostite remaining martensitic orientation.With further increasing of the tempering temperature to530?,the matrix is mainly tempered sorbite remaining martensitic orientation.The average width of martensitic laths is the minimum at the tempering temperature of 230?,and then it increases with the increasing of the tempering temperature.The results of TEM show that the martensitic laths are separated by thin layers of austenite.At the same time,there are a large number of dislocations in the martensitic laths that distribute inhomogeneously.The number of dislocations considerably decreases with the increase of tempering temperature.The carbides mainly precipitate in the martensitic laths,which are needle-like,short rod-like or spherical,and their morphologies depend on the tempering temperature.With the increase of the tempering temperature,the tensile strength?1689MPa?and hardness?49.4 HRC?reach the peaks at the tempering temperature of230?,and then decrease.The impact toughness?U-notch?first decreases and then increases.The minimum value of 26.3 J/cm² is obtained due to tempering at the temperature of 380?.The SEM analysis of impact fracture shows that there are large numbers of small dimples that distributed homogeneously due to tempering at 230?,which indicate ductile fracture.When the tempering temperature is 380?,the mixed ductile-brittle fracture is observed,in which dimple and cleavage plane coexist.With further increasing of the tempering temperature to 530?,it is also ductile fracture.In the case of cyclic quenching,the number of martensite laths increases and the space among the laths decreases with the increasing of quenching times,regardless of the quenching temperatures.The microstructure changes considerably due to 2-time quenching,and the best comprehensive mechanical properties are obtained,i.e.,the tensile strength of 1750 MPa,the hardness of 50HRC and the impact toughness?U-notch?higher than 48 J/cm².Through SEM analysis of the impact fracture images,it is found that they are all ductile fracture.