Study On Microstructure And Mechanical Properties Of 1350MPa Grade High Strength Martensitic Steel

40CrNi3MoV steel is a typical medium carbon CrNiMoV steel,after quenching and high temperature tempering,with high strength and excellent toughness,is widely used in the manufacture of high pressure vessels and other large forgings.Recently,with the increasing demand for weight reduction of large forgings,it is highly required to increase the yield strength level of 40CrNi3MoV steel from current 1000 MPa to 1350MPa.Based on the composition of 40CrNi3MoV steel,the new 50CrNi5MoV steel with yield strength of 1350MPa grade was prepared by increasing the content of C?Ni and Mo elements.In the present study,the effects of tempering temperature on the microstructure and mechanical properties of the conventional 40CrNi3MoV steel and the new 50CrNi5MoV steel were studied by SEM?TEM?EBSD?XRD?Thermo-Calc and phase analysis method.1.On the basis of 40CrNi3MoV steel,increasing the content of C?Ni?Mo,50CrNi5MoV steel after quenching and high temperature tempering after the organization was significantly refined,and the number of carbides increased.The M2C content in 50CrNi3MoV steel is three times of 40CrNi3MoV steel at 550?tempering.2.Compared with 40CrNi3MoV steel,50CrNi5MoV steel in the large angle of the grain boundary and the proportion of retained austenite are increased.The residual austenite in the test steel is granular or massive distribution in the matrix.3?The hardness and strength of 40CrNi3MoV and 50CrNi5MoV tested steel decrease with the increasing of tempering temperature,while the trend of plasticity and impact absorption energy increase with the increasing of tempering temperature.In the 50CrNi5MoV steel,the hardness and strength are increased when the content of C is increased from 0.38%to 0.49%and Mo content increased from 0.43%to 1.06%,and the tempering effect of 500? to 550? is enhanced by secondary hardening of M2C phase.After tempering at 550?,the yield strength of 50CrNi5MoV steel can reach 1350MPa grade,and it has good room temperature toughness and low temperature toughness.At room temperature AK?46J,low temperature(-40?)KV2?23J,KIC?104MPa·m1/2.4.After tempering from 500? to 650?,the tensile strength and yield strength of 50CrNi5MoV steel are higher than that of 40CrNi3MoV steel 210MPa and 140MPa respectively at 350?.5.After tempering at 550?,the decreasing rate of tensile strength of 50CrNi5MoV steel is similar to 4CrNi3MoV steel at 350? t0 500?.However,due to 50CrNi5MoV steel has high strength at room temperature,and the tensile strength is not less than 1000MPa at 500?.Its higher than 40CrNi3MoV steel about 200MPa,which indicates that 50CrNi5MoV steel is more suitable for using at high temperature.To sum up,on the basis of 40CrNi3MoV steel,by improving the content of some alloying elements,successfully developed 1350MPa grade high-strength martensitic steel of 50CrNi5MoV.It has good plastic toughness and high temperature performance,which laying the foundation for weight reduction of high pressure vessels.