Research In Transformation Of CrNiMoV Medium Carbon Steels

Because both of4Cr5MoSiV1H13steel and34CrNi3MoV steel have good mechanical properties, so they have been widely used in making hot work die steels and pressure vessel steels. Researchers have done a lot of experimental research both in H13steels and34CrNi3MoV steels to improve the heat treatment process and organization, but in the study of the mechanism of the phase transition is rarely reported. In addition, the long production cycle, the band structure and the presense of coarse grains has restricted the application of H13steel and34CrNi3MoV steel.In this paper, SEM,TEM,and LSCM have been applied to analyze the microstructure of H13steel and34CrNi3MoV steel at different quenching and tempering temperature. Base on the prior test, the thermal expansion curve of H13steel has been measured, and grain refinement of normalizing of34CrNi3MoV steel has been tested. The test results show that H13steel,34CrNi3MoV steel optimum quenching temperature was1010℃,860℃, too high quenching temperature or too low temperature is undesirable. After quenching, plate martensite and undissolved carbide integration organization have been obtained. For34CrNi3MoV steel, tempered martensite should be observed when it temper at the lower temperature,(80to250℃), at350℃to550℃,it should be tempering troostite organization, when it comes to650℃, organization of sorbite. H13temper at520℃comes to secondary hardening phenomenon, secondary hardening may be caused by the formation of Fe, Cr, C atoms complex segregation area(G.P District). After the H13steel insulate long time near the river district which above the bainite transformation district, phase change don’t happen in matrix,but after a long period of insulation, carbide segregation at the austenite gain boundaries. Segregation of the grain boundary carbides are not conducive to the steel member in the subsequent quenching-tempering process to obtain good mechanical properties, so the temperature range should be avoided in the actual H13steel isothermal heat treatment. When34CrNi3MoV steel normalize in slightly higher than the Aci temperature(800℃), grain refinement is better than it normalize several times repeatedly above the Ac3temperature,(900℃).