Influence Of Carbon Content On High Temperature Properties Of Cr-Mo-V Hot Work Die Steel

The performance requirements of mold materials are getting higher and higher to meet the development of molds in complexity,precision,large size,long life,etc.Hot work die steels work under harsh service conditions,such as high temperature,high pressure and thermal erosion,etc.,which requires the mold materials to have high thermal strength,thermal stability and resistance to cold and hot fatigue at high temperatures.At present,at home and abroad,the high temperature performance of hot work die steel is mainly improved by optimizing the alloy composition and the heat treatment process,and the influence of carbon content on the high temperature performance of die steel is less studied.In this paper,the carbon content is increased on the basis of 4Cr5Mo2V steel.The effects of changes in carbon content on the microstructure and properties of Cr-Mo-V hot work die steels,the evolution of carbides,thermal stability,and the cold and hot fatigue properties were studied by OM,SEM,TEM and self-constrained thermal fatigue testing machine.The following main research results were obtained.（1）The phase transformation characteristics of three different carbon steels were simulated by Jmat-pro thermodynamic calculation software.The results of the analysis and computation show that with the increase of carbon content,the starting temperature of ferrite to austenite is decreasing,the temperature at the end of the transition is not changing,the transition temperature range is expanding,and the complete austenitizing temperature of steel is also gradually increasing.It can be known from the calculation that the carbides precipitated from the three steels are mainly M₂₃C₆ type carbides,and there are also a certain amount of MC and M₆C type carbides.With the increase of carbon content,the number of M₂₃C₆ and MC type carbides is increasing,while the number of M₆C type carbides is gradually decreasing.（2）The effect of carbon content on the phase transition point was measured by a full-automatic Formaster-F dilatometer.It was found that with the increase of carbon content,the Ac₃ and Ms points are gradually decreasing,the Ac₁ point does not change much,and the bainite phase zone is shifted to the left.（3）By measuring the quenching and tempering hardness curves of the three steels,it is found that the overall hardness of the steel in the quenched and tempered state is increased with the increase of carbon content.By observing the quenched structure of three steels at 1030℃,it is found that as the carbon content increases,the grain size and martensite lath width are gradually decreasing,and the amount of precipitated undissolved MC-type carbides is increaseing.In the tempering process,with the increase of carbon content,the number of precipitated fine dispersed carbides is increasing,resulting in an increase in tempering hardness,and the three steels show secondary hardening at 510℃.（4）From the thermal stability curves of three kinds of steels that were kept at 600℃ and 650℃ for 24h,it can be seen that the thermal stability of the steel gradually is increased with the increase of carbon content.The kinetic calculations show that with the increase of carbon content,the activation energy of tempering gradually increases,which makes the tempering transition difficult,and eventually leads to the gradual improvement of the thermal stability of steel.The microstructures of three kinds of steels that were kept at 650℃ for 2h and 24h were observed by SEM and TEM.The results showed that at the same holding time,as the carbon content increased,the degree of martensite recovery gradually is weakened,and the amount of M₂₃C₆ and MC carbides is gradually increasing.The coarsening of M₂₃C₆ carbides is not obvious during the heat preservation process,which results in weaker softening of the material.The reasons for the gradual enhancement of the thermal stability of the steel are the increase in the amount of precipitated MC-type carbides and the dissolution of more carbon into the matrix.（5）The self-constrained thermal fatigue test shows that the cold and thermal fatigue resistance of steel is gradually weakened with the increase of carbon content.The reason is that as the carbon content increases,the steel’s oxidation resistance is gradually weakened.