Study On Microstructure And Mechanical Properties Of Cast Cold Work Die Steel Welded Joints

With the rapid development of automobile industry,the market demand of automobile work die is increasing.New types of die steel have been developed and applied to the manufacture of automobile die.New dies failures in the process of repeated use are inevitable;the study of die steel repair method is also increasingly important.Research on microstructure and mechanical properties of cold work die steel welded joints provides the experimental data and theoretical basis for the renovation and repair of the dies,it is of great significance to the development of dies.In this paper,the cast cold work die steel 5CrMoV was chosen to be investigated.The evolution of microstructure of 5CrMoV steel after heat treatment and the phase transformation of super cooled austenite were studied.The results show that the starting temperature of pearlite to austenite transformation is 773.8 ?,and the end temperature is about 786 ?.The as-cast microstructure was lamellar pearlite and a small amount of ferrite;the as-annealed phase constituent was lamellar pearlite and massive ferrite;the as-quenched steel consisted of fine lath martensite;the main as-tempered steel was tempered sorbite.The products of the super-cooled austenite phase constituents during continuous cooling could be ferrite,pearlite,bainite and martensit.The critical cooling of martensite was at 20?/s and the cooling rate of pearlite was less than 1?/s by JMatPro analysis software,.The microstructural and mechanical characteristics of heat affected zone were studied,then the influence of heat input on the microstructures and mechanical properties was discussed.The results show that overheated zone consisted of acicular ferrite,grain boundary ferrite and granular bainite;recrystallized zone contained polygonal ferrite,acicular ferrite and granular bainite;partially recrystallized zone was composed of ferrite,bainite and temper sorbite;tempering zone was temper sorbite.Softening behavior occurred in the heat affected zone,and the region at the junction of partially recrystallized zone and tempering zone become the weakest part of heat affected zone.The welding heat input had significant effects on microstructures and mechanical properties of welded joints.As heat input increased,acicular ferrite laths become coarser and grain sizes were larger obviously in overheated zone;volume fraction of polygonal ferrite increased and the grain sizes slightly increased in recrystallized zone;the precipitation of carbides separated out and grew up continuously in partially recrystallized zone and tempering zone.The hardness values and tensile strength of heat affected zone decreased and the width of soften zone broadened with heat input increasing.The microstructural and mechanical characteristics of weld metal were investigated,then the effects of heat input on the microstructures and mechanical properties were revealed.Root pass of the weld beam was composed of lower bainite,retained austenite and a small amount of carbides.Filling pass consisted of martensite,retained austenite and lath delta ferrite.Cover pass contained coarse martensite,retained austenite and skeletal delta ferrite.For multi-layer welding,the former layer weld thermal cycle improved the microstructures and performance of the previous layer.The microstructure of the filling layer was even and had better performance.As heat input increased,columnar grain sizes increased and grain sizes were larger obviously in root pass;delta ferrite morphology was changing gradually from long and straight laths to short interruption laths in root pass;volume fraction of delta ferrite decreased and mesh density of skeletal delta ferrite reduced in cover pass.The hardness values of weld metal decreased and the average tensile strength of weld metal specimens exhibited a decreasing trend with the increase of heat input.