Study On Microstructure And Properties Of 5Cr5MoV New Type Cold Working Die Steel Welded Joints

With the rapid development of automobile manufacturing industry,the technical level of die industry is continuously improved.And new type casting die steel is continuously developed and applied.Because of the friction,impact and pressure during the service process,it is possible to be failed.In recent years,the repair research of die steel has become the focus of die manufacturing.It is importance for the development of die industry to study the microstructure and properties of new type casting cold working die steel welded joints.In this paper,the new type casting cold die steel 5Cr5MoV was used as the research material.The changes of microstructure and hardness after heat treatment were studied.The results show that as-cast microstructure of the 5Cr5MoV die steel was lamellar pearlite.The microstructure of steel after isothermal spheroidize annealing was ferrite and granular pearlite.The microstructure of steel after high-frequency induction quenching was mainly martensite.The microstructure observation of 5Cr5MoV die steel SMAW welded joints show that quenching zone was mainly composed of ferrite,tempered martensite,tempered sorbite,residual austenite and a small amount of undissolved carbides.The partial quenching zone consisted of ferrite,tempered martensite,residual austenite and coarse granular pearlite.At the root pass of the welds,there was mainly slatted martensite,residual austenite,a small amount of tempered martensite and tempered sorbite.The microstructure of cover pass was the martensite and residual austenite.The average hardness of the cover pass was 1.15 times of the base metal after high-frequency induction quenching.The tensile strength of welded joints reached 731.5 MPa.The wear resistance of SMAW surfacing specimens was better than that of the base metal,which was more obvious under high load.The surface of the surfacing layer was similar to the wear mechanism of the base metal under different load.The abrasive wear was presented under low load,and serious adhesion wear and oxidation wear was showed under high load.At the same time,the oxidation resistance of surfacing metal was better than that of base metal.The transformation rule of 5Cr5MoV die steel MIG welded joints was studied.The results show that the quenching zone was composed of tempered martensite and residual austenite.With the increase of welding current,the laths of the tempered martensite became coarser,the residual austenite was reduced and there was a small amount of tempered sorbite formed.The partial quenching zone consisted of tempered martensite,residual austenite ferrite and coarse granular pearlite.The columnar dendritic morphology of low current specimens was obvious.The dendrite morphology of high current specimens was more obvious,which growth direction was different,the length of dendrite is small.The root pass of welds was composed mainly of martensite,tempered martensite,residual austenite and a large number of carbide particles,the bottom of the 190 A specimens appeared tempered sorbite.The cover pass of welds was mainly composed of martensite,residual austenite and a large number of fine carbide particles.The particulate in the welds was a kind of complex carbide particle containing Ti,V,Cr,Mn and W elements.According to EDS analysis,the Ti C was the primary phase,and then Ti C acts as nucleates of VC.With the increase of welding current,the hardness of Heat Affected Zone(HAZ)and welds of MIG were decreased.The hardness of cover pass was about 5% higher than that of base metal after high-frequency induction quenching and was about 8.4% lower than that of SMAW surfacing.The tensile strength and elongation of MIG welded joints of different welding currents were higher than that of SMAW welded joints,with the increase of current,the brittleness of welded joints decreased and strength increased.The addition of Ti makes MIG welded joints have higher tensile strength and plasticity.The wear rate of the surface of the surfacing layer with different current showed the same trend with the change of load.The wear rate changed a little under the load between 30N~120N and the wear rate increased significantly of the load exceeded 120 N.Under low load,the wear mechanism of MIG surfacing layer is abrasive wear,wear resistance is better than that of base metal of high-frequency induction quenching.Under high load,the wear mechanism of surfacing layer was oxidation wear and adhesion wear.With the increase of welding current,the oxidation phenomenon was aggravated and accompanied by severe plastic deformation.