Research On High Temperature Solid Solution,quenching And Tempering Heat Treatment Of 4Cr5Mo2V Diecasting Die Steel

4Cr5Mo2V steel has excellent hardenability,toughness,thermal stability and thermal fatigue properties.It has been widely used in manufacture of die casting dies.The spheroidizing annealing process of 4Cr5Mo2V steel is basically the same in different steel mills in China.Two stage annealing process is adopted.However,the high temperature solution treatment is not the same before spheroidizing annealing process,resulting in different annealing microstructures,which directly influences the final quenching and tempering microstructure,and leads to the fluctuation of service performance and unstable life of the dies.By the solid solution+spheroidizing annealing heat treatments,eliminating carbides which precipitated along the slow cooling after forging process.The carbides are dispersed homogeneously in the ferrite matrix,and excellent spheroidizing annealing microstructures are obtained.Good annealing microstructure is not only beneficial to improve the efficiency of rough milling,but also determines the performance and life of die steel.However,there is no reasonable high temperature solution process of 4Cr5Mo2V steel up to now.At the same time,the rough milling of 4Cr5Mo2V steel is difficult in engineering applications.In this study,in order to determine reasonable solution temperature,effect of different solution temperature and holding time with two stages isothermal spheroidizing annealing process on annealing microstructure was studied.Thermal stability,toughness and thermal fatigue properties and end milling performance were also studied.The main research results are as follows:In order to obtain the reasonable solution process,the effect of solution treatment on the spheroidizing annealing microstructure was studied.The solution temperature has a significant influence on the grain size of 4Cr5Mo2V steel.When soluting at1030?,grain size level is 6.However if the solution temperature is higher than 1030?,the grain size level is less than or equal to 5.After solution treatment at 1030?and1060?,the spheroidizing annealing microstructure is rated at AS5 and AS6 by North America Die Casting Association#207-2006?NADCA#207-2006?.Its impact energy is407J and 403J,38%higher than that before solution treatment.By thermal fatigue test,it was found that,the thermal fatigue property of 4Cr5Mo2V steel after solution treatment at 1000?is the worst,and the thermal fatigue property after solution treatment at other temperature have little difference.Therefore,the best solution process of 4Cr5Mo2V steel is 1030?holding 1h.The evolution of carbides in three different states of solid solution,spheroidizing,annealing and tempering of 4Cr5Mo2V steel has been analyzed by XRD.The results show that When soluting at 1030?and 1060?,the undissolved carbides are VC,V₆C₅ and V₈C₇,and with the increasement of solid solution temperature,carbide is almost dissolved in the matrix.The main types of carbides in spheroidizing annealing microstructure are VC?V₆C₅?V₈C₇?M₇C₃ and M₂₃C₆.The analysis of carbides in quenched and tempered 4Cr5Mo2V steel shows that precipitation of VC carbides when tempering at 500?results in the secondary hardening phenomenon when tempering at 520?.By thermal stability test,it is found that 4Cr5Mo2V steel has the best thermal stability when quenching at 1030?.When quenching at 1030?and tempering at600?,hardness of 4Cr5Mo2V steel is 45HRC and impact energy is 347J.The phenomenon of sticking blade occurred in the process of rough milling of4Cr5Mo2V steel,which is the result of many factors,such as annealing hardness,tool selection,milling parameters and so on.In order to obtain high efficiency milling parameters and corresponding tool brand,four brands of end milling tools and four sets of milling parameters are compared.The results show that hardness of annealing state material has no obvious influence on rough milling performance.The final parameters are1)=0.05mm/z,V=126m/min,S=5000r/min,ap=4mm,ae=1.2mm.It has no serious sticking phenomenon occurred during the milling process.The longest life of tool reaches 4h.