Studies Of Heat Treatment And Laser Surface Modification Technologies Of 4Cr5Mo3V Steels

4Cr5Mo3V steel is a kind of hot working die steel which has been independently researched and developed in China.It has good tempering resistance stability,high temperature strength and thermal fatigue resistance.It is used as a thermal forging punch in the forging production of rolling bearing rings.In the process of production,it withstands a high temperature around 1000?,a large impact force and friction,thus thermal fatigue,wear and fracture failure will happen.In order to improve its service life,this paper analyzed the mechanical properties,microstructure,thermal stability and thermal fatigue properties of 4Cr5Mo3V steel at different quenching and tempering temperatures,as well as studies on the laser surface modification quenching process.Metallurgical microscope,scanning electron microscope and hardness tester are used to draw the following conclusions:?1?Quenching was carried out at 980?,1000?,1010?,1020?,1030?,1040?and 1060?,respectively.With the increase of quenching temperature,the hardness value of 4Cr5Mo3V steel showed a trend of increasing first and then decreasing.When the quenching temperature is 1040?,the hardness value reaches the highest 60 HRC.?2?Tempering at different temperatures was carried out under the condition of1030?quenching.It was found that the microstructure was tempered martensite structure at tempering temperature below 400?.With the increase of tempering temperature,the lath martensite further decomposes.After reaching 650?,it is transformed into tempered sorbite structure.Moreover,with the increase of tempering temperature,the hardness value first decreases,then increases and then decreases.Secondary hardening occurs at 550?,with the highest hardness value reaching 52.8HRC.?3?Quenching at 1030?and tempering at 450?,500?,550?and600?,respectively,the impact energy value of the sample first decreases and then increases with the increase of tempering temperature.After 500 and 1000 cycles of cold and hot fatigue,transverse cracks were found on both sides of the specimen.With the increase of cycles,the depth and length of cracks increased significantly.?4?After heat treatment at 1030?+550?for 4-48 h,the hardness decreases by 8.1 HRC,the lath martensite coarsens gradually,and the size and quantity of carbide particles increase slightly.?5?After laser treatment,the surface of quenching layer is relatively flat macroscopically.When the scanning speed is 6 mm/s and 15 mm/s respectively,and the laser power is 3500 W,the highest hardness value is 815.5 HV_0.2.2 and 818 HV_0.2,respectively.The hardness of laser quenching is about four times higher than that of substrate(200 HV_0.2).?6?The laser hardened layer has compact and fine structure,and has no visible defects such as cracks and pores.The hardened layer surface under each power condition is a fused layer structure composed of fine dendrites,which indicates that the sample surface melts at scanning speeds of 6mm/s and 15 mm/s.However,when the scanning speed is 15 mm/s and the laser power is 2500 W,the hardened layer is only composed of hardened layer without fused layer.