Study On High Temperature Service Behaviour And Its Mechanism Of DM Hot Forging Die Steel

To clarify the microstructure evolution of hot forging die steel during service,and improve its service life.The microstructure and performance characteristics of DM steel with high temperature stability were systematically investigated by means of DIL805A linear dilatometer,scanning electron microscopy(SEM),transmission electron microscope(TEM),UMT-3 type high temperature friction and wear test system,thermal fatigue tester and so on.The evolution and kinetics of carbides of the hot forging die steels during tempering process were explained.The mechasim and alloying principle of DM steel with high temperature stability were revealed.The high temperature wear resistances of the hot forging die steels were compared and investigated.The mechanism and influence factors of DM steel were expounded.A numerical model was established to investigate service life during the hot-froging process.In addition,the thermal fatigue properties and micromechanism were analyzed.The main results are as follows:(1)Minium toughness tempering temperature of DM steel is 600 ?.When tempering temperature is lower than 600 ?,the optimum heat treatment process is austenitizing and quenching at 1060 ? then following tempering at 540 ? twice for 2 h,the average hardness is 50.1 HRC.When tempering temperature is higher than 600 ?,the optimum heat treatment process is austenitizing and quenching at 1030 ?then following tempering at 620 ? twice for 2.h,the average impact energy is 196.5J.(2)TEM observations revealed that the substantial amount of M2C carbides precipitates at 650 ?,and responsible for the tempering stability of the hot forging die steels.The activation energies of the M2C carbide precipitation processes in DM,H21,and H13 steels are 236.4,212.0,and 228.9 kJ/mol,respectively.In addition,the results indicated that vanadium atoms both increase the activation energy and delay the evolution of the M2C carbides.Furthermore,combining the differences of balance carbon parameters and the value electron structures in three steels,DM steel with excellent thermal stability shows the best alloying elements ratio and electron structure.(3)High-temperature wear test showed that DM,H21 and H13 steels appear similar wear type evolution.Mild-to-severe oxidation wear transition occurred with an increase of the test temperature and duration.The reason was clarified as the unstable M6C carbides coarsening should be responsible for the severe delamination of tribo-oxide layer.DM steel with high caribides stability has a higher wear resistance than H21 and H13 steels.More importantly,an intense oxidation wear with lower wear rates was found when the experimental temperature reaches 700 ? or after 4 h of test time at 600 ?,which was closely related to the degradation behavior during wear test.Compared H21 steel occurred at 650 ?,high temperature stability delay the transformation of DM steel to the intense oxidation wear.(4)Both the scanning white light interferometer and the scanning electron microscope were used to observe the surface wear states of hot forging die,based on the abrasive wear model developed by Archard,the thermo-mechanical coupled FE model of the hot forging process was built to analyze the wear depth of die.It results demonstrated that three types of wear mechanism exist on the worn surface,namely adhesive wear,oxidation wear and abrasive wear,the maximum wear depth was about 100 ?m.According to the numerical results,when the initial temperature of die was 250 ? and the forging rate was 300 mm/s,the maximum die temperature after forging was lower than the allowable temperature(about 620?),as well as produce the minimum wear depth of die after forging.Also,compared with H13 steel,when the die steel changed to DM,the maximum wear depth of die dropped from 9.6×10-5mm to 4.1×10-5mm.The service life of die was increased from 1041 to 2439 cycles and improved by 2.34 times.While chose H21 steel,increased the wear depth and reduced the service life.The experimental result has a good agreement with the predicted one.(5)Based on the Uddeholm self-restricting thermal fatigue test,the short-time cyclic softening in thermal fatigue was controlled by dislocation rearrangement and annihilation.The long-time softening was affected by the material's temper resistance and strongly depended on the carbide morphology and their over-ageing resistance.Thermal fatigue damage factor of DM steel reaches the minimum value 0.023 and its thermal fatigue property is obviously better.