The Effect Of Mo Content On The Microstructure And Thermal Fatigue Properties Of 5%Cr-based Hot Work Die Steel

Mold is an important corner stone of the developm ent of a country’s manufacturing indu stry,and the develo pment of die has a high er requirement on th e performance of die steel,especially the cold and hot fatigue properti es will directly affect the service life of the mold.This thesis takes 5%Cr hot work die steel as the research object to study the influence of Mo content on the hot and col d fatigu e properties of 5%Cr hot work die steel.The effect of Mo content（1.8%,2.4%and3.1%）on the hot and cold fat igue properties of 5%Cr hot work die steel was studied.The self-constrai ned cold and hot fatigue experimental method s was adopt ed,which combined with scanning el ectron microscope,transmissi on electron microscope,microhardness tester and impact toughnes s test methods to study the effect of three kinds of Mo content on the cold and hot fatigue properties of 5%Cr hot work die steel,and the result s was compared wit h t raditional H13 hot work die steel.The results were as follows:1.The thermodynamic calculati on software of Therm o-Calc was u sed t o simulate the phase transformation character istics of three 5%Cr hot work die steels with different Mo content,and compa red them with H13 steel.Th e results showed that with the increase of Mo content,the ferrite phase region expand s.T he types of carbides in the three experimental steel s an d H13 steels were M ₂₃C₆,MC,M₂C and M6C carbides,respectively.With the increase of Mo content,the number o f M₆C and M₂C carbides in the three experimental steels also gradually in creased.2.The hardness curves of the experimental steels after quenching and tempering were measured.The results showed that th e three experimental stee ls reach ed the second ary hardenin g peak at 510℃,and t he secondary h ardening p eak increased with the increase of Mo content.The seco nd hardening peak of H13 st eel was th e highest.Through the observation and analysi s of the microstructure of the experimental steel,it was found t hat the content of short rod-shaped or ellipsoid al carbides precipitated with the increase o f Mo content.Analy sis of these carbid es b y TEM revealed that t he large-p article carbid es were mo stly M ₆C-t ype carbides.Based on the calcul ation result s,it was specul ated that the fine carbides were mo stly M₂C-type carbid es.TEM analy sis found t hat most of the carbides precipitat ed in H13 steel were MC type carbid es,which made H13 steel hav e a higher hardness.3.Kept the three experimenta l steel s and H13 steel at 6 00℃and 650℃for 24h,it was found that with the increase of Mo content,the resistance to tempering and softening of the test steels increased.Through the observation and analysi s of the microstructure of the three experimental steels,with the increase of Mo content,the number of carbides in th e experiment al steels also in creased,and the m artensite l ath and carbides increased with the incr ease of temperature and the time at high temperature.Compared with the three exper iment al steel s,H13 steel had th e lowest resistance to temp ering and softening.Observing its microstructure,it is found th at its carbide content was less,and the carb ide types were mainly M ₂₃C₆ and MC carbides.4.After 1000 cycles of cold and hot fatigu e（r oom temperature to 600℃）of the three experimental steel s,with the increase of Mo content,the number of surface crack initiation decreases.The number of cracks in the three experimental steel s increased slightly after 2000 cycles.Due to the low toughne ss of the experimental steel of 3.1%Mo and a larg e amount of carbides co arsen ed,when the co arsen ed carbides gath er at the tip of the crack,they became a channel for crack propagation,which cau sed th e crack pro pag ation of the 3.1%Mo experimental steel to b e easier.The growth rate of crack s in H 13 steel after 2000 cycles was higher th an that of th e three experimental st eel s,and the crack dep th was also deeper.