Effect Of Heat Treatmen On Microstructure Of Li7MoV Ductile Iron

Ductile iron is widely used in industry because of excellent mechanical and sasting properties as engineering materials.The existcnce of natural solid lubricant graphite in ductile iron makes it have good antifrietion and wear resistance,so it is possible to be used in making hot working die requiring good wear resistance.It is necessary to study the heat treatment and microstructure of alloyed ductile iron in order to meet the requirements of temperature and mechanical properties in hot working die.In this paper,Ni7MoV alloy ductile iron profiles prepared by horizontal continuous casting were taken as the research object.Graphitization annealing was carried out on the as-cast samples at each position by heating furnace,so as to homogenize the composition and microseructure of the as-cast samples.The homogenized sample is quenched by different processes and tempered.By means of metallography,XRD,SEM、EDS、TEM and hardness tests,the microstructure of as-cast and heat-treated samples were analyzed,and the following conclusions were obtained:1.The as-cast microstructure of Ni7MoV ductile iron consisted of graphite spheres,cementite,martensite and residual austenite.From the heart of thc profile to the edge position,the number of cementite increases significantly,and the size increases,and the number of graphite balls increases,the size decreases,and the hardness of the edge position is higher(HRC59).After the graphitiation annealing at 1050℃ for 3h,the microstructure and composition of the as-cast microstrtucture were basically homogenized:the microstructure was composed of graphite spheres,hainite,martensite and residual austenite.2.The microstructure of the sample after quenching is composed of graphite spheres,twin martensite,residual austenite and trace cementite.With the increase of quenching temperature and cooling rate,the size of martensite increases,the content of residual austenite increases,and the hardness of the material decreases,with the lowest hardness being HRC47.3.Tempering different quenched samples at different temperatures.When tempering temperature is less than 400℃,the microstructure is composed of graphite,twin martensite and residual austenite.When the tempering temperature is 500℃,the microstructure of the material is composed of tempered martensite and secondary quenching martensite.The microstructure of the material at 600℃primary tempering is composed of soxsite and secondary quenching martensite,and the microstructure at 600℃ secondary tempering is pure soxsite.The influence of tempering temperature and tempering times on microstructure and hardness of tempering samples within 400℃ is small,and the influence on microstructure and hardness of tempering samples above 500℃ is great.In addition,with the increase of tempering temperature,the hardness of the material decreases slightly,reaching the lowest value at 400℃,500℃,the hardness of the material increases,and then decreases again at 600℃.In addition,the hardness of the sample with large quenching temperature and cooling speed is lower when tempering within 400℃,and the hardness value is higher when tempering at 500℃ and above.When the water-cooled sample is held at 1050℃ for 3h,its hardness reaches the maximum HRC52 when it is tempered once,and HRC47.5 when it is tempered twice.