Study On Vacuum Carburizing Method And Dispersed Carbides Of 18CrNiMo7-6 Gear Steel

18CrNiMo7-6 steel is a kind of surface hardening alloy structure steel,which has the advantages of high toughness,high strength and good hardenability.After carburizing,not only the surface has high hardness and wear resistance,but also the core has high hardness and toughness.In recent years,the vacuum low-pressure carburizing and high-pressure gas quenching technology has developed rapidly,which can make the carburized structure more fine and precise,and greatly improve the carburizing efficiency and effect on the basis of saving energy and reducing pollution emission.When carburizing,once the carbon potential is slightly higher,the surface will form disadvantageous carbide shape,which will affect the plasticity and toughness of the carburized layer severely.If the carbon potential is low,the plasticity and toughness of the carburized layer can be effectively controlled,but the strengthening effect of the carbide is greatly reduced.Both theory and practice have proved that fine and dispersed carbides are beneficial to the strength and toughness of carburized layer.If we combine vacuum low-pressure carburizing and high-pressure gas quenching technology to study the carburized layer carbide of 18 Cr Ni Mo7-6 steel and give full play to its beneficial role,it will be an economic strengthening technology way.Therefore,firstly,this paper uses the methods of vacuum low-pressure carburizing and high-pressure gas quenching to study the law of microstructure evolution of18 CrNiMo7-6 steel during carburizing and quenching cooling,explore the influence of different vacuum heat treatment processes on the microstructure of 18 Cr Ni Mo7-6carburizing layer,select a reasonable vacuum heat treatment process,and testing the process on the workpiece.The results show that after the vacuum low-pressure pulse carburizing and high-pressure gas quenching which selected 960℃ carburizing temperature,42 min carburizing time,100 min diffusion time,6bar gas quenching pressure,the shaft workpiece with 60.3-62.9HRC surface hardness,35-38.5HRC core hardness,0.90-0.92 mm hardened layer depth and residual austenite and martensite grade no more than grade 3 was obtained.The structure and properties achieved the process requirements.After that,through different isothermal quenching process tests on 18 CrNiMo7-6steel with high carbon content on the surface after vacuum carburizing,the evolution law of carbide structure in different process is studied,and how to obtain carbide with fine particles,round shape and dispersed distribution on the surface of carburized steel is explored.The results show that after austenitizing at 980 ℃ for 30 minutes,holding at600 ℃ pearlite zone for 20 minutes,heating to 830 ℃ and holding at two-phase zone for20 minutes,and vacuum gas quenching to room temperature at a cooling rate of 20 ℃ / s,the dispersed carbides with diameter less than 0.1μm were found in the crystal.At the same time,the massive carbides with diameter of 0.4-1μm were also found.Meanwhile,the length of martensite needle is refined from 12～16μm to 4～8μm of the original structure.The average hardness of the carburized layer of the test steel increased from800 HV to about 850 HV,and the depth of the hardened layer increased from 1.22 mm to1.40 mm.