Study On Vacuum Carburinzing Processes Of M50NiL Steel

Bearing,as a rotating body supporting mechanical work,is widely used in aerospace,ships and automobiles and other major equipments,known as the"joint"of mechanical equipment.M50NiL steel is a new type of high temperature bearing steel,which is mainly used to manufacture aeroengine spindle bearings.The vacuum carburizing process of M50NiL was studied in this paper,in order to obtain the actual production process of bearing rings satisfying the technical requirements（surface hardness to HRC60-63,center hardness higher than HRC40-47,carburizing layer hardness,carburizing layer content 0.85-1.05%,hardening layer thickness 1.8-2.2 mm）.For variable temperature vacuum carburizing,the effects of pulse times,carburizing time and tempering times on the microstructure and properties of the carburized layer were investigated.Decreasing the carburizing time and increasing the pulse number can increase the rate of carbon diffusion to avoid the appearance of coarse carbide network in the carburized layer,but the carbon content no matter on the surface or center all slightly reduced,while the carbon concentration gradient is gentle,and the hardness curve is relatively gentle.With the decomposition of retained austenite,the microstructure is refined and the surface hardness is improved.The grain size,surface hardness and effective depth of carburizing layer all meet the technical requirements,but the surface carbon content does not meet the requirements.During the isothermal vacuum carburizing,the influences of pulse number and diffusion time on the microstructure and properties of the carburized layer were studied.Increasing the number of pulses can increase the carbon content at different positions of the carburized layer,increase the effective depth of the carburized layer and improve the hardness.Decreasing the diffusion time will result in the aggregation of carbon atoms on the surface and a great gradient of carbon concentration in the heart.During the subsequent heat treatment,the carbon concentration gradient will continue to diffuse to the interior,resulting in a relatively gentle carburized layer.The grain size,surface carbon content,surface hardness and effective carburized layer depth of the sample meet the technical specifications.The isothermal vacuum carburizing process 4 meeting the technical requirements is tested.After treatment,the microstructure of the carburized layer of the bearing ring is tempered sorbate and grain size of the core is not less than 4 grades.The surface carbon content is 0.95%,the surface and center hardness are 690 HV₁ and 462 HV₁,respectively.The effective hardening layer depth is 1.9 mm,and the deformation is less than 0.18.The above values meet the technical requirements.