Tailored carburization gradient microstructure and enhanced wear properties of M50NiL steel via introduced prior cold rolling

M50NiL (Chinese grade G13Cr4Mo4Ni4V) steel is widely used in aircraft bearing raceways that require excellent wear resistance under harsh service conditions. However, the impact of cold rolling, as an advanced forming process for bearings, on the carburization process and wear properties is still unknown. Therefore, the gradient microstructure and dry sliding wear performance of the cold-rolled M50NiL disc against Si3N4 ceramic ball have been investigated. The results show that with the increase of cold rolling reduction to 40 %, the carbon concentration of surface is increased by 17.1 % due to the growing percentage of grain boundaries (GB) and dislocation density. The carbide content is increased by 29 % with average size declined by 21.2 %, which contributes to a 7.4 % increase in the surface hardness. Additionally, the wear test indicates that the average friction coefficient and wear rate are respectively reduced by 17.3 % and 47.1 % when the rolling reduction increases to 40 %. After prior rolling, the wear mechanism changes from severe abrasion to adhesion and tribochemical reactions accompanied with the enhancement of wear properties, which is mainly attributed to the uniform distribution of finer carbides with higher fraction on the surface and subsurface.