Formation and Evolution Mechanism of Voids in M50 Bearing Steel During Thermal Deformation

M50 bearing steel is widely used in the manufacture of aeroengine spindle bearings. The voids generated by the thermal processing of bearing steel can easily initiate fatigue cracks and lead to fatigue failure of the bearings. Thus, it is essential to understand the steel production conditions, void distribution in the steel, and effect of the subsequent treatment on the healing process of voids to improve the thermal processing and mechanical properties of the steel. In this work, the thermal deformation of the M50 bearing steel was conducted using a thermal simulation machine. The effects of the strain rate (0.001-1 s(-1)), deformation temperature (1000-1150 degrees C) and strain (10%-50%) on the formation of voids and void healing during the subsequent thermal treatment were systematically studied using OM, SEM, EBSD, and in situ scanning methods. The results show that the formation of voids between the carbide and matrix is attributed to the different hardness values between the matrix and primary M(2)aC and MC carbides. In addition, the carbide fractures can promote the formation of internal voids. The quantitative analysis of the voids indicated that most voids are generated under the following conditions: a high strain rate of 1 s(-1), low deformation temperature of 1000 degrees C, and medium deformation of 30%. Applying a heat treatment after deformation can significantly promote the void healing process, and the Cr element is enriched in the healing zone due to its rapid diffusion in gamma- Fe.