Characterization of Surfaces Obtained by Precision Hard Turning of AISI 52100 in Relation to RCF Life

Traditionally, components of hardened steels, such as bearings, gears, shafts and rails, are finished by grinding process. In this study, Precision Hard Turning (PHT) is proposed as an alternative finishing process to manufacture AISI 52100 bearing components (60-62 HRC), because PHT improves surface integrity and therefore increases the Rolling Contact Fatigue (RCF) life. An experimental design is used, under dry condition using cubic Boron Nitride (c-BN) cutting tools, to investigate the effect of cutting parameters on surface integrity characterised via surface roughness, microstructure analysis and residual stresses. Then, fatigue life tests are performed on a twin-disk machine. SEM observations of transversal cross-sections of all samples show the presence of a very fine and white layer (<1 μm) on the top surface and a thermal affected zone of 40-50 μm in the sub- surface. PHT does not affect quantitatively the percentage of the different microstructural phases and leads to decrease the number of dislocations in the transition zone compared to the bulk material, which is correlated to decrease of nanohardness in the transition zone. Compressive residual stresses are measured in surface and sub-surface and their level is more in compression in the circumferential direction than in tangential direction. Levels of roughness decrease as cutting speed increases. RCF life of bearing steel components machined by PHT reached 5.2 (at Ra=0.11 μm) and 0.32 million cycles (at Ra=0.25 μm). Therefore, RCF life increases as the roughness amplitude Ra decreases.