Three-dimensional (3D) analysis of white etching bands (WEBs) in AISI M50 bearing steel using automated serial sectioning

Dark etching regions (DER) and white etching bands (WEB) form in the subsurface stressed regions of bearing raceways and rolling elements. Also, “butterfly” formations form around non-metallic inclusions and large carbides. The formation of WEBs, DERs and butterfly are dependent on operating conditions, material alloying elements and heat treatment. The DER, WEB and butterfly formations are well studied in two dimensions (2D). The objective of this study was to analyze WEB formation in three dimensions (3D) that will allow better understanding of material transformation after stress cycling. Three rolling contact fatigue (RCF) tested vacuum induction melted vacuum arc re-melted (VIM-VAR) AISI M50 samples (from a bench test, sub-scale test and commercial gas turbine engine) were analyzed using automated serial sectioning to map WEB networks and a butterfly formation in 3D. Automated serial sectioning and 2D optical microstructural data for 3D reconstructions and analysis was done using a fully automated serial sectioning system – Robo-Met.3D®. 3D analysis was performed using Image-Pro Premier 3D. The data set allowed 3D visualization and analysis of the evolution and propagation of WEB and butterfly in M50 bearing steel, including the positive microscopic identification of a ceramic inclusion that was the source of a WEB.