Microstructural characterization of near-surface microstructures on rail wheels in service – an insight into “stratified surface layers”

Background: To decrease maintenance costs and improve safety in rail transportation, the understanding of rail and wheel defects is vital. Studies on “white etching layers” (WEL) on rails and wheels, prone to fatigue crack initiation, have been extensively studied. Recently, a relative named “brown etching layer” (BEL) and its combination, the so-called “stratified surface layer” (SSL), are observed in the field. This study presents an investigation on a rail wheel affected by mechanical and thermal loadings from service with focus on the different evolved layers in the near-surface region. Methods: Optical microscopy is performed on etched cross-sectional cuts to identify different evolved microstructures (WEL, BEL, SSL), further, specific regions are investigated in detail by scanning electron microscopy to evaluate the microstructural characteristics. To analyze the change in mechanical properties, low-load Vickers hardness investigations are executed in distinctive zones. Results: This study highlights the broad variety of evolved microstructures, however, a rough classification of WEL (fine mesh-like microstructure, 900 – 1200 HV0.01) and BEL (globular cementite particles, 400 – 600 HV0.01) is given. Further, results indicate that the BEL is commonly accompanied by a WEL, representing an SSL. Conclusions: The complex loading situation in a wheel-rail contact can lead to the formation of WEL, BEL and SSL. The observation of numerous initiated fatigue cracks within these regions demonstrates the relevance of in-depth studies on evolved microstructures in wheel-rail contacts.