Formation and Evolution of White and Brown Etching Layers and Their Impacts on Rail Degradation

The thin White Etching Layers (WELs) commonly exist on the surfaces of different rail grades in railway networks worldwide. Although the formation of WELs has been the subject of interest for a few decades, precise conclusions on their formation mechanisms are still in argument. Two popular hypotheses for WEL formation have been proposed concerning thermomechanically/thermally-induced and mechanically-induced WELs, respectively. Each type of WELs was formed in different operating conditions and had different microstructure, hardness, composition, and WEL/pearlite interface features. The investigations on ex-service rails reveal that WELs promote crack initiation and propagation due to their brittleness, high hardness, ultra-fine microstructure, residual compressive stress, the difference in thermal expansion, and abrupt transition behaviors at WEL/pearlite interface. The sub-surface cracks developed from the WELs stimulate rolling contact fatigue (RCF) defects, such as squat, stud, and spalling, which is detrimental to railway transportation safety. In addition, WELs on rail surfaces play a critical role in varying the tribology behaviors at the wheel and rail interface. Hence, it is essential to investigate the formation and evolution of WELs on the rails and their impacts on rail degradation, including crack initiation, propagation, fracture failure, friction, wear, etc. Two rail grades, Standard Carbon (SC) rail and Head Hardened (HH) rail, are widely installed for mixed passenger and freight transport. Extensive field monitoring, laboratory experiment, and material characterization were carried out to systematically investigate the formation mechanism of WELs on SC and HH rails. The long-term field monitoring provides evidence that the formation and distribution of WELs are influenced by repeated rolling contact during the service, and HH rail presents a susceptibility to form WELs more than SC rail. The characterization of thermally induced WELs introduced by the laboratory Gas Tungsten Arc Welding (GTAW) experiment indicates that WELs formed on HH rail lead in size and hardness compared to SC rail. The formation kinetics of WELs on different rail grades, such as the austenite growth rate and dissolution rate of cementite, is significantly different due to the interlamellar variation in SC and HH rails.