Severe Plastic Deformed Zones and White Etching Layers Formed During Service of Railway Wheels

Abstract

The near-surface regions of rail wheels experience a complex thermo-mechanical loading, which varies along the location of the tread and causes severe changes to the microstructure. Occasionally, brittle white etching layers (WEL) are formed, representing a high risk of wheel damage. Therefore, we studied the depth-evolution of the surface-near microstructure along different regions of the tread of a wheel being ~ 200,000 km in service. The microstructural variations point toward a higher thermal loading history in the middle of the tread and predominant mechanical loading in the neighboring regions, where a severe plastic deformed microstructure with rolling contact fatigue cracks is present. Varieties of WEL-like microstructures were investigated, the presented analysis contributes to the knowledge of their microstructural characteristics and outpoints these regions in terms of potential crack initiation sites on railway wheels.

Influence of thermal loading parameters and microstructure on the formation of stratified surface layers on railway wheels

“…The settings for the laser system are based on previous work from the authors 19 on laser surface treatments to introduce realistic WELs. The thicknesses of the initiated SSLs (∼100–500 µm) within the current work are related to previous findings of the authors 2,26 when analysing samples from the field.…”

“…39 A comparable effect is suggested for plastic deformation on the wheel treads in service, which leads to grain refinement of the near-surface microstructure. 26,[40][41][42][43] The chemical composition of the steel is expected to affect phase transformations [44][45][46] as it occurs when thermally inducing an SSL. Within this study the sample with the chemical composition of standard wheel steel grade ER7 shows an increased SSL thickness compared to the sample with the chemical composition of wheel steel ER-MA.…”

“…Based on this systematic study the formation process of comparable microstructures observed on wheels and rails from the field 1–4,6,30 can be better understood, even though the thermal loading history of samples from the field is always unclear. However, the presented study is focussing on the thermal formation process, mechanical loadings leading to a massive change of the near-surface microstructure 18,26,31–33 are not considered. Deformation of the near-surface wheel microstructure before/during the thermal induced transformation process is affecting the morphology of the formed WEL and BEL, which is shown in previous work by the authors.…”

“…39 A comparable effect is suggested for plastic deformation on the wheel treads in service, which leads to grain refinement of the near-surface microstructure. 26,40–43…”