Multiscale microstructural investigations of white and brown etching layers initiating the squat formation in pearlitic rail steels

Abstract

This study investigates the microstructural evolution of a pearlitic railway rail affected by squat defects, focusing on the microstructural gradients in the white and brown etching layers (WELs and BELs) below the rail surface. A multiscale characterisation of the rail microstructure is conducted using a combination of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy with Automated Crystal Orientation Mapping (ACOM-TEM) mode, and Atom Probe Tomography (APT). The findings reveal that the microstructural gradient in the transformed layers consists of nanograins of carbon-saturated ferrite, twinned martensite, retained austenite, decomposed cementite, and secondary carbides. The presence of retained austenite suggests a thermal formation of the observed microstructure. The interaction between thermal driving forces and the effects of cumulative plastic deformations caused by wheel–rail interactions is discussed. It is suggested that while thermal forces drive the formation of the WEL/BEL microstructure, the tribological surface transformation (TST) occurs according to local equilibrium conditions. Specifically, the thermal driving force is enhanced by chemical driving forces resulting from the nanostructuring process of the rail microstructure prior to the WEL/BEL formation. Over subsequent wheel–rail contact cycles, the pearlitic microstructure experiences nanostructuring, leading to the formation of a dense network of grain boundaries, the fragmentation and decomposition of cementite, and the diffusion of carbon atoms (C) into ferrite by C-defects interactions. These transformations create a chemical driving force that lowers the energy barrier for austenite nucleation, ultimately resulting in the formation of martensite.