Microstructural evolution of white and brown etching layers in pearlitic rail steels

Abstract

The formation of White (WEL) and Brown Etching Layers (BEL) on rail raceways during service causes the initiation of microcracks which finally leads to failure. Detailed characterization of the WEL and the BEL in a pearlitic rail steel is carried out from micrometer to atomic scale to understand their microstructural evolution. A microstructural gradient is observed along the rail depth including martensite, austenite and partially dissolved parent cementite in the WEL and tempered martensite, ultrafine/nanocrystalline martensite/austenite, carbon saturated ferrite and partially dissolved parent cementite in the BEL. Plastic deformation in combination with a temperature rise during wheel-rail contact was found to be responsible for the initial formation and further microstructural evolution of these layers. The presence of austenite in the WEL/BEL proves experimentally that temperatures rise into the austenite range during wheel-rail contact. This is in agreement with finite element modelling results. Each wheel-rail contact must be considered as an individual short but intense deformation and heat treatment cycle that cumulatively forms the final microstructure, as shown by diffusion length calculations of C and Mn. The presence of secondary carbides in the BEL indicates that the temperature in the BEL during individual loading cycles reaches levels where martensite tempering occurs. Partially fragmented primary cementite laths, enriched in Mn, depleted in Si, and surrounded by a C-gradient and dislocations were found in the BEL. The initial step in the formation of BEL and WEL is the defect-and diffusion-assisted decomposition of the original microstructure.

In situ study on fracture behaviour of white etching layers formed on rails

“…However, recent insights have shown that WELs form by the combination of the temperature rise above the austenite start temperature and the plastic deformation at the rail raceway [17][18][19]. Still, there is considerable debate among the research community concerning the WEL formation mechanism.…”

Pre-cracking development of weld-induced squats due to plastic deformation: Five-year field monitoring and numerical analysis

“…The effects of this evolution on the development of surface irregularities is another topic for further research. Further research should also investigate whether white etching layers occur on rail surface around welds and how they influence the hardness variation since they are sometimes observed on rail surfaces, e.g., [45]. Moreover, the material of WM is considered the same type as the parent rail steel.…”

The role of electric current in the formation of white-etching-cracks

“…As a result, depending on the operation conditions and/or types of steels, WECs can be generated in different ways. For example, in the case of rails, due to elevated temperature and higher lateral shear forces, WEAs (in this field usually called whiteetching-layers, WELs) form on the surface before crack initiation occurs in the WELs or at the interface to the base material [3]. WEAs can also be generated by cyclic heating alone [4], or by adiabatic heating due to ballistic impact [5].…”