Formation and characteristics of white etching layers on austenitic steels

Abstract

Transport by train for passengers and for industrial logistics require advancements on the maintenance and demand a deeper understanding of the damages that influence the railway system. Railways experience significant mechanical and thermal loads at the wheel and rail interface, which demand materials with high durability and to prevent failure mechanisms. While fine pearlitic steel (R260) is the predominant material used in Europe, certain areas such as switches and crossings S&C employ alternative materials like Mn13, Hadfield steel, or bainitic steel to withstand high dynamic impact. By examining the microstructural changes and hardness variations in these railway steels, this study contributes to a deeper understanding of the effects of mechanical deformation and thermal exposure in the wheel and rail interface. These findings can provide information to improve maintenance strategies aimed at preventing failures and increasing the lifetime of railway systems. This project aims to characterise the microstructural changes and evaluate the hardness properties of two railway steels, R260 and Mn13, in the heat-affected region. By subjecting the materials to mechanical deformation and thermal pulses, this study seeks to gain insights into the damage interactions that may occur at the wheel and rail interface. The result confirms that R260 steel transforms to martensite, forming a thin layer known as a white etching layer (WEL) at the surface after thermal pulses. The rolled austenitic manganese steel instead remelts and forms a layer with a combination of dendrite and columnar structure, similar to cast or welded Mn13. Notable observations include the increased presence of segregated carbides along the heat-affected zone, grain boundaries, and solute accumulation in between the dendrites. Microhardness and microstructural evaluations were conducted on both untreated bulk materials and those subjected to deformation and thermal exposure. In the thermally exposed region, the R260 steel shows an increase in hardness, while the hardness instead decreases in Mn13. Insights into what affects the hardness and why the microstructure changes are presented in the report.