The Process of Spalling in Railroad Wheels

Although the term “Spalling” means different things to different disciplines and product types, for railroad wheels the term is used for the process by which tread cracks form as a result of a sliding event. The process includes rapid heating and austenitizing of the tread surface during a slide followed by rapid cooling and transformation to untempered martensite. Preexisting cracks in the area of a slide can grow from shallow and harmless cracks into cracks of greater significance due to high thermal and transformation stresses. Case crushing of the tread caused by high loads can also develop into spalls. Lastly, rolling contacts can cause fatigue cracks to form at the edges of a martensite patch in the heat affected zone. A complex combination of lower material strength and higher residual and applied stresses and the limiting hardenability of carbon steel produce conditions ideal for the formation of fatigue cracks. This investigation uses finite element analyses and laboratory tests to characterize the process of spalling.