Degradation of railway rails from a materials point of view

Doktorsavhandling, 2013

From time to time railway rails need to be replaced. In that case and when new track is built, a decision has to be made about which rail grade that is going to be installed. The intention of this work was to compare different rail steels concerning their suitability from a mechanical durability perspective. The most common steel types used for rails are presented as well as the most common damages that occur as part of the degradation process. The rail-wheel contact is the primary source for damages. Two types of damages were studied in detail, white etching layers (WEL) and rolling contact fatigue (RCF) cracks. The WEL that was investigated originates from a metro rail with heavy traffic. It can be shown, that the WEL is more brittle than the pearlitic base material of the rail. Also, evidence can be provided for a type of crack that is exclusive to WEL in rails. This crack type is perpendicular to the surface of the rail and can initiate or be connected to other cracks in the base material. Retained austenite measurements in the WEL prove that the microstructure was produced under the influence of high temperatures and suggests that there is martensite present. Rolling contact fatigue cracks from field samples and from test rig samples were investigated. Three-dimensional images were produced. Interaction between the plastically deformed microstructure and cracks could be shown as well as shielding of cracks by other cracks or their own branches. When determining which rail grade is best suited, a holistic view must be adopted, including all relevant influencing factors, such as traffic, track geometry, maintenance regime and climate. The problem of rail degradation cannot be solved by material choice alone.