Surface metallurgy and rolling contact fatigue of rail
This thesis presents the results of an investigation into the effect of surface metallurgy on the rolling contact fatigue behaviour of rail. The investigation has used laboratory based rolling/sliding twin disc testing of samples with a surface metallurgical feature simulated on them. The samples used in laboratory testing have been compared with samples of rail removed from track. Two surface metallurgical features have been investigated: decarburisation and white etching layer. Decarburisation is the loss of carbon from the surface of the rail due to oxidation at high temperatures, resulting in a softer layer at the surface (180HV compared to 250HV bulk). The decarburised layer has been simulated in this research by heat treating discs in a laboratory furnace with an air atmosphere. The results show that by increasing the depth of decarburisation the growth rate of cracks within the sample, along with the wear rate, increases. At the maximum depth of decarburisation allowed on rail by the standard (O.5mm) there was little difference in the wear or rolling contact fatigue behaviour with or without decarburisation. White etching layer (WEL) forms on the surface of rail due to the action of the wheels and is a very hard layer (>850HV) up to 250mm deep. White etching layer has been simulated in two ways: spot welding and gross sliding of the discs. The results have shown that cracks initiate preferentially at weak spots at the surface, such as the interface between the WEL and pearlite or along proeutectoid ferrite boundaries. It has been found that the growth of cracks below the surface depends on the strain history of the subsurface pearlite. The results indicate that white etching layer is detrimental to rail life through either the promotion of rolling contact fatigue and/or wear.