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.