Influence of thermal loading on mechanical properties of railway wheel steels

Doctoral thesis, 2012

Material integrity and properties of wheels are critical in railway traffic, as wheels fulfil the important function of transferring load and traction from the vehicle to the rail track. Steels with a pearlitic microstructure are commonly used for wheels due to their height strength, low cost and good wear properties. However, the pearlitic microstructure and behaviour can be altered by thermal and mechanical load being present in the contact between wheel and rail. As very high power is available, a few milliseconds of time where the slip between wheel and rail becomes large can cause small material volumes in the contact to be heated several hundred degrees Celsius. The present work was initiated with the main purpose to investigate the effects of rapid thermal heating and cooling on wheel material. Cyclic and monotonic mechanical testing was performed to study the effects of thermal softening on virgin wheel material and on used wheels taken out from service. Furthermore, material in both pearlitic and martensitic state was investigated during rapid heating and cooling cycles by methods as laser and resistive heating for different loading conditions. It was shown that alloying composition of different wheel steels could decrease sensitivity to thermal loads, while plastic deformation had the opposite effect when the material was subjected to long time thermal loading. For the typically very short heating times present in the wheel rail interface no significant permanent effects on mechanical properties were measured for pearlitic material. However, for martensitic material, substantial permanent hardness decrease progressed within fractions of a second at elevated temperature. This rapid tempering behaviour was observed to progress even faster in the presence of an external load. Moreover, the inherent different behaviours of pearlite and martensite can result in different residual stress fields for the case of local heating on the tread surface, affected by heating rate, peak temperature and duration. Some additional effects of frictional heating and the influence of wear debris within cracks for rolling contact fatigue cracks were also investigated by use of image and chemical analysis.