Search

The development of premium heat treated rail steels with hardness levels of 400HB and above, and the application of these under high axle load conditions, has provided substantial improvements in rail lives. An additional benefit is the potential to increase the intervals between consecutive grinding cycles, where rail grinding is used to control rolling contact fatigue damage. Increased grinding intervals will, however, place more emphasis on the performance of rail welds which are more sensitive to localised rolling contact fatigue damage than parent rail. The increased sensitivity to damage at welds arises from the inherent variability in material characteristics, and may vary between welding procedures, and for any particular welding process, between rail grades. A detailed microstructural examination of the characteristics associated with flashbutt welds in the new condition and after exposure to service conditions which resulted in gauge corner cracking has been performed in order to correlate the development of rolling contact fatigue damage to the underlying microstructure. Of particular interest were the behaviour of the softened (heat affected) zones and the role of grain boundary networks of pro-eutectoid cementite which may develop during welding of hypereutectoid rail steels. Understanding the effects of these microstructures on the material response to wheel rail contact conditions, and using this information to develop improved welding procedures or modifying rail maintenance procedures to minimise their effects, are necessary in order to maximise the benefits obtained through use of these rail steels. Gauge corner cracking was found to initiate in the softened zone associated with spheroidised microstructure. The role of pro-eutectoid cementite network on crack initiation was difficult to evaluate, however, it seems to have only a minor effect on crack propagation. The proposed options to mitigate gauge corner cracking was to make narrow welds to minimise softened zone or to select rail grades that are more resistant to spheroidisation. [ABSTRACT FROM AUTHOR]

• The FaStrip algorithm is modified to estimate elastic-plastic traction distribution. • Softened zones of rail welds experience the highest ratcheting strain rate. • Ratcheting strain rates in the sampled 2000 m radius curved track are higher than those in the sampled tangent track. • The RCF initiation location is highly dependent on the magnitude and pattern of traction distribution. Ratcheting at the rail heads due to cyclic rolling contact stresses is often observed in heavy haul railways, especially when there are large steering forces developed in low-radius curves. Additionally, ratcheting behaviour can be exacerbated when associated with material strength loss in the heat affected zones of rail welds. However, the numerical evaluation of this failure mode requires a sophisticated representation of elastic–plastic contact pressures and traction distributions, which, to the best of the authors' knowledge, cannot be directly measured by any existing tools. In this paper, in order to accurately evaluate the ratcheting performance at rail welds in curved tracks, attempts were conducted to modify the FaStrip algorithm to analytically estimate the traction distributions based on contact pressures and creepages obtained from static finite element analysis and multi-body dynamic simulations, respectively. Cyclic rolling contact was then simulated by repeatedly applying contact pressure and traction on the rail surface consisting of both the rail weld and the parent rail regions. The weld region was divided into 23 sub-zones in the rolling direction to represent the material inhomogeneity within the heat affected zones. Cases on a 2000 m radius curved track and a tangent track were studied for comparison. It was found that the ratcheting strain rate in curved tracks can be significantly elevated due to the higher magnitude of traction, and the location of the elements with the highest ratcheting strain rate was dependent on the traction distribution patterns. The proposed method provides a general tool for predicting rolling contact fatigue initiation life and location, which will benefit the maintenance efficiency of heavy haul rail welds in curved track. [ABSTRACT FROM AUTHOR]