Thermal Crack Development and Wheel Useful Life

The impact of thermal crack development on the wheel useful life was investigated. The actual data from the “Railroad” has been actively and intensely reviewed. Initial efforts were concentrated on the wheel metallurgy, thermal crack development mechanism and maximum allowable operating speeds on each line. Recent efforts have concentrated on the braking control and associated equipment configuration. The effect of worn wheel and repetitive braking was also investigated. However, it is difficult to postulate alternative approaches to managing wheel useful life without completely identifying existing operating policies and maintenance procedures, and without establishing measurable results of such policies/procedures. It is also crucial to understand the physical process involved. This process will be repeated with every brake application in combination with the residual compressive stress (due to rolling and battering of the wheel on the rail and by rim-quenching) causing the cyclic stressing of the material due to thermal expansion, i.e., “thermal fatigue”. Higher brake rate and higher tread to disc brake ratio lead to higher temperature gradient and higher compression stresses (higher portion of stress in plastic range). In addition, the microstructure change in the surface zone material involves additional uncertainty, i.e., the existence of material of unknown physical properties (perlite transformation in to spheroidite). The presence of spheroidite is a good indication that a wheel tread has been subjected to direct thermal damage. It is a common conclusion based on two completely independent research projects that the combination of heat and compression increases hardness and yield strength. The maximum wheel temperature has a major impact on microstructure changes of the wheel surface layer. Also a smaller wheel diameter yields to higher maximum tread temperature. Various studies relating new and worn wheel clearly show the impact.