Abstract
Thermal or thermo-mechanical loading is one of the major causes of wheel surface damage in Australian heavy haul operations. In addition, multi-wear wheels appear to be particularly sensitive to thermo-mechanical damage during their first service life. Such damage can incur heavy machining penalties or even premature scrapping of wheels. The combination of high contact stresses as well as substantial thermal loading (such as during prolonged periods of tread braking) can lead to severe plastic deformation, thermal fatigue and microstructural deterioration. For some high-strength wheel grades, the increased sensitivity to thermo-mechanical damage observed during the first service period may be attributed to the presence of a near-surface region in which the microstructure is more sensitive to these loading conditions than the underlying material. The standards applicable to wheels used in Australian heavy haul operations are based on the Association of American Railroads (AAR) specification M-107/M-208, which does not include any requirements for microstructure. The implementation of acceptance criteria for the microstructure, in particular that in the near-surface region of the wheel, may be necessary when new wheels are purchased. The stability of wheel microstructures during thermo-mechanical loading and the effects of alloying elements commonly used in wheel manufacturing are reviewed. A brief guide to improving thermal/mechanical stability of the microstructure is also provided.
Highlights
Axle loads used in Australian heavy haul operations currently range from 35 to 40 tonnes
For some high-strength wheel grades, the increased sensitivity to thermo-mechanical damage observed during the first service period may be attributed to the presence of a near-surface region in which the microstructure is more sensitive to these loading conditions than the underlying material
The standards applicable to wheels used in Australian heavy haul operations are based on the Association of American Railroads (AAR) specification M-107/M-208, which does not include any requirements for microstructure
Summary
Axle loads used in Australian heavy haul operations currently range from 35 to 40 tonnes. The service conditions impose high contact stresses and severe thermal loads on the wheel rim. It is well known that thermal inputs can be generated from wheel lockup (skid events), friction drag braking, traction forces, curving, hunting oscillation, etc., due to slip or creepage from wheel–rail interaction and wheel to brake block friction [1–3] The combination of these thermal and mechanical loading conditions to the wheel rim can result in degradation or failure from one or more of the following modes:. TMS: Softening of the microstructure due to thermal loads occurs by spheroidisation of the near-surface pearlite, leading to increased sensitivity to cracking under the rolling contact loading conditions. The presence of bainite in the initial microstructure of the wheels has been found to increase the sensitivity to thermal fatigue damage, for example due to brake block contact towards the front rim face (Fig. 2).
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