A thermal-metallurgical model was developed for investigating railway vehicle braking behavior on the potential for spall generation. This work focused specifically on a tread-braked freight vehicle having composition brake shoes where heat generated at the brake block/wheel interface was coupled to the wheel via a thermal/metallurgical analysis. The wheel/rail contact patch caused a rail chill effect on the wheel as a result of the hot wheel rolling over the "incoming" rail. An additional modification over previous thermal-sliding models was made which incorporated finite element heat partitioning which resulted from the wheel/brake block sliding contact, in order to remove analytical uncertainty resulting from the choice of a heat partition factor. Results for typical US freight vehicle braking conditions showed wheel tread temperatures in excess of 1000 °C for extended drag braking conditions, however conjugate metallurgical predictions for wheel tread microstructure indicated that transformations from austenite occurred at rate slow enough to yield a fully pearlitic structure once the braking encounter ended and the rolling wheel was allowed to cool by rail chill conduction, forced convection and thermal radiation. Results of a thermal-metallurgical tribo-energetic FEA model for predicting wheel tread temperatures and microstructures indicate that under braking conditions typical of US freight vehicles, wheel tread surfaces undergo thermal phase transformations to austenite during heating (braking) and back to pearlite during cooling (subsequent rolling without brake application). Although full austenitization of tread thickness sufficient for spall formation was predicted by the model during brake encounters, the combined cooling processes of forced convection, rail chill conduction and thermal radiation typical of moderate environmental conditions (21 °C), were not sufficient to remove the heat at a rate necessary for forming the martensite layers which have been observed to occur on wheel treads.
Thermal modeling of railcar wheel braking
Thermische Modellierung gebremster Güterwagenräder
2010
13 Seiten, 20 Bilder, 19 Quellen
Conference paper
English