Thermal and mechanical behaviour of railway wheel steel

Doctoral thesis, 2001

The wish to increase loads and speeds of trains puts higher demand on construc¬tion, engineering and material than ever before. Therefore it is necessary to con¬tinuously develop technology to meet these increasing requirements. This thesis includes a thorough investigation of one of the most common wheel damages – the damage that arises when a wheel slides along the rail. It also contains an attempt to counteract damage by development and testing of new wheel materials. The wheel slide problem was examined by metallurgical studies, analytical modelling and numerical modelling. A large number of samples and experimental data were available from full-scale experiments on wheel sliding. Frictional heating and subsequent cooling of the material under the contact surface transform a thin layer, usually less than 2.5 mm, to brittle martensite. Cracks often originate in this layer. Detailed studies of grain sizes and deformation led to the conclusion that the surface temperature rapidly rises to a stable temperature level of roughly 800–1000 degrees C. An analytical model of wheel sliding was established showing that cooling rates are fairly constant with depth, and that higher axle loads yield higher surface temperatures and thicker martensite layers. Thereafter, a numerical FE analysis was done including variation of thermal parameters with temperature and phase. The model gave information on temperature fields and phase transformations occurring during sliding. For instance, it was noted that cooling rates are much higher towards the periphery than in the centre of the transformed layer. The influence of initial wheel temperature was investigated. The second part of the thesis treats the development of new materials, more resistant to the mechanical and thermal loads. The experiment includes production and investigation of totally fourteen different wheel materials yielded by different chemical composition and by changes in production process and heat treatment. These materials have been investigated microstructurally, by hardness and tensile testing, fracture toughness testing and low cycle fatigue testing. Results indicate improvement in microstructure and mechanical properties for some materials and it is probable that these materials would endure better in service.