Effects Of Temperature On Friction Coefficient And Wear Of The Wheel/Rail Friction

The friction of wheel/rail is a complicated problem because it involves heat transfer, machinery, material, chemistry and so on. The research on the problem is becoming more and more important with the increase of running speed, axle load and the frequency of braking and acceleration traction during train operation. Stable and adequate friction coefficient is found to be crucial for the safe operation of the train. However, the high temperature induced by actual wheel/rail friction always leads to the abrupt alteration on either microstructure or properties of the material in the vicinity of contact surface. The transformation not only breaks the stability of wheel/rail friction, but also accelerates delamination and wear of material on both surface of wheel and rail. This could result in the hidden troubles and the increased cost of safe operation for train. Thus, the research in the influence of temperature on friction and wear of wheel/rail is necessary to understand friction mechanism of wheel/rail, also is beneficial to prolong of wheel/rail service life and ensure the safe operation of the railway.In present study, we firstly made further research on the theoretical calculation on the basis of previous works and yielded the theoretical relationship between the temperature and the friction coefficient of wheel/rail. Then the friction and wear of wheel/rail at different temperature were observed on a pin-on-disc high temperature tribometer. Finally the results of experiments and theoretical calculation were compared. The results show that:(1) The estimated formulas for the friction coefficient of wheel/rail with varied temperature were obtained based on the studying of Popov and others research. According to the calculated results the variation of friction coefficient with increased temperature could be divided 3 stages, namely slow increase, rapid decrease and slow down.(2) The experimental results showed the complicated variation of the friction coefficient with temperature rise. From room temperature to high temperature, the friction coefficient first grows quickly to the maximum value at 200℃. Then it decreases rapidly to a minimum at 400℃ and then start increases again until the temperature reached 500℃. The friction coefficient gets into stage of slow down between 500℃ and 700℃, and drops rapidly when the temperature exceeds 700℃.(3) A blue brittle phenomena of wheel/rail steel was found between the temperature range of 200℃ to 400℃. It was found that the surface of wheel/rail steel become blue in the temperature range and the tensile strength and hardness of steel varied abnormally with increasing temperature, i.e. increase to a maximum value rather than decrease with increasing temperature. This may be the reason for dramatic variation of friction coefficient of wheel/rail steel in the temperature 300℃ and 400℃. More works on this point should be performed in the future.(4) The coincidence between friction coefficient obtained from theoretical and experimental was found undesirable on either slope-change trend or values at high temperature. This was caused by some abnormal changes, which were not involved in the establishment of estimated formulas, such as blue brittle in wheel/rail steel with varied temperature. Nevertheless, the experimental results in present works will provide help for the further refinement of theoretical calculation model.