Thermal deterioration of railway wheel steels
Paper in proceedings, 2014
In the current work the deterioration of mechanical properties of railway wheel steels is in focus. These are commonly made from medium carbon steels (~0.55 wt.% C) heat treated to a near pearlitic microstructure with some 5–10% pro-eutectoid ferrite. The two steel grades studied here are very common on trains in Europe: the R7T grade is mainly used for freight trains and the R8T grade is mostly used for motorized passenger trains. During operation of trains, high thermal loads are evolved because of recurring acceleration, braking, curving and occasional slippage. It is thus relevant to examine the high temperature performance of wheel material and evaluate the decrease in strength after thermal exposure as well as the degradation of fatigue properties. Samples were extracted from virgin wheels and pre-strained to around 6.5% strain, to also account for the change in properties that is induced by plastic deformation inherent in the wheel tread surface. Both un-deformed and pre-strained material was heat treated for different times in the temperature range of interest, from 250°C to 700°C. Hardening was observed in both conditions around 300°C followed by softening at higher temperatures. Spheroidization of the pearlite started to become visible at 450°C for the un-deformed material and at around 400°C for the pre-strained.