Mechanical properties and fatigue behaviour of railway wheel steels as influenced by mechanical and thermal loadings

During the operation of trains, high thermal loads are evolved because of frictional heating on recurring acceleration, braking, curving and occasional full slippage. Furthermore, since long-term block braking may heat the wheel rim to over 500 °C, it is relevant to examine the high temperature performance of wheel material as well as the decrease in strength after thermal exposure. This work concerns the elevated temperature low cycle fatigue behaviour, and the deterioration of microstructure caused by pre-deformation and heat treatment. The materials examined are two similar medium carbon steels UIC ER7T and ER8T (~0.55 wt% C), heat treated in regular wheel production to a near pearlitic microstructure with some 5-10% pro-eutectoid ferrite in the wheel tread surface. For the study of microstructure deterioration, specimens were extracted from virgin wheels and pre-strained either monotonically to 6.5% strain or cyclically, to imitate plastic deformation developing in the wheel tread surface during operation. Both un-deformed and pre-strained materials were heat treated at various temperatures from 250 °C to 650 °C for various time durations, and thereafter the change in room temperature hardness was measured and the microstructural degradation was analysed. Hardening due to strain ageing was observed around 300 °C while microstructure degradation caused softening at higher temperatures. Spheroidisation of the pearlite started to become visible at 450 °C for the un-deformed material and at around 400 °C for the pre-strained. The elevated temperature low cycle fatigue tests showed a similar increase in strength around 300 °C due to dynamic strain ageing, and decreasing strength at higher temperatures due to increased thermal activation and microstructure degradation. Hold times showed that viscous effects exist already at 250 °C giving stress relaxation, but the influence of hold times on further cycling is small regarding stress-strain relations.