Effect of Temperature on Mechanical Properties of Railway Wheel Steels
Sammanfattning: One of the most important aspects in railway operation is the interaction between rail and wheel. The contact conditions give rise to wear and damage in both components. Medium carbon steels are used in these components due to their combination of high strength and good wear properties in relation to cost. In service, high surface temperatures develop because of frictional heating on traction, braking, curving and occasional full slippage. Furthermore, long-term block braking may heat the wheel rim to over 500°C. It is thus relevant to examine the high temperature performance of wheel material as well as the decrease in strength after thermal exposure. In the current thesis, two railway wheel steels are examined. These are the medium carbon steels UIC ER7T and ER8T (~0.55 wt.% C), heat-treated to a near pearlitic microstructure with some 5–10% pro-eutectoid ferrite in the wheel tread surface. Specimens were extracted from virgin wheels and pre-strained either monotonically or cyclically, to imitate plastic deformation developing in the wheel tread surface in service. Both un-deformed and pre-strained materials were heat treated at various temperatures from 250°C to 650°C for various time durations, and the change in room temperature hardness as measured before and after heat treatment was analysed. Samples were analysed using Scanning Electron Microscopy (SEM), to examine microstructure degradation. Additionally, Electron Backscatter Diffraction Analysis (EBSD) was used to evaluate if orientation gradients in the pearlitic colonies affect the spheroidisation of the pearlitic microstructure that is observed at higher temperatures. Analyses after the room temperature hardness measurements showed that hardening due to strain ageing takes place at around 300°C while microstructural 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 spheroidised areas appear to have lost their initial orientation gradients after spheroidisation and obtain a more uniform orientation. Cyclic tests at elevated temperature revealed cyclic hardening at around 300°C, as an effect of dynamic strain ageing. At higher temperatures, cyclic softening followed due to a combination of increasing thermal activation and spheroidisation.
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