Fracture toughness of nanostructured railway wheels

A kind of carbide free bainite steel was used to produce railway wheels. The alloy design, manufacture process, microstructure and behaviours are summarised. The novel steel is distinctively superior in excellent combination of strength and toughness compared with the traditional pearlite type wheel steels. The details of the microstructure of novel steel have been investigated in nanoscale.

Microstructure and properties of carbide free bainite railway wheels produced by programmed quenching

An improved railway wheel steel containing higher contents of C, Mn, Si, and V than the traditional ER7 steel was developed by alloy design for shoe-braked high-speed train applications. The effects of the thermal load on the microstructure and mechanical properties of a wheel made from this steel were investigated using a combined experimental and numerical approach. The wheel braking was studied using finite element simulations that account for the thermal loading of the wheel in order to find the temperatures reached in the wheel rim. Hardness measurements, tensile tests, toughness tests, fatigue crack growth tests, and microstructural observations were carried out on samples extracted from real wheels, with and without heat treatments simulating the modification of the microstructure due to the shoe braking. The results on the un-treated samples showed that the improved steel has a better combination of strength and toughness than ER7 steel. The results on the heat-treated samples showed that the improved steel maintains acceptable mechanical properties provided the maximum temperature during braking is below the A3 temperature of the steel (around 790 °C).

Experimental and Numerical Investigation of the Thermal Effects on Railway Wheels for Shoe-Braked High-Speed Train Applications

An investigation was carried out to study the effects caused by shoe braking on the microstructure and mechanical properties of ER7, CLASS B, CLASS C, and SANDLOS® wheels as a result of the thermal load. Particular attention was given to cases of exposure to medium and high temperatures, namely from 700 °C to 970 °C, followed by air cooling. Hardness measurements, tensile tests, toughness tests, fatigue crack growth tests, and microstructural observations were carried out on samples extracted from new wheels, with and without heat treatments simulating the microstructural modifications caused by shoe braking. The experiments showed that the hardness, yield strength and ultimate tensile strength of the steels all decrease with the heat-treatment temperature up to 750 °C due to globular pearlite formation. However, after the heat treatment at 970 °C, these properties show a trend inversion, as the globular pearlite no longer appears, while, in some cases, traces of bainite and martensite are observed. On the other hand, the fracture toughness, crack propagation threshold, and rate are not significantly altered by the heat treatments, showing the steels to have a good stability under the thermal loads caused by shoe braking.

Changes in the Microstructure and Mechanical Properties of Railway Wheel Steels as a Result of the Thermal Load Caused by Shoe Braking

We study the microstructure, mechanical properties, and micromechanism of fatigue fracture of high-strength railway-wheel steel (0.63% C and 0.1% V). The necessity of elevation of the cyclic fracture toughness of this type of steel by isothermal hardening and tempering at 500°C is substantiated. It is shown that the cyclic crack-growth resistance of steel decreases for high loading amplitudes as the load ratio (simulating the influence of residual thermomechanical stresses) increases.

Influence of the mode of thermal treatment and load ratio on the cyclic crack-growth resistance of wheel steels

In this study, the failure modes of cartwheel and mechanical properties of materials have been analyzed. The results show that rim cracking is always initiated from stringer-type alumina cluster and driven by a combination effect of mechanical and thermal load. The strength, toughness, and ductility are mainly determined by the carbon content of wheel steels. The fatigue crack growth resistance is insensitive to composition and microstructure, while the fatigue crack initiation life increases with the decrease of austenite grain size and pearlite colony size. The dynamic fracture toughness, KID, is obviously lower than static fracture toughness, KIC, and has the same trend as KIC. The ratio of KID/σYD is the most reasonable parameter to evaluate the fracture resistance of wheel steels with different composition and yield strength. Decreasing carbon content is beneficial to the performance of cartwheel.

H. C. Gu

Papers

Failure modes and materials performance of railway wheels