Journal of Materials Research and Technology (Sep 2024)
Thermo-mechanical response of near-pearlitic steel heated under restriction of thermal expansion
Abstract
Railway wheels experience high temperatures during operation, especially during severe block braking. The thermal expansion of wheel rim material due to frictional heating is limited by the wheel's geometry and size. This study investigated the impact of this combined mechanical and thermal loading on the mechanical properties and microstructure in the tread surface of an ER7T steel railway wheel.The material response below the austenitisation temperature is comprehensively evaluated through thermal cycling at peak temperatures of 300, 400, 600, and 650 °C, simulating severe block braking cycles with varying degrees of thermal dilatation (25 %, 50 %, 75 %, and 100 %). An initial plastic deformation was observed during the first heating cycle for the two lower temperatures at lower restrictions, followed by a predominantly elastic response. However, at higher restrictions, the material showed time-dependent relaxation during heating, and some plastic deformation was observed during cooling. Notably, even at the lowest level of restriction, similar observations were made at higher peak temperatures. Increasing the restriction resulted in large strains and a wider hysteresis loop. The test bars exposed to the two higher temperatures retained large tensile stresses after cooling. This indicates a risk of a significantly altered residual stress state in the rim of an overheated railway wheel, which could adversely affect its fatigue properties. The study's outcomes will aid in designing and calibrating constitutive material models for severe block braking. Furthermore, it will significantly contribute to research into the thermo-mechanical behaviour of pearlitic/near-pearlitic materials during railway operations and maintenance.
