Changes in the Microstructure and Mechanical Properties of Railway Wheel Steels as a Result of the Thermal Load Caused b
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INTRODUCTION
THE microstructural change in the surface layer of a railway wheel under service conditions is a key factor that affects its properties and durability. This work aims to understand how the microstructure and consequently mechanical properties of wheels for high speed service, made of various steels, can change as a result of heating from the brake shoe. The investigation originated with several rolling stock suppliers who were concerned about the application of shoe braking on high speed trains. For this reason, the authors have recently carried out a study into the shoe braking application for emergency purposes when the train speed is lower than 120 km/h.[1] Furthermore, there is also interest for freight transportation and for suburban and metro trains, where shoe braking is widely used. In particular, drag braking in freight transportation along slopes is a condition which applies very severe thermal loading to
MICHELA FACCOLI and ANGELO MAZZU` are with the Department of Mechanical and Industrial Engineering, University of Brescia, via Branze 38, 25123 Brescia, Italy. Contact e-mail: [email protected] ANDREA GHIDINI is with the Lucchini RS, Via G. Paglia 45, 24065 Lovere, BG, Italy. Manuscript submitted September 10, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the wheel tread, whereas shoe braking in metro trains is characterized by frequent stop-and-go operations.[2,3] A shoe-braked wheel designed for high speed service offers the following advantages: reduction of unsprung weight, aggressiveness of the wheel on the rail and lower costs, along with better truck stability, and wheel tread cleaning during braking. Nevertheless, the wheel tread experiences a thermal cycle characterized by the heating due to friction with the shoe brake and by the cooling due to the rail chilling. The thermal loading may cause
high tensile stresses in the wheel rim due to the
nonuniform cooling,[4] significant microstructural modifications near the contact surface, such as pearlite spheroidization or occasionally white etching layer (WEL) formation, which depend on heating and cooling rates, temperature reached, the effect of alloying elements on the austenization temperature, and the kinetics of the pearlite reaction during cooling,[5–12] roughness (corrugation or waviness) by nonevent wear mechanisms on the tread,[13,14] undesired tread profiles due to wear.[15] All these effects may promote the nucleation of cracks on the wheel tread,[4,11,12] most of which get removed by wear. However, due to the rolling contact fatigue (RCF), some cracks may grow several millimeters into
the wheel before deviating towards the surface and causing spalling. Alternatively, these cracks may propagate in the radial direction, causing, in the worst case scenario, wheel failure and the train derailment. Fluids on the wheel tread, such as rain, leaves or grease applied on the rail in curves, promote the fatigue crack propagation due to the hydraulic pressure mechanism.[16,17] In a block-braked wheel, fatigue cracks
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