Experimental and Numerical Investigation of the Thermal Effects on Railway Wheels for Shoe-Braked High-Speed Train Appli
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TRODUCTION
THE wheels of block-braked vehicles have to support thermal load during their life, caused by the sliding of the wheel on the rail and especially to the braking process. Two common braking operations are stop braking and drag braking. The former acts on trains with scheduled stops or in case of emergency, and the latter is used to control the speed over long distances, especially on a slope. The braked wheel tread is subject to heating caused by friction with the shoe brake and cooling due to rail chilling. An example of such thermal loading is shown by Orringer et al. in Reference 1. The authors observed that the heating occurs non-uniformly along the circumference of the wheel concentrating in so-called ‘‘hot spots,’’ where the temperature can reach 540 C. Other studies reported temperatures in the wheel tread around 482 C[2] or over 500 C.[3–5] Recently, Caprioli et al.[6] measured the maximum temperature of approximately 300 C by thermocouples embedded 10 mm below the tread surface of an ER7 wheel. The
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 November 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
subsequent rail contact is characterized by rapid cooling due to high heat transfer from the hot wheel to the cold rail. Furthermore, the cooling is non-uniform, so it causes thermal stresses in the wheel rim.[2] High tensile stresses due to this phenomenon can lead to the nucleation of cracks on the wheel tread.[1,2] The thermal exposure of ferritic-pearlitic wheel steel may also lead to microstructural modifications. Nikas et al.[7] found that the spheroidization phenomenon becomes visible at 450 C in ER8 steel. Fe3C lamellae within the pearlite coalesce to form spheroid particles, so the lamellar pearlite becomes globular pearlite. The strength and the hardness of pearlitic steels containing a small amount of ferrite is principally governed by the pearlite constituent, while the ferrite phase mainly affects the ductility. A consequence of the pearlite morphology change is the softening of the steel, which leads to a reduction of the wear and rolling contact fatigue (RCF) resistance. According to References 8,9, temperatures up to approximately 1000 C can be reached in extreme conditions, when the wheelset is locked and skids along the rail. The locking can, for example, arise from badly adjusted brakes in combination with low friction.[8] The heating to a temperature high enough to promote the austenite phase transformation (above the eutectoid temperature A1 of the iron-iron carbide phase diagram) followed by rapid cooling may induce the local transformation of the original ductile pearlite into the hard and brittle martensite, forming the white etching layer (WEL). The pearlite body-centered cubic (BCC) crystal
structure changes
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