The aluminothermic welding (ATW) process is the most commonly used welding process for welding rails (track) in the field. The large amount of weld metal added in the ATW process may result in a wide uneven surface zone on the rail head, which may, in rare cases, lead to irregularities in wear and plastic deformation due to high dynamic wheel-rail forces as wheels pass. The present paper studies the introduction of additional forging to the ATW process, intended to reduce the width of the zone affected by the heat input, while not creating a more detrimental residual stress field. Simulations using a novel thermo-mechanical FE model of the ATW process show that addition of a forging pressure leads to a somewhat smaller width of the zone affected by heat. This is also found in a metallurgical examination, showing that this zone (weld metal and heat-affected zone) is fully pearlitic. Only marginal differences are found in the residual stress field when additional forging is applied. In both cases, large tensile residual stresses are found in the rail web at the weld. Additional forging may increase the risk of hot cracking due to an increase in plastic strains within the welded area.
This study examines the properties of stratified surface layers on rails in service and presents a hypothesis explaining their origin. The stratified layer consists of a white etching top layer and a brown sublayer. The metallurgical composition and properties of this sublayer are found to match with that of globular bainite. The occurrence of stratification in the surface layer is explained by the thermomechanical cycle for a material point on the rail surface under wheel-rail contact. Difference in the surface and subsurface cooling rates after reaching the austenitisation temperature may lead, depending on the chemical steel composition, to the generation of two different phases (martensite and bainite) and stratification. The exclusive occurrence of sandwich layers on rails that have been in service is attributed to the hardening of the top layer, leading to a reduced thermal conductivity, which gains relevance at an increasing depth. The granular morphology of the bainitic sublayer, exhibiting weak globular inclusions, facilitates the initiation and the propagation of transverse cracks, thus contributing to the development of RCF.
Bainitic steels are gaining popularity for application in the railway switches and crossing, thanks to their better rolling contact fatigue (RCF) and wear resistance. The rail degradation caused by RCF and wear could be also reduced by the use of friction modifiers (FM) through their ability to reduce the lateral load during wheel passage. This paper presents the microstructure and the mechanical properties of newly designed bainitic steel grades; B1400+ and Cr-B potential candidates for switches and turnouts railroad industry. It investigates their friction and wear performances using the HOrizontal twin DIsk Machine. The tribological behavior is evaluated under dry and lubricated conditions in combination with three commercial friction modifiers. Results show a better wear resistance of Cr-B steel in case of abrasive mechanism that dominates the dry experiments. This work defines the efficiency of FM as a low and stable friction coefficient concomitant to a low wear rate. It appears that a good tribological performance is untimely linked to the chemical composition of the friction modifier. In case of lubricated contact, cross-sectioning of wear scare confirms the generation of an interfacial layers. These layers have an impact on the wear mechanism and debris detachment susceptibilities of bainitic steels.
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