Metalografik İncelemelerle Ray Yüzeyindeki Soyulmaların Nedeninin Araştırılması

Abstract

Railways are increasingly used for passenger and freight transportation worldwide. This increased traffic causes more quick and frequent formation of defects on train tracks. In this study, rail sets used in Bursa province, that had been reported to experience rapid wear and scaling, were examined. The rail sets were manufactured from R260 steel and the problem occurs more frequently on curves. For comparison, rail sets from straight lines were also examined. After visual examination, specimens from gauge and field corner regions were taken and prepared for metallographic examination. The microstructure of the specimens was examined under optical and scanning electron (SEM) microscopes. Moreover, Vickers micro-hardness tests were also performed. Micro-chemical analyses were performed using an energy dispersive X-ray spectroscopy (EDS) system attached to the SEM. For macro-chemical analysis, an optical emission spectrometer (OES) was used. The cross section of the rails was macro-etched for macro-examination. The results show that the microstructure of the rails is composed of 100% pearlite. On the other hand, the pearlitic structure degenerates at parts of the rail which are under loading. Due to severe plastic deformation, the pearlitic structure changes into a fiber-like structure composed of sub-micron ferrite and cementite. Those degenerated pearlitic regions have higher hardness due to strain hardening and cause crack initiation. Those cracks can then grow into inner sections of the rail easily due to the presence of non-metallic inclusions in the microstructure. Iron oxide particles were found in the vicinity of cracks; which indicate that cracks do not grow rapidly, they rather grow in time according to usage and atmospheric conditions. The examinations revealed that the wear, cracks and scaling problems were formed due to rolling contact fatigue (RCF). The presence of inclusions enhanced the growth of RCF-cracks and later cause “shelling”. In order to delay or prevent this problem, using higher strength rails at critical locations such as curves, controlling and rating the non-metallic inclusions in accordance with the international standards, checking the lubrication and maintenance strategies of rails and wheels, working on optimization of rail-wheel contact characteristics are recommended. The last recommendation involves developing a sustainable model by identifying technical limitations of the line and cruise depending on vehicle characteristics.