Investigation of Caused by Shelling defects on Rail Surface via Metallographic Examination

Year 2020, Issue: 19, 1 - 18, 31.08.2020

Alper Uğur , Kemal Davut , Rasim Bacacı

https://doi.org/10.31590/ejosat.703669

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.

Keywords

Rail defects, Metallographic examination, Microstructure, Rolling contact fatigue (RCF), Shelling

Metalografik İncelemelerle Ray Yüzeyindeki Soyulmaların Nedeninin Araştırılması

Abstract

Demiryolları yolcu ve yük taşımacılığında dünya genelinde giderek artarak kullanılmaktadır. Bu artan trafik aynı zamanda tren raylarında kusurların daha çabuk ve daha sık görülmesine neden olmaktadır. Bu çalışmada Bursa ilinde kullanılan ve hızlı aşınarak parça kopmaları problemi yaşanan ray setleri incelenmiştir. R260 kalite çelikten imal edilen rayların özellikle kurp bölgesinde problem daha çok görülmektedir. Karşılaştırma yapılabilmesi için daha az sorun yaşanan düz hat bölgesinden alınan raylar da incelenmiştir. Problem yaşanan ray parçalarının yüzeyleri incelendikten sonra ray profilinin mantar bölgesi, yanak (gauge ve field corner) yüzeylerinden numuneler çıkarılarak metalografik inceleme için hazırlanmıştır. Hazırlanan numunelerin mikroyapıları optik mikroskop ve taramalı elektron mikroskobu (SEM) altında incelenmiş, ayrıca Vickers mikro-sertlik testleri yapılmıştır. Taramalı elektron mikroskobuna bağlı enerji dağılımlı X-ışınları spektrometresi (EDS) kullanılarak numuneler üzerinde mikro-kimyasal analizler gerçekleştirilmiştir. Makro-kimyasal analizler için optik emisyon spektrometresi (OES) kullanılmıştır. Ray kesitlerine makro-dağlama yapılarak genel yapı (makro-yapı) kontrol edilmiştir. Yapılan incelemeler sonucunda ray kesitlerinin %100 perlitten oluştuğu, öte yandan yük altında olan yanak bölgesinde ise aşırı deformasyon nedeniyle perlit yapısının bozulduğu gözlemlenmiştir. Perlitik yapı aşırı deformasyon etkisiyle mikron altı büyüklüklerde ferrit ve sementite dönüşerek lifli bir yapı halini almıştır. Deformasyon sonucu pekleşen bu yapı çatlak başlangıcına neden olmuştur. Aynı zamanda yapıda bulunan metal dışı kalıntılar (inklüzyon) çatlakların ray içine doğru ilerlemesini kolaylaştırmış ve sonrasında parça kopmalarına neden olmuştur. Çatlakların etrafında demir-oksit partiküllerine rastlanmıştır. Bu durum çatlakların, aniden ilerlemediğini, zaman içinde kullanıma ve atmosfer koşullarına bağlı olarak ilerlediğini göstermektedir. Ray malzemelerinin kimyasal kompozisyonları ve sertlik değerleri R260 kalite için izin verilen aralık içinde bulunmuştur. Teker teması olan bölgeler aşırı plastik deformasyon nedeniyle pekleşmiş ve sertlik değerleri diğer bölgelere göre yüksek çıkmıştır. Yapılan incelemeler sonucunda raylarda aşınma, soyulma ve parça kopması probleminin yuvarlanma temas yorulması (Rolling contract fatigue- RCF) nedeniyle oluştuğu ve çelik yapısında tespit edilen çok sayıda metal dışı kalıntının (inklüzyon) da RCF sırasında oluşan çatlakların ilerlemesini kolaylaştırdığı ve de soyulmalara (shelling) neden olduğu sonucuna varılmıştır. Problemin oluşumunun geciktirilmesi veya önlenmesi için kurb gibi kritik yerlerde daha yüksek dayanımlı rayların kullanılması, kullanılan ray çeliklerinde uluslararası standartlara uygun olarak inklüzyon kontrolü yapılması, teker ve rayların yağlanma ve bakım stratejilerinin gözden geçilmesi, teker-ray temas karakteristiklerinin iyileştirilmesine yönelik çalışmalar yapılması önerilmektedir. Son öneriye yönelik çalışmalar hat ve seyirle ilgili teknik sınırların araç özelliklerine bağlı olarak çıkartılarak sürdürülebilir bir model geliştirilmesini içermektedir.

Keywords

Ray kusurları, Metalografik inceleme, İçyapı, Yuvarlanma temas yorulması, Soyulma

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