Characterization and tribological studies of rail top materials

Abstract

Rail corrugation is a wave-type wear of the rail surface. There are short and long pitch corrugations. Short pitch corrugation displays as an undulation of the rail surface with typical wavelengths between 25 and 80 mm. For long pitch, wavelengths of up to 1200 mm have been found. Rail corrugations have a critical impact on the maintenance effort and the cost of grinding and replacement work is very high. In order to prevent the generation of rail corrugation, many studies have been reported. However, there are few researches on long wavelength investigation. In 2006, the Kowloon-Canton Railway Corporation (KCRC), identified that undulations with a longitudinal pitch of 3.1 m and a vertical profile of less than 1 mm probably existed in their rail tracks. The possible existence of these undulations, interpreted as a form of long wavelength corrugation, has provided the incentives for the present study. In this research, samples were taken from three different manufacturers (BS 97, DO 02 and PZH 98) to study the topography, morphology and microstructure. In the first part of this thesis, materials characterization was done to observe whether there was any structural change near the rail-wheel contact surface of the rail relating to wear. Some metallurgical investigations as well as the macro- and micro-hardness measurements were done. X-ray diffraction was used to identify the phase composition of crystalline materials of the railways. It was found that the microstructures were typical pearlite phase in the core part and elongated fined pearlite due to plastic deformations at the rail top. The hardness beneath the surface is higher than that at the core part of the rail. The phase composition of crystalline materials at the surface of the rails is different from that in the core. While the surface shows many oxide compounds, the core shows alpha-ferrite and cementite. The second part of the thesis described wear tests on the three different rails. Dry wear tests were carried out to compare wear performance of the rail samples using a pinon- disk machine under 20N, with sliding distance of 600 m at constant sliding speed of 0.1 m/s. Six samples were taken within a four-meter length at regular intervals. The analysis of wear test showed that the wear mechanism was adhesive and the wear rate of the PZH 98 is highest among the three railways. In all samples, the wear rate of the disks is higher than that of the pins. Undulation was not found to be a dominant feature in this study. Nevertheless, the present results indicate that the PZH 98 rail has the potential to develop into long wavelength corrugation, i.e. the undulation problem with KCRC in 2006.