Mechanical Properties and Microstructural Analysis of Rail Thermite Welding Joints

Thermite welding is commonly applied to join running rails together and any failures related to it will result in delay or even catastrophe. In this paper, mechanical properties and microstructures of specimens extracted from plain rail and welding are analyzed to understand the potential factors affecting crack initiation and propagation in the head of rail and welding joints. It is found that there is a sharp decrease of hardness near the heat affected zone (HAZ), likely due to the rail undergoing a heat treatment such as annealing. To verify this assumption, the specimens are analyzed by optical microscope and Scanning Electron Microscope. The microstructure result shows that the region near welding can be clearly divided into four zones: Welding Metal Zone, Mixed Melted Zone, Heat Affected Zone (HAZ) and Unaffected Parent Rail with three subzones for HAZ: Coarse Grained HAZ, Fine Grained HAZ and Inter-critical HAZ based on the grain size and inter-lamellar spacing. The decrease of hardness in HAZ is due to softening because of fully annealing, partial annealing, "spheroidization" in three subzones respectively. This paper also found that the large amount of oxide inclusions in Mixed Melted Zone may cause the ease of crack initiation and propagation.