Microstructural Analysis of Thermite Welds

Thermite welding is a simple and cost-effective process widely used in the field for rail repair and joining. Despite the well-accepted use of this technology, there is a major concern regarding the soundness of the weldments which are often found to be very sensitive to wear and cracking. In order to gain a better understanding of the structural factors that contribute to the performance behavior of thermite welds, systematic microstructural analyses of a series of welds was conducted. Of particular interest in this study was to carefully examine and compare the microstructure of the weld metal, heat affected zone (HAZ) and base metal of a series of thermite welded samples with different carbon content. The results of this work revealed the presence of proeutectoid cementite along the prior austenite grain boundaries at the three locations examined. In addition, microhardness evaluation of the welds revealed that substantial softening takes place in the HAZ, independent of the chemical composition of the rails or weld processing conditions. The presence of proeutectoid cementite along the prior austenite grain boundaries and the softening that takes place in the HAZ are two of the structural factors most likely responsible for the lower than expected wear behavior observed in welded or repaired rail steels. This paper will present and discuss the microstructural and processing factors associated with the formation of proeutectoid cementite and the causes leading to the observed softening.