Figure 1 shows a typical macrostructure of a carbon steel rail thermite weld and the microstructure of the fusion zone, the fusion line, the end of the heat-affected zone (HAZ), and the unaffected rail. Typically, the weld is 100% pearlite with varying degrees of coarseness. This is due to the slow cooling rate that completes transformation before reaching the martensite start temperature and the formation of untempered martensite.