PHYSICAL METALLURGY AND PROCESS IMPROVEMENT OF THERMITE RAIL WELDS.

Abstract

This study was an evaluation of thermite rail welding with the goal of the development of welds with improved mechanical properties. The first part of the study involved an in-depth evaluation of 14 thermite rail welds produced by the Department of Transportation using current production practices. These welds were produced using CrMo, CrV and Cr alloy rails, AREA CC rails (i.e., standard rails) and head-hardened rails which were welded with weld metal produced by the aluminothermic reaction of "standard" and "alloy" thermite charges. Temperature at various locations was measured during welding for both the rails and the weld metal. After welding, mechanical properties, macro- and microstructure, inclusion levels and residual stresses induced by the welding operation were all determined. Low impact properties and ductility (2-6 percent reduction in area) were observed in the thermite rail weld metal. These low properties were attributed to microstructure and, to a lesser extent, inclusion content. In order to improve the process by reducing the inclusion content, attempts were made to filter the molten thermite steel by passing it through zirconia/mullite filters. This was included in the second part of the study in which 9 plate welds we made using "standard" thermite charges. Filtering, at best, was only partly successful. However, it was observed that a 30 percent increase in yield strength and hardness was achieved in weld metal containing approximately 0.55 percent carbon and 0.06 percent vanadium. Normalization of the plate welds resulted in a significant improvement in the tensile ductility of as-cast weld metal. Weld metal of 0.55 percent carbon and 0.06 percent vanadium had ductilities in the range of 10-20 percent when the cooling rate exceeded 37 K(DEGREES)/min. through the transformation range. At cooling rates of four times this level, tensile properties equivalent to those of the "alloy" weld metal were obtained along with the enhanced tensile ductility. It was concluded that it is possible to produce a thermite weld with both improved strength and ductility by the judicious control of composition, the addition of microalloying elements and the application of an appropriate post-weld heat treatment, such as normalization.