The mechanical properties of thermite welds in premium alloy rails

In this paper the results of an investigation into the properties of 13 thermite rail welds produced using CrMo, CrV and chromium alloy rails, American Railway Engineering Association controlled-cooled rails (i.e. standard rails) and heat-hardened rails are reported. Various combinations of the rails were joined with weld metal produced by the aluminothermic reaction of "standard" and "alloy" thermite charges. After welding, the hardness profiles, the tensile properties, the impact energies and the macrostructures and microstructures of the welds were determined. Detailed examinations of the microstructures of the heat-affected zones and of the weld metals revealed that the thermite welding process produced welds compatible with rail service in that no martensite was ever observed in any of the welds regardless of the rail composition or of the weld metal composition. Despite the absence of martensite, very low impact energies (1.5-2.8 J at 293 K) were measured in Charpy V-notch specimens removed from the weld metals. The weld metals also exhibited only a 2%-6% reduction in area for tensile ductility. Predictably, all the weld metal fracture surfaces showed brittle-type transgranular cleavage. It was also shown that alloy weld metal is harder and stronger than standard weld metal. Depending on the preheat time, the Rockwell C hardness of alloy weld metal is approximately 28-34 HRC whereas that of standard weld metal is 20-27 HRC. Correspondingly, the tensile strengths and yield strengths of alloy weld metal were about 100-150 MPa greater than those of standard weld metal. Also studied were the properties and structures of specimens which straddled the region of minimum hardness along the outer edge of the heat-affected zone.