Experimental and numerical investigations: Alleviating tensile residual stresses in flash-butt welds by localised rapid post-weld heat treatment

Flash-butt welding is commonly used in the manufacture of continuously welded rails (CWR). The finished welds typically exhibit high levels of tensile residual stresses in the rail web. In addition, the surface condition of the web may contain shear drag or other defects resulting from the shearing process. When combined with torsional loading of the web under service loading (particularly at high axle loads), these conditions may contribute to fatigue failure of the weld in a horizontal split web mode. The risk of weld failure may be alleviated by reducing the magnitude of the tensile residual stresses. A sequentially coupled thermo-mechanical finite element (FE) model incorporating the phase transformation characteristics of the rail material has been used to predict the residual stress distribution developed during welding of AS60 and AS68 rails, by approximating the thermal distribution after upset from the heat-affected zone (HAZ) characteristics. The effect of localised, rapid post-weld heat treatment on residual stresses in the web region of the weld was also investigated. An experimental program covering measurement of post-weld cooling rates using infra-red thermography, and residual stresses by the strain-gauge and trepanning, was used to validate the finite element model. The results have shown that the FE model can satisfactorily predict the residual stress distribution. In addition, the (tensile) residual stress levels in both vertical and longitudinal directions of the web can be reduced by rapidly reheating the base of the foot directly after welding. Thereafter, both numerical and experimental approaches will be used to develop modifications to the flash-butt welding procedure that should result in improved weld performance under high axle load conditions.