Residual stresses in alumino-thermic welded rails

Abstract

Neutron strain scanning has been used to map the residual stress field that is generated in a railway rail by a standard gap alumino-thermic weld made using routinely specified procedures. The longitudinal and vertical residual stress fields in the sections well away from the weld are characteristic of many unwelded rails, being generally tensile in the head and foot with balancing compression in the web. In the vicinity of the weld the residual stress patterns are very different. At the top and bottom surfaces of the rail the longitudinal residual stress field is strongly compressive, which is generally beneficial in that it would tend to inhibit the initiation and propagation of fatigue cracks from surface defects. Just at the surface the vertical residual stress attenuates to zero but internally, in the web region, both longitudinal and vertical components of the residual stress field are strongly tensile, which increases the susceptibility of that region to crack initiation and propagation from internal material defects. This pattern is consistent with practical operating experience, which is that most of the small proportion of alumino-thermic welds that do fail do so as a result of porosity or inclusions in the weld. It is found that the ‘boundaries’ of the ‘weld type’ residual stress fields do not coincide with the boundary of the weld, nor of the heat-affected zone, but correlate reasonably well with the positions of the extremities of the mould assembly and with the location of the steepest longitudinal temperature gradients.