An experimental study of the residual stresses, and their alleviation, in tube to tube-sheet welds of industrial boilers

Micro-crack development in welds, including repair welding, of the tube to tube-sheet region of industrial boilers is not an uncommon problem. Often s uch cracking is exacerbated by stress corrosion cracking or thermal fatigue , in which the residual stresses associated with welding can play a major r ole. In this paper, a custom built, but robust, air abrasive centre hole dr illing facility was used: which induced negligible machining stresses. This system was modified to attach to the (vertical) tube sheets of boilers on site, and to sample residual stresses in the tube to tube-sheet welds, of b oth conventional circumferential fillet welds of protruding tubes as well a s recessed 'J' configuration type. The experimental test programme evaluate d transverse and circumferential profiles of the residual stresses. Global post-weld heat treatment (PWHT) of a whole boiler, indicated substantial (7 5%) reductions in the tensile peak residual stress. A localised PWHT techni que, using a 1 kW incandescent light sourer inserted into the tube, easily achieved the requisite temperatures and hearing rates, but led to significa nt increases in local residual stresses. The implications of such localised PWHT techniques on residual stress and consequent service life. are theref ore of vital importance with regard to plant integrity and safety. (C) 2000 Published by Elsevier Science Ltd.