The effect of induction heating stress remedies on existing flaws in pipes

One method for reducing stress corrosion cracking problems called induction Heating for Stress Improvement (IHSI) has been successful in producing compressive residual stresses on the inside surface of welded pipes. During the time that the induction heating coil is turned on, however, the process causes tensile stresses at the inner wall, raising questions about the effect that the induced tensile stress field may have on existing flaws in the pipe. To provide a basis for comparing aswelded stress distributions with the stress distributions occurring during the IHSI process, stress intensity factors were computed for part-through, axisymmetric, circumferential cracks in pipes using linear elastic fracture mechanics analyses. Results for as-welded stress distributions agree with the field experience that most of the crack pipes in Boiling Water Reactor piping systems are smaller diameter pipes in the 4-12-in. (102-305-mm) range. Stress intensity factors in the smaller diameter pipes for the stresses during IHSI were comparable to those for the aswelded stresses. For the 26-in- (660-mm-) diapipe, the large changes in the stress distributions during the IHSI process suggest the possibility of a net closing or opening of a flaw existing in the pipe prior to the heat treatment.