METHOD FOR IMPROVING RESIDUAL STRESS OF WELD ZONE OF PIPE

摘要

PURPOSE:To decrease residual stresses in a weld zone or impart compressive residual stress with ease of operation by cooling or heating the weld zone of pipes from the outside to desired temps., then quickly heating or cooling the same thereby giving temp. gradients. CONSTITUTION:Annular cooling devices 13 are disposed near a weld zone 12 of a pipe 11, and liquid nitrogen or the like is flowed to cooling the weld zone 12 and the part near the same from the outside surface. After these parts are cooled down to below the required temp., they are quickly heated by using a high frequency heater 14 or the like disposed on the outside circumferential surface of the pipe 11. Thereby, temperature gradients are given between the outside surface and inside surface of the pipe 11. On account of such temperature gradients, the outside surface of the pipe 11 assumes a compressive state and the inside surface a tensile stress state. If such state is left, the temp. gradients becomes constant, thereby leaving the compressive residual stress in the inside surface of the pipe 11. In the case of reducing the axial residual stress, the weld zone 12 and the part near the same are beforehand heated, after which they are quickly cooled.