パイプ円周溶接部の内圧による溶接残留応力除去効果

抄録

The circumferential weld is one of the most popular methods of fabrication of tubular structures and pipings. Due to the weld, residual stresses (hoop and axial) on the inside of the pipe are highly tensile. The tensile residual stress is one of the factors of intergranular stress cororsion cracking of pipings. Several methods which produce compressive residual stresses along the inner surface of the welded pipe have been investigated. They are the methods using thermal elasto-plastic process; induction heating for stress improvement, backlay welding, heat sink welding, and so on. On the other hand, it may be possible to improve the residual stresses due to the circumferential weld of pipes, using some mechanical process; for example, internal pressure, explosion along the inside surface of the weldment.
This paper deals with the stress release effect of internal pressure on the residual stresses due to the circumferential welds of pipes. In the first part, the analytical procedure is presented. The initial residual stress distribution is described as the sum of the "inherent stress" due to weld shrinkage and the 'stress induced by the "inherent stress". The incremental formula is presented for calculating the state of stresses under internal pressure. In the second part, the analytical results are compared with experimental results. The analytical results show good agreement with the experimental results. The final part of the paper deals with the assessments of stress release effect of internal pressure on the residual stresses due to the circumferential welds of pipes. The assessments are as follows.
1) The influence of welding conditions, dimensions and. yield stress of pipes on.the effect of internal pressure.
2) Especially, the influence of heat-input; the less the heat-input of the last layer of the circumferential weld is, the more effectively the residual stresses are improved by means of internal pressure.