横割れの発生と残留応力，拡散性水素濃度との関係 低温溶接後熱処理による２１／４Ｃｒ‐１Ｍｏ鋼厚板突合せ溶接部の横割れ防止について Ｉ

Abstract

This research was conducted in order to clarify the cause of transverse cracks, which were observed in heavy section weldments by submerged arc process, and to investigate the possibility of prevention of them by use of low temperature postweld heat treatment. In the investigation, cracking tests were carried out on restraint test specimens with the thickness range of 50 mm through 200 mm. The characteristics of cracks were observed. Moreover, magnitude of residual stresses and concentration of diffusible hydrogen just after welding as well as after low temperature postweld heat treatment, both of which were considered to have close connection with the cause of the cracks, were measured experimentally and their effects on the occurrence of a crack were examined.The outline of the results obtained is as follows;(1) Transverse cracks initiate in the weld metals just below the final layer of welds and gradually propagate both to the top surface and to the bottom surface. Intergranular fracture generally appears in the initiation area of the crack and quasi-cleavage fracture appears in the propagation area.(2) The location of the largest residual stress parallel to the weld line corresponds to that of crack initiation and so does the location of the highest concentration of diffusible hydrogen just after welding. This implies that the transverse crack is a cold cracking induced by longitudinal residual stress and diffusible hydrogen.(3) The highest longitudinal residual stress is nearly constant in the range of plate thickness over 50 mm. Meanwhile, the highest concentration of diffusible hydrogen just after welding increases with an increase in plate thickness. It decreases with an increase in interpass temperature during welding or in temperature of postweld heat treatment and thereby the cracks can be arrested. Judging from the result, prevention of these cracks can be made possible only by decreasing diffusible hydrogen concentration through postweld heat treatment under lower temperature as compared with a conventional intermediate stress relief annealing.