Formation Mechanism of Folding in Multi-Layer Welds with Single-Bevel Groove

Very small flaws are occasionally observed in the root of the penetration beads (uranami beads) of multi-layer welds with single-bevel groove. The formation mechanism of these flaws has been studied to elucidate whether the flaws are defects or not toward the increased severity of safety requirement under earth-wide enviromental issues.The results show that the flaw is not a weld crack but a folding to be grown up from a buckled weld zone at the root of non-symmetric throats in the first layer of penetration beads, where stresses are concentrated by the thermal contractions of weld metals after the second layers. It is demonstrated to prevent effectively the formation of foldings by the introduction of newly developed welding method.