強度ミスマッチの影響を考慮した鋼多層溶接熱影響部の破壊靭性評価

Abstract

HAZ-notched CTOD tests have been conducted for multipass welds of high strength steels with SMYS 420 to 460 MPa for offshore structural use. The weld metal strength overmatch brings about different fracture behaviors depending upon the actual CGHAZ toughness. In the case that the CGHAZ is completely embrittled, the weld metal overmatch leads to lower bound critical CTOD value. This is due to the elevation of the local stress in the CGHAZ caused by the constraint effect by the overmatch weld metal. The fracture surface is generally flat and brittle fracture originates from the CGHAZ sampled by the fatigue crack front. Larger fraction of the CGHAZ along the crack front gives smaller critical CTOD value. On the other hand, in the case that the CGHAZ has moderate toughness, the weld metal overmatch can produce a higher critical CTOD value at brittle fracture initiation. This is attributed to the crack path deviation toward the base metal. The plastic strain accumulates preferably to a larger extent in the softer base metal side, before the critical stress condition for brittle fracture is attained at the CGHAZ. This asymmetrical plastic deformation promotes the deviation of ductile crack growth from the crack tip CGHAZ. In such a case, the critical CTOD value does not always reflect the CGHAZ toughness itself.Notch tip location much closer to weld metal sometimes causes fracture initiation at the weld metal even if the fatigue crack tip samples the CGHAZ. Such test data can not tell the real CGHAZ toughness.The meaning of critical CTOD value obtained in the test must be identified in the fracture toughness eveluation of the weld CGHAZ. This paper presents an evaluation procedure for CTOD test results of HAZ-notched welds taking the mis-matching effect into account.