高張力鋼溶接部限界COD値に及ぼす諸影響因子の検討

概要

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Factors controlling critical COD value (δ-c) of multi-pass submerged arc welded joints of 500MPa class high-tensile strength steels were investigated through metallographic examination of fatigue crack-tip location and fracture initiation points in the COD test specimens. Main results obtained are as follows : Brittle fractures with low δ-c were initiated mostly from the grain-coarsened zone that was reheated between A_<c1> and A_<c3> transformation temperatures and secondarily from the grain-coarsened zone that was reheated below A_<c1> by the subsequent welding passes. Occurrence of the fatigue crack-tip to the grain-coarsened zone was a prerequisite for low δ_c. Critical COD value was not related with the sum of the lengths of the grain-coarsened zones crossing the fatigue crack-tip front but decreased monotonically with increasing size of the grain-coarsened zone from which fracture was initiated. This result was explained by the following reasons : (1) Larger size of the grain-coarsened zone causes lower reheating temperature of the subsequent welding pass, which results in lower toughness of this zone. (2) Higher tensile stress at the crack-tip is achieved at larger size of the grain-coarsened zone because the grain-coarsened zone has higher hardness than the surrounding regions. Higher crack-tip tensile stress promotes brittle fracture initiation at smaller crack-tip deformation.