Evaluation Method for Fracture Performance of Offshore Structural Steel Welds

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Welded joints produced by multi-pass welding have a marked variation in microstructure due to multiple thermal cycles. Frequently regions with considerably reduced fracture toughness, which are called local brittle zones (LBZ), appear in the vicinity of fusion line. In this study a method for determining the reliability of welded joints of offshore structural steels has been developed with taking the significance of LBZ into account.HAZ-notched CTOD test on multi-pass welds of TMCP steel, YP 420 MPa class for offshore structural use, revealed that fracture initiation in multi-layered HAZ is controlled by the coarse grained HAZ (CGHAZ) and that a lower bound fracture toughness of multi-pass welds is determined by the toughness of CGHAZ. The CGHAZ toughness is mainly controlled by the metallurgical properties, such as chemical composition and making process, of base steel plate and weld thermal cycles subjected. Decrease in Si and Al content brings about increasing CGHAZ toughness and a low Al-B-treated condition exhibits excellent CTOD performance even at low temperatures. Tempering effect by the succeeding weld passes also leads to the improvement in the CGHAZ toughness, due to the decomposition of the embrittled microstructure (MA-constituent) into the fine ferrite and cementite. These results enable us to guarantee a sound toughness of offshore structural steel welds. Moreover, by using the weakest link model considering both the metallurgical heterogeneity and the variation of fracture mechanics parameter along the crack front, the procedure was proposed for determining the fracture probability of welded joints from the CTOD test results.