Evaluation Procedure for High Strength Steels for Offshore Structures Considering Strength Mis-Matching in Welds

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This study developed a procedure for assessing fracture performance of welded joints of high strength steels in the light of design solution of weldments. Major concern was an influence of strength mis-matching between the base and weld metals on the fracture mechanical behavior of welded joints. TMCP steels for offshore structures with yield point 420 to 500MPa class were employed. The transferability of 3-point bend test results to fracture performance assessment of wide plates was discussed with attention to strength mis-matching effect.Strength overmatch of the weld metal gave rise to different effects on the fracture behavior of the heat affected zone (HAZ)depending on the toughness level of the HAZ.In the case that the HAZ had a moderate toughness, the overmatch condition resulted in deviation of ductile crack extension toward a softer material. The resultant high critical CTOD at unstable fracture does not necessarily mean the actual toughness of the HAZ itself. On the other hand, in the case of the embrittled HAZ,the overmatch condition promoted fracture initiation in the HAZ and lowers the fracture toughness of the HAZ.This was due to the elevation of plastic constraint in the HAZ.Strength overmatching is not always beneficial The specimen geometry also affects the degree of constraint in the crack tip region. The deep notch bend specimen produces the geometrical constraint to the largest extent. The geometrical constraint in the deep notch bend specimen was more intense than the constraint caused by 25% strength overmatching for the center notched wide plate subjected to tension. The present study developed the procedure to transfer toughness results to fracture assessment of wide plates with taking constraint effects into account. A new methodology for fracture toughness requirement of welds was proposed as a function of the required deformability of weldnents.