Failure types and integrity management of bimetallic weldment in view of engineering critical assessment

Life management and structural integrity assessment of bimetallic welds, as presented in codes and procedures, relies on simplistic strength analyses and practical methods from years of experience in operation. Complex loading conditions result in an intricate interaction of microstructural regions with gradients in material properties. This has limited the capabilities of currently existing methods to construct the assessment on the basis of actual failure mechanisms. Current work addresses the fracture mechanics approach combining characteristic failure modes to the failure types exhibited in actual service conditions. Two prime failure types were identified: cracks located at the outer heat-affected zone (HAZ) lead to unstable crack deflection towards the fusion boundary, whilst for cracks positioned near the fusion boundary, a low-toughness ductile fracture process occurred. The fracture types are found to correlate with characteristic properties of the different regions, particularly, with inequalities in hardening of the HAZ and the buttering. The compatibility of small-scale material characterization and engineering critical assessment results are evaluated applying the "SINTAP" structural integrity engineering procedure, emphasizing the identified failure modes and their effects to the evaluation.