Fabrication Issues Associated with Butt-Welded Branch Connections in Thick-Walled Superduplex Stainless Steel Piping

Abstract During a recent fabrication project, cracking and intermetallic formation were encountered in the weld metal of butt-welded branch connections in thick-walled superduplex stainless steel (UNS S32750 / S32760). This was not evident during weld procedure qualification, which, in line with ASME IX and DNV RP B204, was performed on girth welds.1,2 However, the combination of thick sections in branch weld designs and the need for reliable inspection resulted in a design with high weld volume, comprising multiple layers. This paper provides details of the investigation carried out to understand the causes of the embrittlement and cracking. Together with a review of all processes and practices involved, the program included microstructural characterization of a range of connection geometries, employing advanced characterization techniques to augment conventional microscopy, mechanical testing and numerical modelling. The investigation revealed that cracking had occurred by a predominantly brittle mechanism. Low levels of sigma phase were consistently detected in the root weld metal of the assemblies examined, but, in assemblies incorporating multiple connections, higher levels of sigma were recorded in the root and mid-thickness, and associated with a pronounced drop in impact toughness. The causes of the embrittlement are discussed together with recommended modifications to the fabrication approach to minimize the risk to future fabrications.