Failure of an AISI 8630-Mod low alloy steel to C-Mn steel dissimilar metal weld, made using nickel alloy UNS N06625 welding consumables

摘要

Approximately one year after installation, it was discovered that a failure had occurred, in deep water, at a joint between an elbow from a water injection flowline and a forged connector to a subsea manifold. The line had separated at the welded joint between the AISI 8630-Mod (8630M) low alloy steel connector and the ASTM A694/A694M-03 - Grade F65 (F65) C-Mn steel elbow, made using nickel alloy UNS N06625 (Alloy 625) welding consumables. Although this type of joint is commonly employed subsea, there have been a few failures and this combination of materials is no longer preferred for this type of service. Both halves of the fractured joint were retrieved from the seabed for failure investigation. A detailed investigation was undertaken, including extensive visual inspection, metallography and fractography, using scanning electron microscopy, to characterise the fracture event, fracture morphologies and corresponding microstructures. It was also attempted to estimate the loads which might have been responsible for fracturing the joint. In addition, supporting tests and examinations were carried out on the materials, to assess the assembly against the original construction specifications. The paper presents the observations made and discusses the failure mechanism and its relationship with the environmental conditions encountered.