Double-layer technique for welding subsea joints without post welding heat treatment

Abstract

Welded joints for subsea systems are usually buttered with a nickel alloy, heat-treated and then joined together. Although the majority of these joints have been successful throughout their service, some have suffered catastrophic failure, leading to investigations into the possible causes. The use of heat treatment has been considered the primary cause for failure in this type of joint. The main objective of this article is, with background on the methodology used on Double-Layer Controlled Deposition Technique – DL-CDT, along with the LPTS decision diagrams, to determine the relations between the buttering parameters that allow the welding of subaquatic joints, and the low need for heat treatment. MIG single-bead-on-plate welding was executed on samples of AISI 8630M steel. The welded samples were submitted to metallographic analyses of their geometric and microstructural characteristics and the confection of decision diagrams. Among the 200 possible relationships between the buttering parameters, only one presented positive gaps over all criteria. Grain refinements were observed in the HAZ of joints buttered for the parameters approved by the decision diagrams, while for the joints submitted to the Post Welding Heat Treatment (PWHT) peaks of microhardness on the buttering interface were found, which may result in fragilization of that region.

Key-words:
Subsea joints; PWHT; Double layer technique; LTPS criterion