Abstract
A comprehensive corrosion test program was conducted at high temperature and slightly sour service to qualify supermartensitic stainless steels (SMSS) with 2.5%Mo as flowline material for a new sub sea project. Girth welds were subjected to crack-resistance corrosion testing in the form of four- point bend (FPB) testing at 160°C. The test program involved pre-qualification, a robustness study of the post weld heat treatment (PWHT) procedure, welding procedure qualification, and finally a longterm test. The main concern was intergranular stress corrosion cracking (IGSCC).
IGSCC was observed in the SMSS welds in the as-welded condition at 160°C both in simulated formation water (FW) and in simulated condensed water (CW) solutions. No IGSCC was however observed in welds subjected to PWHT of 630°C for 5 minutes. Short transgranular cracks were observed in the heat-affected zone (HAZ) of welds that had been subjected to PWHT. Crack-like features observed in the optical microscope at large magnification made it difficult to distinguish between pre-existing cracks and cracks promoted by a corrosion process, but it was still obvious that a type of stress corrosion occurred in the initial test period, causing some crack growth. No further crack growth was however observed at 160°C in FW during 3-months and 6-months long-term tests. The test program showed that SMSS pipes containing 2.5%Mo was qualified for the Kristin conditions, but that 160°C probably is a borderline temperature for SMSS alloys regarding corrosion cracking.
