Fracture toughness of a 9% Ni steel pipe girth welded with Ni-based superalloy 625 filler metal operating in a sour environment

Abstract

9% Ni steel was adopted, for the first time, to be used in the CO2 injection units in oil wells. It operates at 550 bar in sour environment (CO2 + H2S), in addition to being subject to abrupt temperature variations. Owing to the lack of knowledge on the behavior of 9% Ni steel under these operating conditions, crack tip opening displacement (CTOD) and J-integral tests were performed on base metal (BM), coarse grain heat-affected zone (CGHAZ), and weld metal (WM) of pipe joints girth welded with Ni-based superalloy 625 filler metal, both in air (at 27 °C and −90 °C) and sour environment (CO2 + H2S). The microstructure of the welded joints was characterized by optical and scanning electron microscopy, X-ray diffraction, and microhardness. In air, all welded joints exhibited high toughness and ductile fracture; however, in sour environment, BM experienced a sharp drop in toughness with a quasi-cleavage fracture. Although the CGHAZ was the hardest region of the welded joints, it was less influenced by the sour environment because it was close to WM (Ni-based superalloy 625); however, it exhibited a brittle fracture. Despite the decrease in the fracture toughness in sour environment, the welded joints met DNV-OS-F101 standards.