Correlative analysis of digital imaging, acoustic emission, and fracture surface topography on hydrogen assisted cracking in Ni-alloy 625+

Abstract

Failures of Ni-based alloy components, used in oil and gas environments have been attributed to hydrogen embrittlement (HE). This has motivated the development of HE resistant Ni alloys without compromising their superior mechanical and corrosion properties. An understanding of crack initiation and propagation mechanisms is essential for improving the Ni alloy design and its resistance to HE. The objectives of this paper are: to elucidate the fracture events involved during a slow strain rate test (SSRT) of a Ni-based alloy, UNS N07716, under an electrochemical hydrogen (H) charging environment, and; to correlate these events with in-situ and offline failure analysis techniques. The in-situ techniques involved continuous monitoring of fracture events during a SSRT using a high-resolution digital camera and an acoustic emission (AE) sensor. The offline techniques include Fracture Surface Topography Analysis (FRASTA) and Finite Element Analysis (FEA). The results obtained from these analyses were correlated with the stress-strain behaviour to determine the crack initiation stress. It is observed that the ductility (based on elongation to failure) of UNS N07716 was reduced significantly under H environment when compared with its ductility in an inert environment. Furthermore, the fracture morphology of a sample tested in H was characterised by intergranular facets in contrast to dimples describing void coalescence in the inert environment. The failure analysis techniques used suggest that the time taken for crack initiation event is at variance between the individual techniques; however, it is clear that the fracture processes in an H environment include early brittle crack initiation with limited plastic deformation, in comparison with the uniform elongation leading to cup-and-cone ductile fracture in air. Early crack initiation is primarily attributed to the loss of fracture toughness in the H affected region, and the local fracture toughness is evaluated using FRASTA. The average value of local fracture toughness in the H induced fracture region was 55 MPa √m as compared to 270 MPa √m in air.