Hydrogen Embrittlement of High Strength Precipitation Hardenable Nickel Alloys
By M Dodge and M Gittos
The high strength and corrosion resistance of nickel-chromium-iron alloys, such as Alloys 718 (UNS N07718), 945 (UNS N09945) and 945X (UNS N09946), make them particularly good candidates for use in demanding environments in the upstream oil and gas industry. These materials generally perform well where resistance to sulphide stress cracking and chloride stress corrosion cracking is required. However, while these alloys are considered ‘NACE compliant', environmentally-assisted failures can still occur.
It is generally accepted that for hydrogen cracks to initiate, threshold conditions of stress, susceptible microstructure and hydrogen concentration must be exceeded. It stands to reason that complete eradication of any of these variables would prevent failure altogether. Of course this is not always practicable in the field, and a simpler approach is often to understand how these variables interact such that the risk of failure can be managed.
The effect of stress, hydrogen concentration and microstructure has been explored in isolation by a number of authors; however, there does not appear to be a unified source of information on the interaction between each variable. In this project, the effect of microstructure is explored by heat treating Alloys 718, 945 and 945X to standard and non-standard conditions. Tensile specimens were slow-strain-rate-tested in air and under cathodic protection (CP) to explore sensitivity to hydrogen embrittlement. Finally, the effect of a severe stress concentration, in the form of a sharp notch, was used to determine whether there is an enhanced susceptibility to hydrogen embrittlement due to the presence of local stress raisers. The results are compared with tests undertaken by other authors under various hydrogen-charging conditions.
SEM images of Alloy 945X specimen, solution treated condition, notched, tested under CP:
Testing of specimens under CP did not result in significant reductions in proof strength. However, there was a clear relationship between increasing material strength in air and reduced ultimate tensile strength (UTS) and elongation when tested under CP.
- The materials revealed an increased notch sensitivity when tested in the presence of hydrogen, particularly for the higher strength materials, such as Alloy 945X. Notch sensitivity in hydrogen was manifested mainly by reduced UTS. The increased notch UTS sensitivity in hydrogen is attributed to the interplay between strain localisation within the notch and the propensity for hydrogen to diffuse towards highly strained and plastically deformed regions.
- Macroscopically, the fracture morphology of the hydrogen-charged specimens consisted of a ring of brittle faceted fracture which corresponds to the area into which hydrogen has diffused during pre-charging and testing. Towards the centre of the specimen the fracture morphology became increasingly ductile;
- High magnification inspection of the embrittled portions of the fracture surface revealed the ‘brittle’ intragranular facets to be populated by slip band traces, the intersections of which were shown to be nucleation sites for micro- and nano-void formation. At high strains it is anticipated these voids will coalesce, resulting in hydrogen crack propagation. Most importantly, these results show that hydrogen embrittlement of these alloys, while macroscopically brittle, is fundamentally a high strain and dislocation activated plastic process.
a) Low magnification overview of the specimen’s fracture surface and notch;b) Higher magnification image of the centre of the specimen showing microvoid coalescence (MVC);c) Higher magnification image of the edge of the specimen.