To evaluate the influence of stress concentration and microstructure on susceptibility of precipitation hardenable nickel alloys to hydrogen embrittlement.
Unnotched and acutely notched circular cross-section tensile specimens were tested in air and under hydrogen-charging conditions in a solution of 3.5% sodium chloride with CP, to simulate subsea service. After testing, the fracture surfaces were examined at high magnification to determine the fracture morphology at various radial positions.
The materials were also characterised using a combination of metallography, hardness testing, and light and scanning electron microscopy.
Testing of the unnotched specimens under CP did not result in a significant reduction in proof strength. However, there was a clear relationship between increasing material strength, as measured 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 (UNS N09946). 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. It would appear as though the onset of plasticity was the point of divergence in material properties, and that the materials behaved similarly in equivalent tests in air within the elastic regime.
Figure 1 shows the fracture morphology of the hydrogen-charged specimens consisted of a ring of brittle faceted fracture which corresponded to the area into which hydrogen had 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-voids. 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, whilst macroscopically brittle, is fundamentally a high strain and dislocation activated plastic process.