The Influence of Stress Concentration and Plastic Strain on the Resistance of Precipitation-Hardened Nickel Alloys to Hydrogen Embrittlement
TWI Industrial Member Report 1160-2022[pdf/ 6587 KB]
By David Griffiths and Mike Dodge
Precipitation-hardened nickel alloys (PHNAs), such as Alloy 718 (UNS N07718), are commonly used in oil and gas subsea service for components such as bolts, fasteners and gaskets, which require high strength and corrosion resistance. Numerous subsea failures of these materials have been attributed to hydrogen embrittlement (HE), caused by hydrogen evolved by cathodic protection (CP) systems, and the presence of geometrical design features, e.g. grooves, notches, threads etc. acting as stress raisers. In addition, some cases have been correlated with a compromised microstructure associated with inappropriately controlled alloy manufacturing processes. Although the coexistence of a susceptible microstructure, sufficiently high stress and hydrogen content is deemed necessary for cracking to occur, the significance of these parameters, and their complex interrelationship, is not understood. The lack of such understanding calls into question the relevance of the widely-used test methodologies for designing with PHNAs, and qualitative (e.g. QA/QC) and quantitative assessments of the resistance of PHNAs to HE, as it has not been established what combination of parameters is of highest significance to be monitored and measured. While failures due to HE of PHNAs have been a primary concern for the oil and gas industry, HE and subsequent failure of PHNAs is also a concern for all parts of the emerging hydrogen economy in which PHNA components come into contact with hydrogen containing environments.
Previously, TWI and other researchers have widely adopted the use of slow strain rate tensile (SSRT) testing of un-notched specimens, primarily as a qualitative screening method, to rank the HE susceptibility of various alloys in different microstructural conditions (see Dodge and Gittos, TWI CRP Report 2019). In some cases, attempts have been made to use the test and its results to obtain a quantitative insight into the cracking sequences and stress/strain thresholds. While the majority of the test output has been inherently qualitative, observations of slip bands, on the brittle fracture surfaces of SSRT test specimens, suggest that the mechanism of HE might be associated with a degree of plastic strain and strain localisation, prior to crack initiation. Given the potential existence of such a strain threshold, and the observation that most failures occur at stress concentrators, this study explored the role of stress raisers and their localised straining effect on the resistance of Alloy 718 and Alloy 945X (UNS N09946) to HE, using notches with different stress concentration factors (SCFs) introduced into the test specimens. SSRT testing and incremental step load testing were combined with finite element modelling to investigate the usefulness of each approach in terms of providing a robust, quantitative assessment and design criterion for HE resistance, based on a hypothesis that crack initiation might be due to the lack of local strain accommodation in the microstructure compromised by hydrogen.
- Taken in isolation, environmental-mechanical ISL test data, from an API 6ACRA:2015 compliant UNS N07718 alloy, would suggest the material is only susceptible to HE when stress raisers are present. However, fractographic evidence for HE was present in the absence of stress raisers in both the ISL and SSRT tests. This suggests that the two environmental test methodologies employed are not sufficiently sensitive, in their conventional form and interpretation, to determine significant embrittlement
- The resistance of UNS N07718 to HE, when notched with two different notch geometries (i.e. SCFs of 12 and 20), was found to be equivalent, when tested with the SSRT method. However, ISL testing indicated the more acute notch was marginally more susceptible to HE.
- The resistance of solution heat treated and aged UNS N09946 to HE was found to be equivalent to that of UNS N07718, when notched specimens were tested by ISL under CP; however notch acuity was not found to significantly affect HE resistance in UNS N09946.
- It is not possible to identify crack initiation, which is proposed to be the dominant factor controlling susceptibility to HE, using the current or conventional SSRT or ISL testing set-ups.
- Data from interrupted SSRT tests of notched specimens suggest that an equivalent plastic strain threshold must be exceeded for hydrogen-induced crack initiation.
An EBSD map showing a transgranular hydrogen cracking at the notch root of an interrupted SSRT test specimen