The susceptibility of nickel-base alloys to hydrogen-assisted cracking — known as HISC (Hydrogen-Induced Stress Cracking) — is historically assessed through metallographic examination in accordance with standard API 6A CRA. Alloy 718 heats that had been qualified under this standard nevertheless suffered several in-service failures attributed to HISC. The aim of this study was therefore to develop a more discriminating test method capable of distinguishing between heats.
The work was initially initiated in collaboration with an industrial partner. The relevance of using a slow strain rate technique was demonstrated by working on different heats of alloy 718, some of which had experienced HISC failures and others had not.
Based on these initial results, a JIP (Joint Industrial Project) funded by several major oil industry partners was carried out to verify the applicability of this technique to other nickel-base grades and to conduct inter-laboratory testing. This JIP was carried out in collaboration with SwRI (USA) and CSM (Italy).
Finally, based on the JIP results, the test was standardised. It is now described in Annex C of standard NACE TM0198.