This paper discusses the results of an on-going investigation to rank the hydrogen embrittlement susceptibility of precipitation strengthened Ni-based superalloys using a rising step load (RSL) technique in four-point bending. Alloys characterized include UNS N07716, UNS N09925, and UNS N07718 heat treated to various strength conditions. Hydrogen was introduced either in-situ via an applied cathodic potential (external hydrogen) or prior to testing by thermally charging in gaseous hydrogen (internal hydrogen).
The RSL method measures the stress intensity required to cause propagation of a pre-existing flaw with and without the presence of hydrogen. Another method used to evaluate hydrogen embrittlement is the slow strain rate test, which characterizes material through the relative loss in ductility due to hydrogen exposure. While both methods provide a relevant measure of hydrogen embrittlement susceptibility, fracture mechanics testing such as the RSL method offers more useful information for the design of components.
Results indicate hydrogen embrittlement susceptibility is related to both the microstructure and the composition of the alloy. Measured fracture toughness values are also affected by the method of hydrogen exposure, however general trends among the materials are seen. In light of these results, the relative merits of the RSL testing method compared to slow strain rate testing is addressed, with a focus on overall applicability.
Failures attributed to hydrogen embrittlement of Ni-based superalloys in off-shore wells have emphasized the need to identify the limitations of these alloys in environments where the potential for hydrogen embrittlement exist.
