Precipitation hardenable (PH) nickel (Ni) alloys are often the most reliable engineering materials for demanding oilfield upstream and subsea applications especially in deep sour wells. Despite their ...superior corrosion resistance and mechanical properties over a broad range of temperatures, the applicability of PH Ni alloys has been questioned due to their susceptibility to hydrogen embrittlement (HE), as confirmed in documented failures of components in upstream applications. While extensive work has been done in recent years to develop testing methodologies for benchmarking PH Ni alloys in terms of their HE susceptibility, limited scientific research has been conducted to achieve improved foundational knowledge about the role of microstructural particularities in these alloys on their mechanical behaviour in environments promoting hydrogen uptake. Precipitates such as the γ′, γ′′ and δ-phase are well known for defining the mechanical and chemical properties of these alloys. To elucidate the effect of precipitates in the microstructure of the oil-patch PH Ni alloy 718 on its HE susceptibility, slow strain rate tests under continuous hydrogen charging were conducted on material after several different age-hardening treatments. By correlating the obtained results with those from the microstructural and fractographic characterization, it was concluded that HE susceptibility of oil-patch alloy 718 is strongly influenced by the amount and size of precipitates such as the γ′ and γ′′ as well as the δ-phase rather than by the strength level only. In addition, several HE mechanisms including hydrogen-enhanced decohesion and hydrogen-enhanced local plasticity were observed taking place on oil-patch alloy 718, depending upon the characteristics of these phases when present in the microstructure.
This article is part of the themed issue ‘The challenges of hydrogen and metals’.
Precipitation hardenable (PH) nickel (Ni) alloys are often the most reliable engineering materials for demanding oilfield upstream and subsea applications especially in deep sour wells. Despite their ...superior corrosion resistance and mechanical properties over a broad range of temperatures, the applicability of PH Ni alloys has been questioned due to their susceptibility to hydrogen embrittlement (HE), as confirmed in documented failures of components in upstream applications. While extensive work has been done in recent years to develop testing methodologies for benchmarking PH Ni alloys in terms of their HE susceptibility, limited scientific research has been conducted to achieve improved foundational knowledge about the role of microstructural particularities in these alloys on their mechanical behaviour in environments promoting hydrogen uptake. Precipitates such as the γ′, γ″ and δ-phase are well known for defining the mechanical and chemical properties of these alloys. To elucidate the effect of precipitates in the microstructure of the oil-patch PH Ni alloy 718 on its HE susceptibility, slow strain rate tests under continuous hydrogen charging were conducted on material after several different age-hardening treatments. By correlating the obtained results with those from the microstructural and fractographic characterization, it was concluded that HE susceptibility of oil-patch alloy 718 is strongly influenced by the amount and size of precipitates such as the ϒ' and ϒ'' as well as the δ-phase rather than by the strength level only. In addition, several HE mechanisms including hydrogenenhanced decohesion and hydrogen-enhanced local plasticity were observed taking place on oil-patch alloy 718, depending upon the characteristics of these phases when present in the microstructure. This article is part of the themed issue "The challenges of hydrogen and metals'.
Low-alloyed, high-strength steels with yield strength above 1200 MPa are frequently used in the automotive industry, in order to reduce the weight of the components. However, hydrogen embrittlement ...susceptibility is often a concern for this group of materials. Hydrogen can be produced on the metal surface, as a result of electrochemical corrosion reaction during service. The aim of this study is to evaluate the impact of the various loading procedures according to the existing technical specifications for environmental hydrogen embrittlement (EHE) testing. The hydrogen embrittlement susceptibility of high strength fasteners (strength class 14.8) was tested according to the well-established testing specifications: DIN 50969-2, DIN EN ISO 7539, ASTM F1624-12 in 5 wt.-% sodium chloride solution acidified to pH 3 with hydrochloric acid. Open circuit potential measurements were carried out during the testing procedure to gain understanding of the damage process in each of the testing approaches. Under the chosen testing conditions, rising step load (RSL) testing approach, specified in ASTM F1624-12, appeared as the most sensitive method for the evaluation of the material's hydrogen embrittlement susceptibility.
The resistance against stress corrosion cracking of a metallic material strongly depends on the quality of the present oxide layer as well as material resistance to hydrogen induced cracking. In case ...of nickel alloy UNS N07718 (alloy 718), the material susceptibility to stress corrosion cracking has been assumed to depend in particular on the material hardness.
A systematic study of the corrosion properties of alloy 718 has been conducted with the main objective to investigate the nature of the correlation between aging temperature and the material corrosion performance as this correlation has been reported in the literature.
Electrochemical experiments conducted on samples aged at different temperatures indicated a higher localized corrosion resistance for those samples which have been heat treated at lower temperature (8h at 760 °C) compared to those heat treated at higher temperature (8h at 870 °C). However, the observed differences in the corrosion behavior could not be attributed solely to the precipitation of the intermetallic phases, such as γ'+γ'' and δ. Complementary investigations of the oxide layers revealed differences in their composition as a result of the aging temperature. Further, the presence of atomic hydrogen in the material bulk was found to reduce the material ability to build a protective passive layer. The degree of the impairment increased with an increasing heat treatment temperature.
No correlation between the hardness of the alloy 718 and its hydrogen embrittlement susceptibility could be demonstrated. In addition, besides the recognized detrimental effect of orthorhombic phase precipitation δ on the hydrogen embrittlement resistance of alloy 718, the hydrogen embrittlement susceptibility of the material was found to correlate with the size and fraction of γ'+γ'' precipitates. Among the tested material variants, the double aged material (8h at 720 °C followed by 8h at 620 °C) revealed the lowest degree of susceptibility. Thus, it is the morphology of the strengthening phase but not the hardness (as suggested in the API specification) that provides the significant contribution to the degradation mechanism of alloy 718 in hydrogen containing environments.
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