Alloy UNS N07718 (known as Alloy 718) is a precipitation hardening nickel alloy containing additions of chromium, niobium, titanium, aluminum and molybdenum. This combination of elements provides an alloy with a combination of high yield strength and corrosion resistance required in sour service applications. Through the precipitation hardening heat treatment, the alloy precipitates the intermetallic Gamma Prime (ordered fcc Ni3Al) and Gamma Double Prime (bcc tetragonal Ni3Nb) phases, which are responsible for elevating the yield strength of the material. Additionally to the importance of these both phases in the hardening process, previous studies showed that the microstructure of Alloy 718 may also have a direct influence on its susceptibility to Hydrogen Embrittlement.
Laboratory melts with modified compositions based on Alloy UNS N07718 were produced and tested to correlate both mechanical and hydrogen embrittlement properties. The testing plan included mechanical testing, Slow Strain Rate Tensile (SSRT) tests under cathodic protection and numerical thermodynamic simulations.
Its outstanding mechanical performance and its very good corrosion resistance have turned Alloy UNS N07718 into the most preferred applied nickel alloys in the Oil & Gas industry.1 However, recent field component failures reported the occurrence of hydrogen embrittlement,2 which can be a serious limitation to the material application. The continuing development of oil and gas production industry pushes the needs to develop new materials technology for applications involving high temperatures, high pressures and increasingly aggressive service environments.3
The outstanding mechanical features of Alloy UNS N07718 are resultant of the precipitation of the intermetallic phases Gamma Prime (ordered fcc Ni3Al) and Gamma Double Prime (bcc tetragonal Ni3Nb), which is allowed by the presence of the alloying elements niobium, aluminum and titanium.
Previous studies show that the strengthening phases Gamma Prime and Gamma Double Prime play an important role on the corrosion resistance of the alloy UNS N07718.4-10 Gosheva et al. have made important contributions clarifying the impact of microstructure on the hydrogen embrittlement susceptibility of UNS N07718.8 Their studies concluded that the amount of hydrogen stored in the material during cathodic hydrogen charging was predominantly dependent on the strengthening precipitates and their interface with the Gamma matrix. The precipitates act as trapping sites, slowing down the diffusion process. Klapper et al. showed that the amount and size of precipitates such as Gamma Prime and Gamma Double Prime, as well as the Delta phase, rather than the strength or hardness level only, predominantly affected the hydrogen embrittlement susceptibility and defined the type of embrittlement mechanism.9 According to his studies, HEDE- (Hydrogen Enhanced Decohesion) and HELP (Hydrogen Enhanced Localized Plasticity)-assisted shear localization may occur on oil-patch N07718 depending on the amount and localization of Delta phase.
