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Precipitation hardened (PH) Ni-alloys are widely used in the oil and gas industry since they provide an excellent combination of corrosion resistance and mechanical strength. Their use in the manufacture of API1 6A pressure-containing and pressure-controlling components is subject to the stringent requirements of specification API Standard 6ACRA. However, matching the requirements of API Standard 6ACRA does not preclude susceptibility of some PH Ni-alloys to hydrogen embrittlement (HE) and this in some cases has led to premature and unexpected failures of components made from suchmaterials.(
Hydrogen embrittlement susceptibility of precipitation hardened Ni- alloys is a complex function of several factors such as chemical composition, deformation mode, grain size and grain boundary precipitation of carbides and intermetallic phases. Understanding the role of these factors is fundamental in identifying metallurgical conditions that can enhance resistance of precipitation hardened Ni-alloys to hydrogen embrittlement.Analysis of hydrogen embrittlement data and a new test program have been carried out to further investigate the role of such factors and determine metallurgical conditions capable of improving hydrogen embrittlement resistance of UNS N09955 and UNS N07716 alloys. Results of the study are presented and discussed in this paper.
The high strength and corrosion resistance of nickel-chromium alloys such as Alloy 718 and nickel-iron-chromium alloys such as Alloys 945 and 945X make them particularly good candidates for use in demanding environments in the upstream oil and gas industry. These materials generally perform well where resistance to sulphide stress cracking and chloride stress corrosion cracking is required. However whilst these alloys are considered ‘NACE compliant' environmentally-assisted failures can still occur.It is generally accepted that for hydrogen cracks to initiate there must be a critical combination of stress susceptible microstructure and hydrogen concentration. In this project the effect of microstructure is explored by heat treating Alloy 718 945 and 945X to standard and non-standard conditions. Tensile specimens were slow-strain-rate-tested in air and under CP to explore sensitivity to hydrogen embrittlement. Finally the effect of a severe stress concentration in the form of a sharp notch was used to determine whether there is an enhanced susceptibility to hydrogen embrittlement due to the presence of local stress raisers. The results are compared with tests undertaken by other authors under various hydrogen-charging conditions.
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Materials properties that are used in specific oil and gas environments are de-rated due to the risks associated with hydrogen embrittlement cracking. In oil production environments the concern is for the onset of stress corrosion cracking (SCC), while in seawater environments the concern is for Hydrogen Induced Stress Cracking (HISC). Both are hydrogen embrittlement phenomena with the distinction being the source of hydrogen for each. In SSC the source of hydrogen is from the presence of H2S in the oil production fluids, and in HISC the source of hydrogen is from the dissociation of water from the cathodic protection system. This paper is focused on the latter phenomena and aims to characterize the susceptibility of carbon alloy steels as applied in fastener applications, in a seawater environment under cathodic protection.