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On the Susceptibility of Precipitation Hardened Nickel Alloys to Hydrogen Assisted Cracking

Picture for On the Susceptibility of Precipitation Hardened Nickel Alloys to Hydrogen Assisted Cracking
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Precipitation Hardened (PH) Nickel alloys have been widely used in the Oil & Gas Industry for decades as these materials offer high strength and outstanding corrosion resistance in aggressive environments. They are commonly used in high-strength components in downhole wellhead subsea and Christmas tree equipment. However high profile failures of equipment have occurred including tubing hangers cross-overs and subsea bolts with alloys such as UNS N07718 UNS N07716 or UNS N07725. In all these cases the mechanism identified was Hydrogen Assisted Cracking (HAC) as the result of the interaction between atomic hydrogen adsorbed by the alloy and its microstructure.PH Nickel alloys are all subject to precipitation of secondary and tertiary phases which if processed improperly (particularly during hot working and heat treatment) may adversely affect the material properties required for the intended application. Despite the number of scientific and technical contributions produced over the last years the interaction between these complex microstructural features and atomic hydrogen is still not understood and is further complicated by variations in testing approaches used to simulate severe hydrogen charging conditions. The present paper provides insights on the HAC failure mechanism for API 6ACRA PH Nickel alloys comparing findings from numerous studies. In addition implications for currently adopted standards and emerging specifications are also presented and discussed.

Product Number: 51319-12846-SG
Author: Roberto Morana
Publication Date: 2019
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Picture for Hydrogen Embrittlement of High Strength Precipitation Hardenable Nickel Alloys
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Hydrogen Embrittlement of High Strength Precipitation Hardenable Nickel Alloys

Product Number: 51319-13161-SG
Author: Michael Dodge
Publication Date: 2019
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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.
Picture for The Effect of Hydrogen on Plain and Notched Test Specimens of PH Nickel Alloys
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51317--9655-The Effect of Hydrogen on Plain and Notched Test Specimens of PH Nickel Alloys

Product Number: 51317--9655-SG
ISBN: 9655 2017 CP
Author: Stephen McCoy
Publication Date: 2017
$20.00

This paper reviews previous work and shows laboratory results using different test techniques to demonstrate the influence of yield strength and microstructure on the resistance to Hydrogen Stress Cracking of precipitation hardenable Nickel alloys N07718, N09925, N07725, N09945 and N09946.

Picture for Susceptibility to Hydrogen Induced Stress Cracking of Alloy 718 and Alloy 725 Under Cathodic Polarization
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Susceptibility to Hydrogen Induced Stress Cracking of Alloy 718 and Alloy 725 Under Cathodic Polarization

Product Number: 51315-5597-SG
ISBN: 5597 2015 CP
Author: Gaute Stenerud
Publication Date: 2015
$20.00

 A test project to examine the susceptibility of Hydrogen Induced Stress Cracking (HISC) has been executed. In this project hydrogen charged samples of Alloy 718 and Alloy 725 have been exposed under tensile stress to establish critical stress levels for initiation of HISC.
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