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Additive manufacturing (AM) is a transformative technology that has opened areas of design space that were previously inaccessible by enabling the production of complex, three-dimensional parts and intricate geometries that were impractical to produce via traditional manufacturing methods. However, the extreme thermo-mechanical conditions in the AM build process (e.g., cooling rates ranging from 103 K/sto 106 K/s and repeated heating/cooling cycles) generate deleterious microstructures with high residual stresses, and extreme compositional gradients.
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The Alloy UNS(1) N07718 is among the most used alloys in the oil and gas industry. Due to the presence of the alloying elements niobium, aluminum and titanium, this alloy is precipitation hardenable by the formation of the phases Gamma’ and Gamma’’. Although presenting excellent strength properties and good resistance in sour gas applications, this material is known to be susceptible to hydrogen embrittlement and most field failures are related to this limiting property.
The use of Alloy 718 (UNS N07718) for oil & gas applications is regulated by the API(2) 6ACRA1 standard and it is available in three different grades, the 120K, with minimum 120 ksi yield strength, the 140K, with minimum 140 ksi yield strength, and the 150K, with minimum 150 ksi yield strength. Previous studies showed that, due to the different hardening heat treatment parameters, each of the available grades presents a different precipitation behavior in terms of distribution and amount of precipitates, and the obtained microstructure is directly related to the resistance of the material to hydrogen embrittlement.
Alloy UNS N07718 (hereafter abbreviated as 718) is one of the most versatile precipitation-hardened nickel-based corrosion-resistant alloys (CRAs) used for both surface and sub-sea components in oil and gas production service. API 6ACRA provides heat treatment windows and acceptance criteria for 718 in these oil and gas production environments, in which the heat treatment is intended to homogenize the microstructure and obtain the correct microstructure for targeting the desired mechanical properties. For fabricating high temperature materials via additive manufacturing (AM), alloy 718 is a primary focus due to its widespread applications in the past 60 years and excellent weldability in either age hardened or annealed condition.
Over the past twenty years, additive manufacturing (AM) has gradually emerged as an important commercial manufacturing technology for the production of components, particularly complex and highvalue metallic components. AM enables the layer-by-layer rapid manufacturing of near-net shapes using 3D computer-aided design data and typically minimizes raw-material wastes.
Alloy UNS N07718 (hereafter abbreviated as 718) is one of the most versatile precipitation-hardened nickel-based corrosion-resistant alloys (CRAs) used for both surface and sub-sea components in oil and gas production service. API 6ACRA1 provides heat treatment windows and acceptance criteria for 718 in these oil and gas production environments, in which the heat treatment is intended to obtain high strength and to minimize the formation of δ-phase at grain boundaries. As pointed out in NACE MR0175 Part 32 (Table 1), field failures of 718 components in sour service are primarily related to stress corrosion cracking (SCC) at elevated temperatures and hydrogen embrittlement in the lower temperature range. The latter is specifically called galvanically induced hydrogen stress cracking (GHSC or GIHSC), which is typically caused by atomic hydrogen uptake from galvanic corrosion or cathodic protection (CP) when 718 is used with steel components in a seawater environment. CP is normally used to protect steel component from corrosion in subsea environments.
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.
Parts produced via additive manufacturing (AM) are being adopted broadly among many industries andused in an array of applications. AM parts are attractive to these industries for several reasons. Complexgeometries that cannot be manufactured using traditional, subtractive methods can be producedadditively.