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UNS N07718 (Alloy 718) is a precipitation-hardened Nickel alloy widely used for various components in oil and gas production service where a combination of high strength, good cracking and corrosion resistance is needed. API 6ACRA provides heat treatment windows and acceptance criteria for wrought Alloy 718 in these oil and gas production environments, in which the heat treatment is intended to obtain high strength desired for applications in combination with good environmental performance.
Additive Manufacturing (AM) is increasingly becoming a source of design, fabrication of complex components where machining from wrought material would be very cumbersome or introduced complicated welding processes.
Additive Manufacturing (AM) technology can be beneficial for complex parts which can be difficult to machine. The two main AM methods used for metallic material components are Powder Bed Fusion (PBF), Directed Energy Deposition (DED). UNS N07718 (Alloy 718) is a common precipitation hardenable Nickel Alloy used for various components in oil and gas especially needing good corrosion resistance. AM of Alloy 718 components and their performance characterization is of significant interest in the oil and gas application space as combining the novel manufacturing method and a highly corrosion resistant grade that can be heat treated to high strength can lead to applications that have complexity and need good long-term service. In this paper, properties of Alloy 718 additively manufactured via laser based, powder fed DED process is investigated. The Alloy 718 parts after building were solution annealed and aged per API 6ACRA UNS N07718-120 specification. Characterization included testing of mechanical properties, hardness, microstructure evaluation and corrosion testing. Stress Corrosion Cracking testing was done using SSRT (Slow Strain Rate Test) per NACE TM01982 and C-Ring testing via NACE TM01773 to evaluate performance limits in sour (H2S) service of this AM Alloy 718. Additionally, some pitting corrosion and acid testing was done to understand general corrosion resistance of AM Alloy 718. For comparison SSRT, pitting, acid testing was also done Wrought Alloy 718 bar per API 6ACRA UNS N07718-120.
Alloy 718 is a common oilfield material for permanent and service equipment in need of high-mechanical ratings and resistance to corrosion especially environmentally-assisted cracking in sour gas wells. In past decade Alloy 718 production from traditional and newer mills has greatly increased in response to global demands; independently yet driven by similar market growth additive manufacturing (AM) has expanded beyond rapid prototyping to become an industrial production process namely in the aerospace. Today 718 bar stocks as per API6CRA are produced by over a dozen mills worldwide;similarly 718 powder products are increasingly offered by both traditional and newer mills with intents to servea multitude ofAM technologies. Due to the rise of new economic forces in the O&G there are today needs for evaluating (ultimately qualifying) newer 718 producing mills as well as 718 powders in combination with various AM technologies. Due to concerns overraw-material properties a study was conducted to analyze 718 materials from these various origins utilizing (1) mill cert big-data analyses (2) third-party recertified mechanical test data (3) a multitude of sour service test results outside the traditional NACE MR0175/ISO15156 operational service limits among others. The later raw-material test implemented in the early 2010s for screening and qualification purposes aims at quantitatively comparing 718 production heats of various origins and with additive manufacturing also generating interests since the early 2010sthe same tests have also beenextended to determine how layer-by-layer deposited materials compare to bar stock materials.
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The overall goal was to determine if a set of additive manufacturing (AM) parts could comply with the testing requirements of wrought or welded materials for sour service as outlined in NACE MR0175 / ISO 15156-3:2015. Recommendations for a qualification pathway for AM parts in sour service are included.
The main scope of this work is to explore different process configurations to understand the corrosion response of additive manufactured alloys and to suggest the parameters to be controlled for future qualification in sour environment.