Additive manufacturing (AM) provides a new approach to the design and manufacture of components from metal powder and provides unique advantages over traditional manufacturing. An industry joint project was recently conducted to investigate the performance of AM’d alloy 718 (UNS N077168) in sour conditions specified in NACE(1) MR0175/ISO(2)15156. The evident variation of properties and performance was noticed on three batches of AM 718 samples from different vendors, even though they were solution annealed and aged individually to meet the same specification of API(3) 6ACRA 150K grade. A thorough microstructure characterization were performed on three AM 718 materials to reveal the differences in γ grains, strengthening γ’ and γ" precipitates, other precipitates, and microstructural features. All three AM 718 materials showed different microstructures from each other, and they were also different from wrought 718 150K grade. The results suggest specific heat-treatment for AM 718 to achieve comparable microstructure and thus properties to wrought 718.
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 homogenize the microstructure and obtain the correct microstructure for targeting the desired mechanical properties.2 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.3 The development of AM technologies and commercialization over the last decade not only makes the powder materials and machinery more affordable but also enables building quality metal parts in short turn-over time. Due to the rapid melting and solidification from AM processing, the as-built microstructure will vary significantly from wrought material.4 In addition, alloy 718 is a precipitation hardened nickel-based alloy and thus solution treatment and aging (STA) are typically used to minimize the formation of δ-phase at grain boundaries and obtain high strength by forming γ’ and γ" strengthening precipitates.
