Microstructure Characterization Of Additively Manufactured Alloy 718

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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.

Product Number: 51323-19419-SG

Author: Liu Cao, Dian Li, Sydney Fields, Yufeng Zheng

Publication Date: 2023

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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 MR0175/ISO 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 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.