Assessing the Performance of Additively Manufactured Alloy 718 after Optimized Heat Treatment for Oil and Gas Applications

Abstract Alloy 718 is attractive for many additive manufacturing (AM) applications, because it exhibits a desirable combination of strength, corrosion resistance, and excellent weldability, but the processing conditions in AM generate considerable residual stresses, compositional gradients, and microstructural heterogeneities that degrade properties and performance. The protocol for solution anneal prescribed in the American Petroleum Institute (API) 6ACRA standard does not sufficiently homogenize the as-printed AM microstructure, thus the required properties cannot be reliably achieved. In wrought material, precipitates, such as δ-phase and carbides, are known to alter the local electrochemical conditions, and the concomitant modulus mismatch with the surrounding matrix is also known to affect the crack propagation resistance. Since the growth of the δ-phase is of particular concern within the oil and gas industry, the question is whether AM processing increases the susceptibility to environmentally assisted cracking. An effective post-build heat treatment was developed specifically for AM-Alloy 718 and applied to a set of tensile specimens. Slow strain rate tensile tests were used to evaluate the influence that AM processing may have on cracking resistance. AM and wrought samples were tested in an inert reference environment and in an acidified environment under galvanostatic control. The results revealed that the performance of the AM alloy was equal to, or better than, the wrought material in both environments, which verified the effectiveness of the optimized heat treatment protocol.