Abstract
The combination of strength, corrosion resistance, and excellent weldability makes Alloy 718 an attractive alloy for additive manufacturing (AM) applications, but the AM build process generates large compositional and microstructural heterogeneities. The formation of the δ-phase is of particular importance within the petroleum and natural gas (PNG) industries and a reduced Nb content is one method currently in use to control δ-phase growth in wrought IN718. However, it is not clear how effective that strategy will be in AM components as the growth kinetics of δ -phase are exceptionally sensitive to the build parameters used in the AM processing. Since the API 6ACRA treatment protocol used for wrought Alloy 718 does not produce the same properties in AM 718, a refined post-build heat treatment is required to relieve residual stresses and produce uniform microstructures and properties. An integrated computational materials engineering (ICME) framework was adopted for this work to develop an effective heat treatment protocol for AM-processed IN718 that consistently achieves the requisite performance metrics while minimizing the δ-phase growth. for oil and gas industry applications. The results revealed that even though the wrought heat treatment does not completely remove all the AM solidification microstructure, it was sufficient to precipitate γ’ to achieve the 1035 MPa (150 ksi) strength level. Precipitation of γ’’, which governs the 850 MPa (120 ksi) strength level, is far more difficult to achieve without significant co-precipitation of the δ-phase. Additional characterizations and model refinements are in progress to optimize the γ’ and γ’’ precipitation and to control the precipitation of the δ-phase.
