Abstract
The widely adopted temperature for solid solution heat treatment (ST) for the conventionally fabricated Inconel 718 is 1100 °C for a hold time of 1 h or less. This ST scheme is, however, not enough to dissolve Laves and annihilate dislocations completely in samples fabricated with Laser metal powder bed fusion (L-PBF) additive manufacturing (AM)-Inconel 718. Despite this, the highest hardness obtained after aging for ST temperatures (970–1250 °C) is at 1100 °C/1 as we have ascertained in our previous studies. The unreleased residual stresses in the retained lattice defects potentially affect other properties of the material. Hence, this work aims to investigate if a longer hold time of ST at 1100 °C will lead to complete recrystallization while maintaining the hardness after aging or not. For this study, L-PBF-Inconel 718 samples were ST at 1100 °C at various hold times (1, 3, 6, 9, 16, or 24 h) and aged to study the effects on microstructure and hardness. In addition, a sample was directly aged to study the effects of bypassing ST. The samples (ST and aged) gain hardness by 43–49%. The high density of annealing twins evolved during 3 h of ST and only slightly varies for longer ST.
Highlights
Inconel 718 is a high-strength corrosion-resistant nickel-based superalloy used in the temperature range of −252 to 704 ◦ C [1]
Solid solution heat treatment can lead to recovery, recrystallization, and grain growth depending on the temperature and/or hold time
Complete strain-free recrystallized microstructure can be obtained after solid solution heat treatment for longer than 7 h, but most likely less than 9 h at 1100 ◦ C
Summary
Inconel 718 is a high-strength corrosion-resistant nickel-based superalloy used in the temperature range of −252 to 704 ◦ C [1]. The optimum microstructure and mechanical strength of Inconel 718 is generally achieved after performing two steps of post-fabrication heat treatments. These are solid solution heat treatment (ST) and aging ( known as precipitation hardening). The solid solution heat treatment is often performed in the temperature range of 970 to 1250 ◦ C (the fusion temperature of Inconel 718 is ≈1300 ◦ C) for a few hours to dissolve Laves phase in order to release age-hardening constituents (Nb, Ti, and Al) into solid solution in the matrix [2,3,4,5,6].
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