metals Article Heat Treatment Effects on Pristine and Cold-Worked Thin-Walled Inconel 625 Gabriel Demeneghi *, Skylar Elliott, Ellen Rabenberg, Ayman Girgis, William Tilson, Annette Gray and Gregory Jerman   Citation: Demeneghi, G.; Elliott, S.; Rabenberg, E.; Girgis, A.; Tilson, W.; Gray, A.; Jerman, G. Heat Treatment Effects on Pristine and Cold-Worked Thin-Walled Inconel 625. Metals 2021, 11, 1746. https://doi.org/10.3390/ met11111746 Academic Editor: Frank Czerwinski Received: 29 September 2021 Accepted: 26 October 2021 Published: 31 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). NASA Marshall Space Flight Center, 4602 Martin Rd SW, Huntsville, AL 35808, USA; engineergirl47@gmail.com (S.E.); ellen.m.rabenberg@nasa.gov (E.R.); ayman.m.girgis@nasa.gov (A.G.); william.g.tilson@nasa.gov (W.T.); annette.r.gray@nasa.gov (A.G.); gregory.a.jerman@nasa.gov(G.J.) * Correspondence: gabriel.demeneghi@nasa.gov Abstract: Thin-walled Inconel 625 sheet metal was sectioned into tensile specimens, plastically strained, and then heat treated. Specimens were pulled to a targeted strain, unloaded, and then sub- jected to one of two heat treatments with the goal of restoring the full ductility and total plastic strain capability of the material. Post-heat treatment tensile testing was performed at room temperature to evaluate the heat treatment efficacy and then followed by hardness and microstructural analysis. The results showed the amount of material recovery was affected by the initial amount of plastic strain imparted to the tensile specimen before heat treatment. Although recrystallization was not observed, grains did elongate in the load direction, and the Kernel average misorientation (KAM) increased with heat treatment. Furthermore, specimens prestrained to 40% and heat treated at 980 ◦ C successfully recovered 88% of pre-heat treatment strain capability prior to fracturing. Keywords: Inconel; thin wall; tensile; microhardness; microstructure; heat treatment 1. Introduction Inconel 625 is a solid solution strengthened nickel-based superalloy commonly used to manufacture components requiring high temperature strength, excellent corrosion resis- tance, good weldability, and processability in harsh environments [1–3]. Inconel 625LCF is a special grade of Inconel 625 used in applications requiring improved low cycle fatigue (LCF) properties. The improved LCF properties are the result of controlled microstructures with fine grain sizes (American society for testing materials (ASTM) No. 5 or finer), which are achieved by appropriate heat treatments and tight control of impurities [4]. Because of Inconel 625’s outstanding properties, it is a material of choice for numerous applica- tions across several industries, including aeronautical, aerospace, chemical, petrochemical, marine, and nuclear [1,3,5]. During manufacturing, the material might undergo several machining, forming, and shaping processes to obtain its final shape. These processes intro- duce significant deformation in the part, leading to hardening, with an associated increase in strength and reduction in ductility, thus presenting properties that may significantly differ from the baseline material properties. Metallic materials subjected to prestrain cold-work are well known to display an increase in strength and decrease in ductility due to work hardening [6–10], while heat treatments generally reduce strength and increase ductility [2,11,12]. Heat-treatment effects on Inconel 625 were studied under different thermo-mechanical conditions (i.e., as solution treated, cold rolled, and cold rolled and annealed) [13]. It was determined that cold rolling followed by annealing at 800 ◦ C for 30 min resulted in a strength increase from 291 to 676 MPa and a ductility reduction from 75 to 50%. The strength increase was attributed to grain boundary and twin boundary strengthening [13]. Additionally, a study of the influence of prestrain and heat treatment on aluminum sheet mechanical properties found that deformed and heat-treated specimens exhibited higher strength when compared to Metals 2021, 11, 1746. https://doi.org/10.3390/met11111746 https://www.mdpi.com/journal/metals