# The Influence of Accidental Overheating on the Microstructure and Hardness of the Inconel 718 Alloy
## 4 Conclusions  
The evolution of the Inconel 718 superalloy under overheating conditions was investigated within the temperature range of 400-1000 \({}^{\circ}\)C over 20 overheating cycles. * Structural Stability: At temperatures below 700 \({}^{\circ}\)C, the alloy retains its forged structure, characterized by twinned polyhedral grains and rows of hard phases aligned with the deformation direction (Figure 3a). * Microstructural Changes at Higher Temperatures: In the 700-1000 \({}^{\circ}\)C range, the alignment of hard phases becomes less distinct, while twinned polyhedral grains persist but with increased grain size (Figure 3b,c). Additionally, secondary-phase precipitates decrease in number and grow in size due to dissolution and coalescence phenomena. * Delta-Phase Formation: At 800 \({}^{\circ}\)C, after 20 overheating cycles, delta-phase precipitates were identified. * Surface Layer Integrity and Corrosion Resistance: The protective surface layers formed on the alloy remain compact and continuous at lower temperatures, particularly the chromium-rich layer, which mirrors the underlying metallic surface and enhances corrosion resistance. The diffusion and positioning of alloying elements in the oxide  
Figure 13: The evolution of microhardness with temperature was measured along the height of the sample, from the surface opposite to the one exposed to thermal shock, at temperatures ranging from 700 to 1000\({}^{\circ}\)C, over 20 overheating cycles. layers have been highlighted. However, increasing the temperature initiates degradation processes in these layers, reducing their compactness and continuity, which subsequently lowers corrosion resistance. * The experimental data show the diffusion of carbon into the oxide layer, which, according to the research conducted by [PERSON] and others [62], can diffuse from the bulk metal towards the surface layer, contributing to the formation of compounds that affect the corrosion behavior of the alloy. * Hardness Evolution: In the 400-600 \({}^{\circ}\)C range, surface hardness values are generally lower than those of the reference sample, except at 600 \({}^{\circ}\)C, where aging and secondary-phase precipitation begin, yielding hardness values close to the reference. In the 800-1000 \({}^{\circ}\)C range, significant hardness reductions occur near the surface subjected to thermal shock. At 1000 \({}^{\circ}\)C, this decrease extends almost through the entire sample thickness, from the exposed surface to the base. * Operational Limitations: The severe hardness reduction at high temperatures suggests that even short-term overheating can significantly impair the mechanical performance of Inconel 718. Therefore, its use in high-stress applications at elevated temperatures is not recommended. * For applications in the energy, aerospace, and other highly demanding fields, managing phase equilibrium and structural transformations can prevent processes that lead to the loss of mechanical strength and increased brittleness. * Continuing research with longer holding times at high temperatures could provide essential data for developing effective thermal regulation strategies, thereby improving the durability and reliability of materials used in extreme conditions. Conceptualization, M.A., E.R.A.U., A.-D.R. and D.A.N. ; methodology, E.R.A.U., D.-C.A., D.A.N., A.H. and M.A. ; software, D.A.N. and E.R.A.U. ; validation, E.R.A.U., D.A.N. and M.A. ; formal analysis, D.A.N. ; investigation, E.R.A.U., S.G.M., D.A.N., D.-C.A., A.-D.R., A.H. and L.M.C. ; resources, E.R.A.U. ; data curation, M.I.P. and R.S. ; writing--original draft preparation, E.R.A.U. and M.A. ; writing--review and editing and visualization, D.A.N. and M.I.P. ; supervision, M.A. and D.-C.A. ; project administration, M.A. All authors have read and agreed to the published version of the manuscript. This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 823802.