Citation: Rajendran, N.; Yurgel, C.C.; Misiolek, W.Z.; Alves de Sousa, R. Hot Forging Die Design Optimization Using FEM Analysis for Near-Net Forming of 18CrNiMo7-6 Steel Pinion Shaft. Metals 2023, 13, 815. https://doi.org/10.3390/ met13040815 Academic Editors: Zbigniew Pater and Umberto Prisco Received: 1 March 2023 Revised: 1 April 2023 Accepted: 17 April 2023 Published: 21 April 2023 Copyright: © 2023 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/). metals Article Hot Forging Die Design Optimization Using FEM Analysis for Near-Net Forming of 18CrNiMo7-6 Steel Pinion Shaft Nijenthan Rajendran 1 , Charles Chemale Yurgel 1 , Wojciech Z. Misiolek 1 and Ricardo Alves de Sousa 2, * 1 Loewy Institute, Materials Science and Engineering Department, Lehigh University, Bethlehem, PA 18015, USA 2 Center for Mechanical Technology and Automation, Department of Mechanical Engineering, Campus de Santiago, University of Aveiro, 3810-183 Aveiro, Portugal * Correspondence: rsousa@ua.pt Abstract: The objective of the presented work was to develop a new forging process for a pinion shaft as a component of a wind turbine. A study of near-net-shape forming using Deform 3D software was performed to reduce operational cost, time, and material scrap; enhance specific properties; increase productivity. Near-net forged products have good dimensional accuracy and continuous metal flow lines, which are characteristic of improved mechanical properties. To avoid the traditional trial-and-error experimental method, the process and tool design were accomplished with a careful and detailed numerical simulation approach. In the present work, the Finite Element Method was used to develop a process model for the existing hot forging process of the 18CrNiMo7-6 steel pinion shaft used in a wind turbine. The developed numerical process model was validated via experiment including a comparison of the metal flow lines from the FEM model with the metallography results of the forged part. Two new die designs were proposed, and the simulation results were compared to the actual process to achieve improved geometry. The results for the new geometries showed improvements in terms of the die cavity filling for the new proposed dies and better results in grain flow orientation. Compared to the initial non-optimized die, the new designs improved the mechanical properties and savings associated with the lower volume of required raw material and fewer finishing operations. Considering the applied stresses and wear in the new near-net shape, the die geometry shall be updated to accommodate more severe solicitations. Naturally, all the improvements carried out are dependent on other factors such as the conditions of the equipment, operator skills, lubrication, and other variables. A surface heat treatment is also suggested for stress relief as a reliability improvement. Keywords: finite element method; hot forging; pinion shaft; microstructure; Brinell hardness; near-net shape 1. Introduction Hot forging is a metal forming process whose main objectives are not only to change the shape but also improve the mechanical properties of the forged parts measured by ultimate tensile strength and ductility. Hot forging is defined as the process in which metal is plastically deformed above the recrystallization temperature [1,2]. The metal workpiece is heated up to the desired temperature and deformed with the impact energy of tooling in the closed-die hot forging process. In this process, the service life of the forging dies is especially important due to economic reasons and, also, to the quality of forged components. The flash formation during forging requires high levels of effective stress because of highly localized yield stress and high active friction surfaces. The small thickness of the flash due to the high surface-to-volume ratio is much cooler than the bulk of forging, resulting in a higher material yield stress value [2,3]. In the hot forging processes, the final-part geometry impacts process parameters due to complex interactions between tooling and ingots, which can result in inhomogeneous Metals 2023, 13, 815. https://doi.org/10.3390/met13040815 https://www.mdpi.com/journal/metals