Two Step Forging of Alloy 718

Abstract

A two step forging route was employed for Alloy 718 pancake forging. The effect of forging parameters on the evolution of microstructures within the pancakes during deformation was evaluated by metallographic observations. With the critical forging parameters of the dynamic recrystallization obtained by compression tests, FE simulation can effectively be utilized to predict detailed variations of the microstructures due to dynamic recrystallization for all forged pancakes. The distribution of forging parameters within a work piece is shown to be important in determining final grain sizes. Introduction One or two step forging processes are routinely employed for superalloy disk forging. Two step forging may be used especially for complex geometry forging of a superalloy disk with a uniform microstructure and properties. Depending on the forging temperature and post treatment, Alloy 7 18 can be grouped into three different types; i.e, standard, high strength and direct age versions[l]. Standard 718 is finish-forged well above the 6 (Ni3Cb) phase solvus temperature between 1038% and 1066% to make use of its reasonably good forgeability. The resulting grain size is ASTM 4-6. High Strength 718 is forged initially above the 6 solvus temperature, but finished in the end slightly below the 6 solvus temperature, resulting in a final grain size of ASTM 8 or finer. Direct-age 718 is finish-forged even at lower temperatures below the 6 solvus temperature followed by the conventional direct aging treatment, resulting in a finer grain size less than ASTM 10 with various mechanical properties; such as tensile strength and LCF properties. The selection of the different forging routes is determined by the design requirements of microstructures and mechanical properties. The change in microstructures is closely related with dynamic and static recrystallization during forging or post heat-treatment[2]. The recrystallization behavior is influenced by process variables such as the forging temperature, strain rate, initial grain size and amount of deformation[3]. The aim of this research is to investigate the effect of strain distribution on the final microstructure in the two step forging process. FEM simulation was carried out to predict the evolution of microstructures. Material Experimental Procedure The material used in this study was the premium quality Alloy 718 of high Nb content. The Superalloys 7X3,625,706 and Various Derivatives Edited by E.A. Loria The Minerals, Metals &Materials Society, 1997