Abstract
Alloy 718 is one of the most useful heat-resistant alloys for important device components that require high-temperature properties. In order to obtain excellent mechanical properties, it is necessary to form fine grains, for which the pinning effect of the δ phase can be used in some cases. To precipitate a sufficient amount for the pinning effect, time-consuming isothermal heat treatments are required. Thus, a metallurgical method with a shortened holding time would improve production efficiency considerably. Our goal is to optimize the forging process to control grain size by utilizing the δ phase, and the purpose of this study was to investigate the influence of the initial microstructure of the precipitated γ″ phase on δ-phase precipitation behavior in Alloy 718. As a solute treatment, Alloy 718 was heated at 1050 °C for 4 h, followed by heating of some samples at 870 °C for 10 h to precipitate the γ″ phase. The specimen with precipitated γ″ phase showed more precipitated δ phase than that under the solute condition by comparing results of heating at 915 °C. This suggested that utilizing the γ″ phase promoted δ-phase precipitation, and it is thus expected to shorten the heat treatment time for δ-phase precipitation.