Grain refining process of nickel-based superalloy 718

• Main Authors: Wen-HsinHou, 侯文星
• Other Authors: Hwa-Teng Lee
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/42972163320009428654

博士 === 國立成功大學 === 機械工程學系碩博士班 === 100 === This study investigated the process for grain refinement of superalloy 718 with various fine grain levels. Post heat treatment fails to refine the grain structure of superalloy 718. The fine grain grade of ASTM No.6~8 for general industry application should be controlled during manufacturing by inducing full dynamic recrystallization through means of a carefully-controlled hot forming process performed in a powerful and precise forging machine. This work presents an alternative method for obtaining a fine and uniform grain structure through means of static recrystallization and proper control of the δ phase formation. In the proposed method, the component is cooled in water immediately after forging to suppress δ phase precipitation and preserve the internal strain energy produced by the hot deformation process. The component is then heated to a temperature 30°C lower than the δ solvus temperature of 1030°C, resulting in continuous recrystallization of the microstructure. Our results demonstrate that the proposed static recrystallization method yields a fine microstructure with an average grain size of ASTM No.7 (31.8 μm). For acquiring a finer grain structure, Inconel 718 is traditionally refined by aging treatment, and a high volume fraction of acicular δ phase precipitates before forming. During the following static or dynamic recrystallization process, the existing δ phase inhibits recrystallized grain growth and acquires a fine grain structure. In the proposed approach, the Inconel 718 specimens are re-solution heat treated at a temperature higher than the δ solvus temperature to ensure thorough dissolution of the precipitated δ phase into the austenitic matrix and produce a niobium oversaturated matrix. Finally, the specimens are cold compressed to produce a dislocation saturated matrix and are then recrystallized at 750~950°C to induce the rapid precipitation of fine δ phase. The δ phase precipitates exert a strong grain-boundary pinning effect, and thus a fine grain of less than 2.5μm is obtained despite the high recrystallization temperature of 950°C. The average grain size in the refined microstructure is found to be 200nm~2.5 μm, which is finer than the finest grain size of 4μm reported in the literature, achieved by using the conventional process. Hardness testing and tensile testing at 25°C and 650°C revealed that various fine grain structures have their own characteristics.