Influence of precipitated phase formation on recrystallization behavior of superalloy 718

Post heat treatment fails to refine the grain structure of superalloy 718. Thus, for components with demanding grain size requirements, the forging process should be performed within an extremely limited temperature region just below the δ solvus temperature, yet higher than that required to induce dynamic recrystallization. The grain size of superalloy 718 is generally 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 a 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 1030°C; resulting in a continuous recrystallization of the microstructure. Experimental results indicate that γAA phase precipitation and δ phase precipitation dominate the dynamic and static recrystallization behaviors observed in the conventional and proposed grain refinement methods, respectively. Our results further demonstrate that the proposed static recrystallization method yields a fine microstructure with an average grain size of ASTM No. 7 (31.8 μm). Thus, the proposed method provides an inexpensive and technically straightforward alternative to the conventional hot forming grain refinement method.