Annealing Treatment Methods and Mechanisms for Refining Mixed and Coarse Grains in a Solution Treatment Nickel-Based Superalloy

The annealing treatment methods and mechanisms for refining deformed mixed and coarse grains in a solution treatment nickel-based superalloy are investigated. The single-stage annealing tests are done first, and it is found that the mixed and coarse grains are hardly refined to about 10 mu m by a direct annealing at 980 degrees C due to lack of delta phase. Then, two-stage annealing tests, in which the deformed sample is first heated to 900 degrees C for 12 h for precipitating some delta phases and then reheated to 980 degrees C for a recrystallization annealing treatment (RT), are designed. It is found that the recrystallization is still not complete although a long-time RT is done. The main reason is that the distances between adjacent delta phases precipitating during direct aging are so small that the recrystallization nuclei are hard to form. So, three-stage annealing tests, including a pre-recrystallization annealing treatment (PRT) at 980 degrees C, an aging annealing treatment (AT) at 900 degrees C, and an RT at 980 degrees C, are designed. It is found that the PRT has a favorable influence on the refinement of mixed and coarse grains during the subsequent AT and RT. The mixed and coarse grains can be refined after the three-stage annealing treatment of 980 degrees C x 3 min/WQ + 900 degrees C x 12 h/WQ + 980 degrees C x 120 min/WQ.