Hot forging of a nickel-base superalloy

Compression tests were carried out to investigate the hot-deformation behavior of nickel-based Alloy 718. The tests were conducted in the temperature range 982–1066°C and the strain rate range 5×10−4–5.0 s−1. Pancakes were also made by multi-step forging and their microstructures were compared with specimen-base compression tests. Hot-deformation behavior was led mainly by dynamic recrystallization as well as by dynamic recovery. Dynamic recrystallization as well as meta-dynamic recrystallization plays a significant role in determining final microstructures for the given temperature ranges. Once the dynamic recrystallization is finished, no significant stress relaxation attributed to meta-dynamic recrystallization occurs and stress hardening following the intermittent dwell time between the first and the second compression tests occurs even at 1066°C due to the fact that grain growth becomes prominent. In such two-step compression tests, large second strains produce second dynamic recrystallization, which leads to fine grain sizes. To control the grain size through dynamic recrystallization in multi-step forging, one needs to consider the strain partitioning in each forging step for the given temperature and strain rate ranges. FEM simulation was used to evaluate the microstructural variation during pancake forging, taking into account the variation of grain sizes following dynamic recrystallization.