Microstructural evolution of Nimonic 80a during hot forging under non-isothermal conditions of screw press

Abstract

The microstructural evolution of nickel base superalloy Nimonic 80a during hot forging was studied in order to explore the suitability of this alloy to be hot forged under non-isothermal conditions of screw press, considering aspects such as deformation inhomogeneity, flow localization and shear bands formation. Hot forging trials in screw press with double truncated cones were carried out at three different temperatures, 950, 1050 & 1150°C. The microstructure in the as-preheated condition was analysed in order to understand the impact of soaking temperature on the evolution of both grain size and precipitates/carbides prior to hot forging operations. In the as-forged condition, the impact of forging temperature, strain, strain rate and chilling effect of the dies on the microstructural evolution of Nimonic 80a was studied. No evidences of shear bands or flow localisation associated to adiabatic heating were found in the hot forged double truncated cones, indicating the suitability of Nimonic 80a to be processed in screw press. However, the non-isothermal conditions of screw press resulted into the development of heterogeneous structures across the thickness. Highly or fully unrecrystallized structures were observed in those areas in contact with bottom die, in contrast with top positions, where highly recrystallized structures were found. Soaking treatments play a very important role on microstructural evolution during hot forging for Nimonic 80a due to its strong impact on both grain growth and dissolution/precipitation of precipitates/carbides. For the present work, discontinuous dynamic recrystallization (DDRX) was found to be the main recrystallization mechanism for Nimonic 80a. Clear evidences of nucleation of new DRX grains by the formation of bulges in serrated grain boundaries with the subsequent growth by twinning were observed at the three forging temperatures. Particle-stimulated nucleation mechanism (PSN) is also operating in Nimonic 80a enhancing the heterogeneous nucleation of new grains in the interior of deformed grains.