Comparative study of the hot processing behavior in advanced Ni-based superalloys for use in A-USC applications

Three Ni-based superalloys, Haynes 214, Haynes 230 and Inconel 740H, have been analyzed for their mechanical behavior under conventional industrial hot-processing conditions. These and other comparable alloys are expected to be used in sCO(2) heat exchanger cycles, as proposed in the Advanced UltraSupercritical (A-USC) program, due to their unique combination of mechanical, corrosion and creep properties. However, these alloys are difficult to form, specifically for sCO(2) components, such as header pipes and manifolds, requiring optimal forming conditions. In this study, hot-deformation testing was conducted using a Gleeble 3500, to simulate temperatures and strain rates expected during industrial forming operations. The effect of using as-received material on the deformation behavior was analyzed, emulating microstructures under real conditions more accurately. The flow-stress curves for all three alloys showed the occurrence of dynamic recrystallization (DRX) with some signs of dynamic recovery, for the slowest strain rates and higher temperatures. Additionally, the influence of secondary phases on the hot deformation behavior was analyzed. The M6C-carbide in Haynes 230 showed the strongest effect, promoting recrystallization necklacing and inhibiting DRX growth. The activation energies for plastic flow were 246, 302 and 217 kJ mol(-1), for Haynes 214, Haynes 230 and Inconel 740H, respectively. Hot-processing maps were generated for all alloys and suggested specific domains for optimal workability under the tested conditions. For Haynes 214, that is epsilon <= 0.1/s and T >= 1050 degrees C. For Haynes 230, it is 1050 degrees C <= T <= 1150 degrees C and 0.01/s <= e <= 0.5/s. For Inconel 740H, it is 1010 degrees C <= T <= 1075 degrees C and 0.01/s <= epsilon 0.1/s. (C) 2019 Elsevier B.V. All rights reserved.