Microstructural control in hot working of IN-718 superalloy using processing map

Abstract

The hot-working characteristics of IN-718 are studied in the temperature range 900 °C to 1200 °C and strain rate range 0.001 to 100 s⁻¹ using hot compression tests. Processing maps for hot working are developed on the basis of the strain-rate sensitivity variations with temperature and strain rate and interpreted using a dynamic materials model. The map exhibits two domains of dynamic recrystallization (DRX): one occurring at 950 °C and 0.001 s⁻¹ with an efficiency of power dissipation of 37 pct and the other at 1200 °C and 0.1 s⁻¹ with an efficiency of 40 pct. Dynamic recrystallization in the former domain is nucleated by the δ(Ni₃Nb) precipitates and results in fine-grained microstructure. In the high-temperature DRX domain, carbides dissolve in the matrix and make interstitial carbon atoms available for increasing the rate of dislocation generation for DRX nucleation. It is recommended that IN-718 may be hot-forged initially at 1200 °C and 0.1 s⁻¹ and finish-forged at 950 °C and 0.001 s⁻¹ so that fine-grained structure may be achieved. The available forging practice validates these results from processing maps. At temperatures lower than 1000 °C and strain rates higher than 1 s⁻¹ the material exhibits adiabatic shear bands. Also, at temperatures higher than 1150°C and strain rates more than 1s⁻¹, IN-718 exhibits intercrystalline cracking. Both these regimes may be avoided in hotworking IN-718.