Hot forging of IN718 with solution-treated and delta-containing initial microstructures

A systematic study of the effect of δ phase precipitate morphology on the hot deformation behavior and microstructural evolution in nickel superalloy Inconel 718 is presented. Isothermal compression tests at fixed nominal strain rates and temperatures relevant to industrial forging (0.001–0.3 s^-1 and 990–1040 °C) were used. Three distinct initial microstructures have been examined: (I) solution treated, (II) a microstructure with finely dispersed particulate δ precipitates, and (III) a microstructure containing dense network of intragranular and grain boundary δ platelets. The peak flow stress associated with these various microstructures has been rationalized using a single, temperature-compensated power law. This clearly demonstrates opposition of the external applied stress by an internal back stress related to the initial δ phase morphology and apparent delta solvus temperature. Post-peak flow softening is attributed to dynamic recrystallization, aided by the dissolution of finer precipitates in material containing particulate δ phase, and to a certain degree of mechanical grain refinement caused by distortion and offsetting of grain segments where a dense δ-platelet structure exists.