Hot Forging of IN718 with Solution-Treated and Delta-Containing Initial Microstructures

H M Lalvani,J W Brooks

doi:10.1007/s13632-016-0299-4

Abstract

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.

Highlights

IN718, a nickel-based superalloy, is widely used in aeroengine applications due to its strength and stable microstructure at elevated temperatures
Post-peak flow softening is attributed to dynamic recrystallization, aided by the dissolution of finer precipitates in material containing particulate d phase, and to a certain degree of mechanical grain refinement caused by distortion and offsetting of grain segments where a dense d-platelet structure exists
The overall behavior is in broad agreement with that usually observed for IN718: A distinct peak stress, tending to be more pronounced at higher strain rates, is followed by significant post-peak flow softening that is generally attributed to recrystallization and/or adiabatic heating [6,7,8,9,10]

Summary

Introduction

IN718, a nickel-based superalloy, is widely used in aeroengine applications due to its strength and stable microstructure at elevated temperatures. Significant research has been directed toward characterizing the precipitation and dissolution kinetics of the d phase in IN718 during heat treatment and aging, e.g., [1,2,3,4,5], and several experimental investigations have explored recrystallization associated with high-temperature compressive flow in solution-treated material, e.g., [6,7,8,9,10], but there has been rather less focus on the specific role of d precipitates during hot deformation. The area fraction of un-recrystallized grains was calculated from the binary image, and the recrystallized fraction was deduced from this information These measurements were taken along the specimen axis, at a distance h/3 from one end of the specimen, where h is the deformed specimen height, assumed to be representative of the bulk strain [21]

Results and Discussion

Conclusions

20. ASTM Standard E112-96