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

Metallography, Microstructure, and Analysis, Jul 2016

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.

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

Metallogr. Microstruct. Anal. Hot Forging of IN718 with Solution-Treated and Delta-Containing Initial Microstructures H. M. Lalvani 0 1 2 J. W. Brooks 0 1 2 0 The University of Birmingham , Edgbaston, Birmingham B15 2TT , UK 1 Advanced Forming Research Centre, University of Strathclyde , Glasgow PA4 9LJ , UK 2 & H. M. Lalvani A systematic study of the effect of d 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 d precipitates, and (III) a microstructure containing dense network of intragranular and grain boundary d 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 d 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 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. Bulk deformation; Delta phase; Nickel-based superalloys; Thermomechanical processing; Recrystallization Introduction IN718, a nickel-based superalloy, is widely used in aeroengine applications due to its strength and stable microstructure at elevated temperatures. These high-temperature properties of IN718 are attributed to slow growth kinetics of c00 precipitates and make IN718 a prime candidate for forged turbine disks. 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., [ 1–5 ], and several experimental investigations have explored recrystallization associated with high-temperature compressive flow in solution-treated material, e.g., [ 6–10 ], but there has been rather less focus on the specific role of d precipitates during hot deformation. However, it is known that prior aging in the d stability field can have a significant effect on stress–strain response [ 11, 12 ] and recent studies have highlighted the complexity of the mechanisms involved, including dynamic dissolution and reprecipitation [13], sub-grain formation [ 14 ], and platelet spheroidization [ 15 ]. A key concern in the manufacture of critical components is how the variability in initial and evolving microstructure might be reasonably characterized and quantified for incorporation into process models of high-temperature forging operations. A systematic study of the flow behavior and microstructural evolution in IN718 obtained during small-scale compression experiments using three distinct microstructures has been presented in this paper. A brief synopsis of flow stress and microstructure comparison between a solution-treated and acicular d-containing microstructure has been published by authors [ 16 ]. However, the current paper provides an in-depth analysis of the role of the d phase in influencing dynamic recrystallization and flow softening in IN718. Furthermore, a constitutive law has been formulated in terms of an internal back stress that takes into account the initial d phase morphology and distribution. Material and Experimental Methodology The material used in the present work was supplied as standard billet of 178 mm diameter that had undergone a typical annealing and aging cycle. It was heat-treated at 980 C for 1 h and water-quenched followed by an aging treatment at 720 C for 8 h. The billet was then furnacecooled to 620 C, aged for a further 8 h, and finally aircooled. The chemical composition (in weight %) of the billet contains 50.50 Ni, 19.10 Cr, 18.78 Fe, 5.28 Nb, 4.15 Mo, 1.06 Ti, 0.613 Al, 0.130 Co, 0.115 W, 0.039 V, and 0.033 C. Microstructure analysis of the billet material revealed a c-matrix grain size of 40–60 lm and densely distributed d precipitates with varying morphology across the billet diameter: predominantly fine, blocky d particles near the outer billet diameter, becoming increasingly acicular toward the billet center. Since the precipitation and dissolution kinetics of the d phase have a complex dependence on the time, temperature, phase morphology, and local Nb concentration [ 2 ], a single value for the solvus temperature is difficult to determine [ 4 ]. The equilibrium delta solvus calculated using thermodynamic software JMatPro was 1033 C. However, in order to plan a testing program, an approximate ‘apparent’ d solvus temperature, derived using material con (...truncated)


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H. M. Lalvani, J. W. Brooks. Hot Forging of IN718 with Solution-Treated and Delta-Containing Initial Microstructures, Metallography, Microstructure, and Analysis, 2016, pp. 392-401, Volume 5, Issue 5, DOI: 10.1007/s13632-016-0299-4