716-Fine Grains Forming Process, Mechanism of Fine Grain Formation and Properties of Superalloy 718

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Text of 716-Fine Grains Forming Process, Mechanism of Fine Grain Formation and Properties of Superalloy 718

  • Fine Grains Forming Process, Mechanism of Fine Grain Formationand Properties of Superalloy 718

    Hwa-Teng Lee1,+1 and Wen-Hsin Hou2,+2

    1Department of Mechanical Engineering, National Cheng Kung University,No. 1, University Road, Tainan 701, Taiwan, R. O. China2Research & Development Center, Gloria Material Technology Corp., Taiwan, R. O. China

    The mechanical properties of Inconel 718 superalloy are determined primarily by its microstructure and grain size. The grain structure ofInconel 718 is traditionally rened by aging treatment, and a high volume fraction of acicular phase precipitates before the structure forms.During the following static or dynamic recrystallization process, the existing phase inhibits recrystallized grain growth and acquires a ne grainstructure. In the proposed approach, the Inconel 718 specimens are re-solution heat treated at a temperature higher than the solvus temperatureto ensure thorough dissolution of the precipitated phase into the austenite matrix and produce a niobium oversaturated matrix. The specimensare then cold compressed to produce a dislocation saturated matrix and are nally recrystallized at 950C to induce the precipitation of ne phase. The phase precipitates exert a strong grain-boundary pinning effect, and thus a ne grain structure is obtained despite the highrecrystallization temperature. The average grain size in the rened microstructure is found to be 23 m, which is around half that of the grainsize in the specimens prepared using the conventional process. Hardness testing and tensile testing at 25 and 650C revealed its superiormechanical properties. [doi:10.2320/matertrans.M2011337]

    (Received October 31, 2011; Accepted December 26, 2011; Published February 22, 2012)

    Keywords: superalloy, phase precipitate, recrystallization, cold forming, ne grains

    1. Introduction

    Superalloy 718 possesses an advantageous combination offavorable mechanical properties and good corrosion resist-ance at high temperatures. Thus, it is extensively usedthroughout the aerospace, petrochemical, and oil and gasindustries. The properties of Inconel 718 are determinedprimarily by its microstructure, particularly the grain size.Studies have shown that the ne grain size of Superalloy718 results in both a signicant increase in strength andan enhanced low cycle fatigue capability.1,2) Furthermore,given the presence of ne and stable equi-axed grains with asize of less than 10 m, Inconel 718 demonstrates a super-plastic property at elevated temperatures and intermediatestrains.24)

    Superalloy 718 comprises an austentic matrix. Thus, themicrostructure of forged Inconel 718 components cannot berened using the same heat treatment processes as those usedto rene the grain size in conventional metal forgings.5)

    Camus et al.6) showed that un-recrystallized grains in themicrostructure of Inconel 718 hot-worked components maynot fully recrystallize when subjected to further thermaldeformation. Therefore, dynamic recrystallization has at-tracted signicant attention as a means of producing a neand evenly-distributed grain structure in Superalloy 718specimens following hot forming processes.712)

    The grain renement of Inconel 718 is commonly achievedusing the phase grain boundary pinning effect to inhibit thegrain growth.1315) The initial ingot is reduced to anintermediate billet diameter, and the billet is then aged at atemperature of around 890920C in order to precipitate ahigh volume fraction of acicular phase. Hot working, coldforging, or cold rolling is then performed. In cold forming,

    during the following static recrystallization treatment, theexisting phase exerts a strong grain-boundary pinning effectand prohibits the growth of recrystallized grains, and a negrain structure is obtained. In hot working, thermomechanicalprocessing is then conducted at a temperature slightly lowerthan the delta solvus,1) causing the morphology to becomespherical and to inhibit recrystallized grain growth via agrain-boundary pinning effect. These grain renementmethods are known as the Delta Process.The proposed method also uses phase precipitation to

    rene grain size. However, the main difference is that theaging treatment is used in place of solution heat treatment at atemperature higher than the solvus temperature to ensurethorough dissolution of the precipitated phase into theaustenite matrix and produce a niobium oversaturated matrixbefore cold forming. Therefore, the following static recrys-tallization process prompts a large number of spherical ne phases to precipitate and inhibit recrystallized grain growthvia a grain-boundary pinning effect. Thus, grains that aremore uniform and ne are obtained.The conventional process of hot forming has been

    successfully used to produce Inconel 718 alloy samples witha grain size of as little as ASTM 10 (11.2 m) without theneed for solution annealing.11) However, maintaining the phase during the forging process while minimizing its contentin the nal component is a major challenge.16) To prevent thedissolution of the phase, and to ensure the occurrence ofdynamic recrystallization, the deformation process must beperformed under enough strain and low strain rate conditionsat a temperature between the dissolution temperature ofthe phase and the dynamic recrystallization temperatureof Inconel 718.12) The initial grain size of Inconel 718components is largely dependent upon the nish forgingtemperature. Although a lower deformation temperature isbenecial in reducing the grain size, it also prompts areduction in the volume fraction of the recrystallized

    +1Corresponding author, E-mail: htlee@mail.ncku.edu.tw+2Graduate Student, National Cheng Kung University

    Materials Transactions, Vol. 53, No. 4 (2012) pp. 716 to 7232012 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

  • grains.17) Therefore, in many 718 forgings, dynamicrecrystallization may be incomplete.Lu14,15) produced an Inconel 718 sheet with a ne grain

    size of ASTM 12 (5.6 m) via integration of a great volumeof existing phase, cold rolling, and recrystallization.However, a large volume fraction of phase precipitateswas precipitated, leading to a reduction in the tensile strengthas a result of the consumption of the strengthening elementniobium (Nb). Moreover, in Refs. 14,15), the specimencondition before the ne grain treatment, the grain renementmechanism, and the tensile strength of the nished productwere not discussed.Accordingly, the present study proposes an alternative

    method for rening the grain structure of Inconel 718:solution heat treating the initial specimens (i.e., hot rollingand then solution heat treatment at 968C) at high temper-ature to ensure a thorough dissolution of the precipitated phase and then performing a cold forming process to producea dislocation saturated matrix with high strain energy. Finally,a recrystallization process is performed to prompt theprecipitation of ne phase, thereby restricting graingrowth by means of the grain-boundary pinning effect.The mechanism by which the grain structure is rened isexamined. In addition, the effect of phase precipitation onthe tensile strength of the nished component is evaluated byperforming tensile tests at temperatures of 25 and 650C.

    2. Experimental Section

    The composition of the Superalloy 718 used in the presentexperiments is shown in Table 1. In the experimentalprocedure, Inconel 718 ingots with a diameter of 400mmwere fabricated using a Vacuum Induction Melting (VIM)process followed by Vacuum Arc Remelting (VAR). Anopen-die pre-forging process was performed to reduce thediameter of the ingots to 350mm. A nal forging processwas then performed using a four-hammer precision forgingmachine to reduce the circular ingots to square billets withdimensions of 133mm 133mm. Each billet was hot rolledto a diameter of 22mm, solution heat treated at 968C for 1 h,and then peeled to a diameter of 19.84mm.The materials were then compressed at room temperature

    using a 50T universal tensile testing machine. The recrystal-lized structure of the compressed sample was evaluated by

    performing metallographic tests. Finally, standard metallo-graphic techniques were used to prepare specimens forobservation via an optical microscope (OM), an imageanalyzer, a Transmission Electron Microscope (JEOL JEM-2100F CS STEM), and a Scanning Electron Microscope(SEM). Mechanical properties were determined by micro-hardness and a 10T universal tensile testing machineequipped with a high-temperature furnace.

    3. Results and Discussion

    3.1 Observations of annealed microstructureIn the proposed grain rene method, before cold forming,

    the specimen was re-solution treated at a temperature higherthan solvus, and the grain structure transformed to coarsegrains. Normally, coarse grains will decrease the performanceof grain renement. To investigate the effects of coarsegrains and the timing of phase precipitation on the grainrenement results, the initial annealed samples were furthersolution heat treated at temperatures of 1030 and 1060C,respectively, for 1 h and then quenched in water. Figure 1(a)presents an SEM micrograph of the microstructure of theinitial Inconel 718 sample solution heat treated at 968C for1 h. As shown, the sample has a typical austenitic micro-structure, with twin boundaries located within the individualgrains. The grain size was found to be around 18 m. Inaddition, the micrograph shows the presence of rod-like phase precipitates at the grain boundaries. Figures 1(b) and1(c), illustrating the microstructures of the samples solutionheat treated at 1030 and 1060C, respectively, show thatgrain growth occurred in both specimens. The grain size ofthe sample solution heat treated at 1030C was around60 m, while that of the sample heat treated at 1060C wasaround 68 m. In addition, in both samples, the