Superalloy 718 and Derivatives: Melting, Forging, and Wrought Processes
Program Organizers: Joel Andersson, University West; Chantal Sudbrack, National Energy Technology Laboratory; Eric Ott, GE Additive; Zhongnan Bi, Central Iron and Steel Research Institute
Monday 8:00 AM
May 15, 2023
Room: Admiral
Location: Sheraton Pittsburgh Hotel at Station Square
Session Chair: Andrew Wessman, University of Arizona; Paul Jablonski, National Energy Technology Laboratory
8:00 AM Introductory Comments
8:05 AM Invited
Segregation and Freckles in Alloy 718 and Related Alloys: Alec Mitchell1; Stephane Hans2; 1University of British Columbia; 2Aubert Duval
The defect known as “freckle”, normally identified on billet sections, has been extensively studied by modeling and experimentation. It appears to form as an upward-flowing channel due to gravitational forces acting on the segregated liquid between the growing dendrites. This liquid is less dense than the bulk liquid in many superalloys. The Nb-containing superalloys (and several other complex alloys) also form freckles, but examination shows that they are channels of liquid which has flowed in a downward direction as the segregated liquid becomes more dense than the bulk. The mechanism proposed for the “upward” freckles does not fully explain the phenomenology of the “downward” freckles in several key aspects. In this work we propose that a different series of events leads to the “downward” freckle formation; a mechanism which more fully explains the observed facts. The conclusion is that the present direction of alloy development, that of composition manipulation to reduce the segregation density changes, is likely to be only partially successful and that changes in other process parameters will also be required.
8:35 AM Invited
Manufacturing Large Superalloy Pipe Bends: John De Barbadillo1; Brian Baker1; 1Special Metals
The U.S. Department of Energy, through its’ office of Fossil Energy has funded programs to develop materials technology for advanced energy systems such as Advanced Ultra-supercritical (AUSC) coal-fired steam boilers and turbines. The AUSC plant would operate in the 700-800°C temperature range which requires the use of nickel-base alloys to meet design creep -rupture life requirements for welded structures. Specific components include large-diameter, heavy-wall pipes, induction bends and forged fittings. INCONEL alloy 740H is a γʹ-strengthened nickel-base superalloy that was developed for this application and down-selected for the manufacturing demonstration program. Work was recently completed on the Phase 2 program under DOE contract DE-FE0025064 that had the goal of demonstrating the ability of US industrial supply chain to manufacture full-scale components. Alloy 740H contains 16-20% γʹ and is sensitive to thermal stress cracking and auto-aging which affects each stage of the manufacturing operation. This paper describes the manufacture of these components at Special Metals and various subcontractors, the resulting material properties, and the mitigation of process cracking encountered in this work. The test articles are now stored at Oak Ridge National Laboratory awaiting future programs for more detailed material characterization.
9:05 AM
Local Assessment of Mechanical Properties in Forged Alloy 718 Components Based on the Simulation of the Microstructure Evolution During Production: Christian Gruber1; Peter Raninger2; Aleksandar Stanojevic1; Flora Godor1; Hans-Peter Gänser2; Stefan Marsoner2; Martin Stockinger3; 1voestalpine Böhler Aerospace GmbH & Co KG; 2Materials Center Leoben Forschung GmbH; 3Montanuniversität Leoben
The characterization of local and global fracture mechanical properties is carried out with destructive testing methods and is increasingly required in the specifications of forgings. Especially in the case of alloy 718 aircraft parts, the numerical estimation of local material properties is essential for lightweight design, geometry optimization and a significant reduction of development and experimental characterization costs. This leads to a demand for numerical models to capture initial microstructural inhomogeneities, describe the forming history and reflect the local microstructure and properties of the final product.Therefore a digital twin for the complex forging process was developed in order to reproduce and evaluate the resulting local microstructure across the complete process chain. Since the microstructure determines the mechanical properties like yield stress and fracture toughness, a dedicated model was implemented to describe the local evolution of the relevant microstructural features.
9:25 AM
Towards Enhancing Hot Tooling to Form High -𝛾′ Superalloys: Arthi Vaasudevan1; Enjuscha Fischer2; Thomas Witulski2; Catherine Rae1; Enrique Galindo-Nava3; 1University of Cambridge; 2Otto Fuchs; 3University College London
Ni-superalloys are overwhelmingly relied upon in aerospace, power generation and automotive sectors, yet, are seldom considered as materials for hot tooling. The operational conditions of hot forming dies potentially exceed those experienced by aircraft turbine discs. Fortunately, new disc alloys have pronounced elevated temperature capabilities and the current study focuses on implementing two advanced alloys, VDM 780 and Haynes 282 (H282) as hot tool materials. There is, however, inadequate evidence of their life-limiting properties and mechanisms in the in-service temperature regime of 700 – 900 ºC. Thus, realistic operating conditions were replicated by combining interrupted short and long-term thermal-mechanical tests. To begin, isothermal ageing in the furnace, assisted in comparing the extent of 𝛾′ coarsening between the alloys, and subsequent in-situ ageing and compression testing reflected the accompanying loss in strength. Compression creep testing at stresses near appropriate yield point (250 – 750 MPa) revealed accelerated creep rates at high temperatures with creep behaviour directly transitioning from primary to tertiary regime, the steady-state creep region being absent. The results indicated that even as exposure duration, temperature, and applied stress all influence microstructural evolution, the exposure temperature was pivotal in determining the effective life of these 𝛾′-strengthened alloys. Dissolution kinetics of γ' around near-solvus temperatures was crucial and was governed by elemental additions. As a result, the research paves the way for better understanding and design of superalloys with improved thermal integrity for hot tooling.
9:45 AM Break
10:15 AM Invited
The Effect of Microstructure on the Strength of VDM Alloy 780: Mark Hardy1; Masood Hafez2; Christos Argyrakis1; Ross Buckingham1; Andrea La Monaca1; Bodo Gehrmann2; 1Rolls-Royce Plc; 2VDM Metals International GmbH
Nickel-base alloys have been developed, which offer higher temperature capability from ingot metallurgy than Alloy 718. However, these alloys have higher volume fractions of gamma prime precipitates, require further processing steps and consequently, have higher material processing costs, show a greater propensity for freckle and are not readily electron beam (EB) welded. There is an appetite for an alloy that shows improved forgeability, that is EB weldable and can be used at temperatures of up to 700°C. This study examines VDM Alloy 780, first using laboratory compression tests and heat treatment experiments to determine suitable thermo-mechanical processing (TMP) conditions. Subsequently, pancakes that were 18-20 mm in height and 137-144 mm in diameter were forged from 70 mm in diameter x 76 mm high bars, which were extracted from mid-radius locations of 8-inch diameter billet. From these, test piece blanks were extracted, and heat treated for tensile tests at temperatures between 20 and 750°C. Results from these tests were compared with microstructure and data from 30 mm thick pancake forgings that received different TMP conditions from earlier work at VDM Metals International. The combined experiences provide an insight into the effect of forging and heat treatment conditions on the microstructure and tensile test properties of VDM Alloy 780. They show that strength levels for fine grain Alloy 720Li can be achieved if specific sizes of gamma grains and gamma prime precipitates can be produced.
10:45 AM
In-situ HT-EBSD Measurements and Calibration of Multi-class Model for Grain Growth and δ-phase Dissolution Kinetics of Alloy 718: Peter Raninger1; Christian Gruber2; Walter Costin1; Aleksandar Stanojevic2; Ernst Kozeschnik3; Martin Stockinger4; 1Materials Center Leoben Forschung GmbH; 2voestalpine BÖHLER Aerospace GmbH & Co KG; 3TU Wien, Institute of Materials Science and Technology; 4Montanuniversität Leoben
In the aerospace industry the microstructure evolution of alloy 718 during forging and heat treatment and the resulting mechanical properties are decisive in view of the high quality requirements of aircraft components. During thermomechanical processing the temperature control and adiabatic heating lead to grain growth and, if δ-solvus temperature is exceeded, to the dissolution of the δ-phase, which further results in accelerated grain growth. To describe the history of the microstructure in terms of grain size during and after forging or heat treatment an existing multi-class microstructure model is optimized with focus on grain growth kinetics and parameterized by experimental results. The multi-class model describes the microstructure and the coarsening during processing more precisely in terms of the grain size distribution than previously used single-class models. This is crucial in order to be able to predict mechanical properties such as tensile strength, fracture toughness and creep resistance, which are topics for other but related work. The experimental data basis for the presented work stems from in-situ high-temperature electron backscatter diffraction (HT-EBSD) investigations and supporting experiments. The experimental setup and the results are discussed in detail.
11:05 AM
Abnormal Grain Growth Maps of Wrought Ni-base Superalloys: Michael Fahrmann1; David Metzler1; 1Haynes International
Abnormal grain growth (AGG) upon sub-solvus annealing of hot worked material has been reported for a number of cast & wrought and powder metallurgy superalloys, irrespective of the nature of the grain boundary pinning phase (e.g., delta phase in alloy 718 or gamma-prime phase in Rene 88DT). A common feature are large overgrown grains that exhibit the original precipitate distribution as well as a high density of annealing twins (Fig. 1). It has also been established that these phenomena occur only for certain hot working conditions. Identifying these special hot working conditions a priori has proven difficult.Taking a more industry-oriented approach, we studied the (sub-solvus) annealing response of a sizeable number of hot-worked product forms of two very different wrought Ni-base superalloys: HAYNES 244 alloy and HAYNES 233 alloy. The main grain boundary pinning phase in 244 alloy is an intermetallic Mo-rich phase with a solvus temperature of approximately 2050oF (1120oC). In contrast, the key pinning phase in 233 alloy are secondary Cr- and Mo-rich carbides that precipitate in sufficient quantities below 2100oF (1149oC).
11:25 AM
Alloy Design and Development of a Novel Ni-Co Based Superalloy GH4251: Hongyao Yu1; Hailong Qin1; Xizhen Chen1; Guangbao Sun1; Bin Gan1; Yu Gu2; Teng An2; Jinglong Qu2; Jinhui Du2; Zhongnan Bi1; 1Beijing Key Laboratory of Advanced High Temperature Materials; 2Gaona Aero Material Co. Ltd.
The need of developing new high temperature materials has increased significantly in the last decade owing to the demand of higher engine operating temperature. This demand has motivated the development of a new Ni-Co based superalloy GH4251 with service temperature up to 700°C~800°C. Based on disk used alloy U720Li, the GH4251 alloy is designed by adjusting the content of Co, Cr, Ti, Nb and other elements. On the one hand, by increasing the Co content to 25 wt.%, the stacking fault energy is effectively reduced, which makes it easy to form nano-twins and other substructures that strengthening the alloy together with γ′ precipitates. The yield strengths of the alloy can achieve 1100MPa at 750℃ and the creep-rupture life is more than 500h at 750℃ under 530MPa with fine grain size (ASTM 8), which is superior than U720Li. On the other hand, a certain Nb element is added to replace Ti element, which can reduce the γ′ solvus temperature and its precipitating dynamic, along with a changed thermal deformation behavior caused by lower stacking fault energy, leading to a significant better hot work ability and weld ability compared with U720Li. Beside being used as disk or ring forgings, this novel GH4251 alloy can also be well processed by additive manufacturing due to its low cracking tendency. Keywords: Ni-Co based superalloy, alloy design, disk forging, additive manufacturing
11:45 AM Conference Luncheon