Proceedings of the 9th International Symposium on Superalloy 718 & Derivatives: Energy, Aerospace, and Industrial Applications

Inhaltsverzeichnis

1. Frontmatter

2. Superalloy 718 & Derivatives: Keynotes

   1. Frontmatter

   2. Age Hardenable Nickel-Based Alloy Developments and Research for New High Temperature Power Cycles

      John P. Shingledecker, John A. Siefert

      Abstract

      Advanced Ultrasupercritical (A-USC) steam Rankine cycles and Supercritical Carbon Dioxide (sCO₂) Brayton cycles are under intensive development to enable low carbon generation of electricity. These high-efficiency power cycles, aimed at fossil and in some cases renewable energy, require higher temperatures and pressures compared to traditional steam cycles for pressuring retaining components such as tubing, piping, heat exchangers, and turbine casings. Extensive research and development to produce and characterize age-hardenable nickel-based alloys containing Al, Ti, and Nb in judicious amounts have allowed designers to now consider supercritical fluid temperatures up to ~760 °C which is much greater than today’s supercritical steam technology based on steel metallurgy up to ~610 °C. This paper will focus on the alloys developed around the world to enable these advanced power cycles, and a discussion on their key properties: long-term creep strength (100,000 h+), fabricability, and weldability/weld performance. Most of these alloys contain less than 25% gamma prime, such as alloy 740H, 263, and 282, due to the need for heavy section weldability, unique to these applications. While welding processes have now been developed for many of these alloys using a variety of filler metals and processes, key research questions remain on the applicability of processes to field power plant erection, the potential for cracking to occur during service, and the long-term weld creep and creep-fatigue performance.

   3. Superalloy 718: Evolution of the Alloy from High to Low Temperature Application

      Shailesh Patel, John deBarbadillo, Stephen Coryell

      Abstract

      Alloy 718 (UNS N07718) was the culmination of a research project started in the mid-1950s to develop a stronger pipe alloy for coal-fired power plants. It was never used for that application, but it was quickly adopted for aircraft turbine engines because of its very high strength, thermal stability, formability and weldability compared to the γʹ-strengthened alloys available in the 1960s. Alloy 718 derives its unique combination of strength and fabricability from a coherent ordered tetragonal phase γʺ and the slow diffusion rate of its main constituent, niobium. Very early in its commercial life, alloy 718 was recognized as having attributes for both ambient and cryogenic temperature uses as well. Impact toughness, aqueous corrosion resistance, and non-ferromagnetic properties were important attributes. Alloy 718 replaced established age-hardened iron/nickel-base alloys such as A-286, K-500 and X-750 as well as martensitic steels in a wide range of components in the space launch, oil and gas, marine, nuclear and superconducting magnet industries. As the applications became more specialized, so did the heat treatments and microstructures to accentuate specific properties. A common thread through these low temperature applications has been hydrogen embrittlement and a substantial body of literature documents our increasing awareness of its role in service performance. In recent decades, new alloys based on alloy 718 and the γʺ strengthening phase have been introduced, especially for oil and gas production equipment. This paper describes the early development of alloy 718 for these important applications, along with alloy, microstructure and heat-treatment evolution and current status.

3. Alternative Processes

   1. Frontmatter

   2. Alloy 718: Laser Powder Bed Additive Manufacturing for Turbine Applications

      R. Kelkar, A. Andreaco, E. Ott, J. Groh

      Abstract

      Alloy 718 has been utilized successfully in both static and rotating turbo machinery applications for four decades. The combination of high strength, fatigue capability, rupture strength, corrosion and creep resistance at temperatures through 650 °C are key attributes of this alloy. Conventional manufacturing routes include cast, wrought, sheet, joining and fabrication by welding and brazing, powder metallurgical processing and metal injection molding. Recent investigation of aerospace materials like Alloy 718 produced by additive manufacturing technology has provided an opportunity for disruptive component manufacturing methods, geometries, and component capabilities which expand design space for complex applications. At GE Aviation (GEA), development of laser powder bed direct metal laser melting (subsequently referred to as DMLM) Alloy 718 was a natural choice following the successful commercial application of DMLM CoCrMo in GE Aviation and Safran’s LEAP platform fuel tip component and the GE90 T25 sensor part. GEA’s DMLM 718 development started with demonstrator military applications, and now has expanded to include multiple commercial engine applications across the size range of the GEA product line. The additive (laser) process development for Alloy 718 involved a combination of laser processing parameter investigation and heat treatment development to produce both acceptable build geometry and metallurgical microstructures. Initial developments started with 20 μm build layer thickness, and expanded to 50 μm thicknesses for improved build productivity. From the onset, the materials engineering focus was on isotropic, fine grain, pore free and fully developed microstructures for Alloy 718 by heat treat design. Mechanical characterization included consideration of build direction, machine type, machine to machine variation and processing gas effects. This paper will discuss various metallurgical challenges and related, microstructure & mechanical characterization of DMLM Alloy 718 .

   3. Progress in the Processing and Understanding of Alloy 718 Fabricated Through Powder Bed Additive Manufacturing Processes

      Michael M. Kirka, Alex Plotkowski, Peeyush Nandwana, Anil Chaudhary, Suresh S. Babu, Ryan R. Dehoff

      Abstract

      This paper reviews currently available information on the processing and understanding of Alloy 718 fabricated through powder bed additive manufacturing processes, specifically selective laser melting, electron beam melting, and binder jet additive manufacturing. In each instance, the microstructures formed exhibit attributes unique to the process used. Through post-processing, these materials are capable of achieving property behaviors similar to that of the long utilized wrought material. While AM processes are complex, computational modeling has been successfully applied to capture the heat and mass transfer, microstructure evolution, and constitutive response of the material.

   4. Impact of Powder Variability on the Microstructure and Mechanical Behavior of Selective Laser Melted Alloy 718

      Chantal K. Sudbrack, Bradley A. Lerch, Timothy M. Smith, Ivan E. Locci, David L. Ellis, Aaron C. Thompson, Benjamin Richards

      Abstract

      Powder-bed additive manufacturing processes use fine powders to build parts layer-by-layer. Alloy 718 powder feedstocks for selective laser melting (SLM) additive manufacturing are produced commercially by both gas and rotary atomization and are available typically in the 10–45 or 15–45 µm size ranges. A comprehensive investigation was conducted to understand the impact of powder variability on the microstructure and mechanical behavior of SLM 718 heat treated to Aerospace Material Specification (AMS) 5664. This study included sixteen virgin powders and three once-recycled powders within the 10–45 and 15–45 µm size ranges that were obtained from seven direct source suppliers and one reseller. Although alike as highly regular spheroids, these powders showed distinct differences in composition (especially Al, C and N contents), particle size distributions, and powder features such as degree of agglomeration, fusion and surface roughness. Compositional differences expectedly had the strongest impact on microstructure. High N and C contents formed TiN-nitrides and/or (Ti, Nb, Mo)-C carbides on the grain boundaries, prevented recrystallization during heat treatment, and resulted in retained (001)-scalloped shaped grains that ranged from 19 to 41 µm in average size. In the absence of this particle pinning, the average grain size of the heat treated SLM 718 ranged from 51 to 90 µm. Room temperature tensile and high cycle fatigue (HCF) testing compared as-fabricated (AF) and low stress ground (LSG) surface conditions. Tensile testing revealed consistent behavior between the two surface conditions and amongst the powder lots. The finer grained SLM 718 builds displayed the lowest tensile properties. A SLM 718 build fabricated from a powder with eight times lower C content showed statistically better tensile properties presumably due to enhanced coarsening of δ-Ni₃Nb precipitates. The specimens from once-recycled powders had slightly higher tensile strengths and slightly higher ductility compared to their virgin equivalents; once-recycling also did not substantially degrade the mean HCF life. The LSG fatigue lives agreed with conventionally manufactured 718 data, while AF lives exhibited a knock-down due to surface roughness. The fatigue lives of AF specimens were statistically equivalent across powder lots except for one and failures typically initiated at stress concentrators associated with SLM surface asperities. Fatigue testing of low stress ground specimens result in both transgranular and within facet crack initiations. More than half of the cracks initiated from these facets for the machined condition; however, these facets appeared to be within grains that were larger-than-average in size. A nitrogen-atomized powder with fine prior particles of TiN-nitrides and M(Ti, Nb, Mo)C carbides from atomization on powder surfaces resulted in the best fatigue performance with segregation of these particles to the SLM 718 grain boundaries leading to higher resistance to early-stage crack propagation. Typically the fine-grained builds with minor phases along the grain boundaries did not perform well in fatigue, whereas a larger-grain build with lower carbon content and coarser δ-Ni₃Nb precipitates showed the next best HCF response. Further details of the build microstructure and its impact on tensile and fatigue behavior was considered.

   5. The Effect of Location and Post-treatment on the Microstructure of EBM-Built Alloy 718

      Sneha Goel, Jonas Olsson, Magnus Ahlfors, Uta Klement, Shrikant Joshi

      Abstract

      Additive manufacturing (AM) of Ni-based superalloys such as Alloy 718 may obviate the need for difficult machining and welding operations associated with geometrically intricate parts, thus potentially expanding design possibilities and facilitating cost-effective manufacture of complex components. However, processing AM builds completely free from defects, which may impair mechanical properties such as fatigue and ductility, is challenging. Anisotropic properties, microstructural heterogeneities and local formation of undesired phases are additional concerns that have motivated post-treatment of AM builds. This work investigates the microstructural changes associated with post-treatment of Alloy 718 specimens produced by Electron Beam Melting (EBM) for as-built microstructures at 3 build heights: near base plate, in the middle of build and near the top of the build. Two different post-treatment conditions, hot isostatic pressing (HIP) alone and a combined HIP with solutionising and two-step aging were examined and compared to the results for the as-built condition. The influence of various post-treatments on minor phase distributions (δ, γ″, carbides), overall porosity, longitudinal grain widths and Vickers microhardness was considered. The HIP treatment led to significant reduction in overall porosity and dissolution of δ phase, which led to appreciable grain growth for both post-treatment conditions. The variation in hardness noted as a function of build height for the as-built specimens was eliminated after post-treatment. Overall, the hardness was found to decrease after HIP and increase after the full HIP, solutionising and aging treatment, which was attributed to dissolution of γ″ during HIP and its re-precipitation in subsequent heat treatment steps.

4. Applications

   1. Frontmatter

   2. ICME Based Additive Manufacturing of Alloy 230 Components

      Suresh Sundarraj, Sion Pickard, Alonso Peralta, Anil Chaudhary, David Snyder, Jeff W. Doak, Suraj Rawal, Ray Xu, Sesh Tamirisakandala, Albert Contreras, John Meyer, Andrzej Wojcieszynski, Derrick Lamm, Edwin Schwalbach

      Abstract

      Metal additive manufacturing (AM) is an innovative and enabling manufacturing technology that is also pervasive/cross cutting in terms of system applications, dual-use potential and interest from multiple agencies. AM technologies build near-net/net shape components, one layer at a time, using digital data from 3D CAD models. In addition, AM has the potential to enable novel product designs that could not be fabricated using conventional subtractive processes. The goal of this Metals Affordability Initiative (MAI) project (HON-9 Agreement Order Number FA8650-14-2-5204) is to create a cross-functional team focused on developing the necessary Integrated Computational Materials Engineering (ICME) based framework, knowledge and supporting models to enable powder bed AM production of nickel-based superalloy aerospace and space components. An Activity Integrated Project Team (AIPT) comprising of Honeywell Aerospace (Lead), Aerojet Rocketdyne, ATI Powder Metals, Carpenter Powder Products, Lockheed Martin, Northrop Grumman, Rolls-Royce Corporation, Arconic Inc. along with Applied Optimization and QuesTek as major subcontractors was formed. The AIPT successfully completed the concept feasibility demonstration for additively manufactured Alloy 230 components. A focused series of design of experiments (DOE) related to machine parameters and post processing operations were designed and implemented within Concept Laser Cusing M2 machine. The collected empirical data was used to optimize process parameters, calibrate ICME models, and improve tool maturity level (TML) of the ICME framework for AM of Ni superalloy components. A preliminary business case was developed for parts from Honeywell Aerospace, Aerojet Rocketdyne, Rolls-Royce Corporation, Northrup Grumman and Lockheed Martin.

   3. Simulation of Co-precipitation Kinetics of γ′ and γ″ in Superalloy 718

      Fan Zhang, Weisheng Cao, Chuan Zhang, Shuanglin Chen, Jun Zhu, Duchao Lv

      Abstract

      In this paper, we will study the co-precipitation kinetics of phases in Superalloy 718 using the simulation tool we have developed using the CALPHAD approach. This tool considers concurrent nucleation, growth and coarsening of these precipitates. Furthermore, it is directly integrated with thermodynamic calculation engine to obtain instant update of phase information, such as the composition of the matrix and the nucleation driving force for each precipitate. In addition to the average particle size, the more advanced KWN (Kampmann and Wagner Numerical) model was implemented to allow for predication of the full evolution of the particle size distribution (PSD). In this paper, we will perform virtual experiments using this tool to simulate the co-precipitation of the γ′ and γ″ phases under different heat treatment conditions. Simulation results, such as temporal evolution of volume fraction, number density, and mean size of the precipitates, as well as the final particle size distribution will be presented and discussed. The impact of δ precipitate and the initial microstructure will also be briefly discussed. These virtual experimental results can be used to understand the microstructural features of Superalloy 718 and serve as guidance for further optimization of heat treatment schedule.

5. Corrosion

   1. Frontmatter

   2. Performance of Wrought Superalloys in Extreme Environments

      B. A. Pint

      Abstract

      As power generation systems move towards higher efficiency operation above 700 ℃, wrought superalloys are the leading structural alloy candidates, including precipitation strengthened (PS) alloys 740 and 282 for the highest temperatures. To evaluate the performance of these alloys for these applications, a range of 500–5000 h evaluations have been conducted in environments including steam, supercritical CO₂ (sCO₂) and simulated combustion exhaust with H₂O and/or SO₂ at 700–800 ℃ and compared to baseline exposures in laboratory air and 1 bar CO₂. These alloys primarily rely on the formation of an external Cr-rich oxide layer or scale for environmental protection and the reaction rates in all of these conditions are similar and relatively low. However, compared to a conventional solid solution strengthened alloy, like 625, the mass gains are higher for PS alloys due to the internal oxidation of the γ′ forming additions, Al and Ti. Post-exposure characterization has quantified the reaction products and the depth of internal oxidation is not a concern and does not appear to increase above the baseline behavior in laboratory air. Likewise, there is no indication of internal carburization in the sCO₂ environment at 750 ℃/300 bar. The addition of 0.1% SO₂ in CO₂-10% H₂O at 800 ℃ actually suppressed the internal oxidation at 1 bar but SO₂ may be a concern when the total pressure is higher.

   3. Corrosion and Carburization Behaviour of Ni-Cr-Mo-Nb Superalloys in a High Temperature Supercritical-CO2 Environment

      Sung Hwan Kim, Chaewon Kim, Gokul Obulan Subramanian, Changheui Jang

      Abstract

      Two Ni-Cr-Mo-Nb superalloys (Alloy 625 and Alloy 718) were corroded in high temperature supercritical-CO₂ (S-CO₂) at 700 ℃ (20 MPa) for 500 h and compared in terms of oxidation and carburization behavior. A continuous chromia (Cr₂O₃) layer was formed on the surface of Alloy 625, whereas Ni- and Fe-rich oxide nodules were also formed with chromia on Alloy 718. Meanwhile, the extent of carburization by formation of an amorphous C layer at the oxide/matrix interface was comparatively low for Alloy 625. This difference did not seem to stem from oxide type or underlying microstructure, and was thought to be associated with oxide properties. In terms of mechanical properties, only Alloy 625 exhibited decrease in ductility after exposure to S-CO₂. This was ascribed to the microstructural evolution of the alloys during the high temperature exposure.

   4. High Performance New Ni-Base Alloy AF955 (AF955) for Oil and Gas Industry

      Luca Foroni, Louis Lherbier, Carlo Malara

      Abstract

      A precipitation hardened Ni-base alloy has been developed to fulfil the recent stringent requirements of the oil and gas industry. The new alloy is commercially designated as AF955 and the Unified Numbering System (UNS) assignment of N09955. The new alloy is patented and accepted for inclusion in NACE MR0175/ISO 15156. It is produced at strength levels of 827 MPa (120 ksi) 0.2% offset minimum yield strength (MYS) and 965 MPa (140 ksi) MYS with very good ductility and toughness, and with a microstructure characterized by fine γ′ and γ″ strengthening precipitates, uniform and equiaxed grain size distribution, minimized secondary phase precipitation and free of continuous grain boundary precipitates. It exhibits good corrosion resistance and low susceptibility to hydrogen embrittlement. These properties make alloy AF955 a very promising material for widespread applications from oil and gas industry to power generation, chemical applications and many others. Details of the manufacturing process and properties of AF955 are presented and discussed in this paper.

   5. Hydrogen Influence on Crack Propagation and Stress-Strain Evolution of Alloy 718

      Sergey Kolesov, Robert Badrak, Aleksey Shakhmatov

      Abstract

      Nickel base alloys such as 718 are used for many applications in the drilling, completion and production segments in the Oil and Gas Industry. The alloy selection is based on high strength levels while exhibiting resistance to embrittlement and environmental cracking. Hydrogen embrittlement can be a limiting factor to applications and this investigation was undertaken to better understand the mechanisms and characteristics of hydrogen in 718. Saluted hydrogen into metal could be presented in different conditions: diffuse-active and trapped by different defects and structure elements. Fatigue was used in current work as a tool for (1) the generation of structure defects and (2) hydrogen effects on crack growth. The following items were studied: (1) hydrogen solubility into different versions of 718 alloy; (2) effects of increased surface and volume defects density on hydrogen solubility; (3) hydrogen effects on stress-strain evolution; and (4) effects of hydrogen at different locations within the structure on crack growth rate. The specifics of each type of hydrogen location within the structure on crack propagation including diffusion-active and trapped by different defects and structure elements were discovered and presented.

   6. Isothermal Oxidation Behavior of EBM-Additive Manufactured Alloy 718

      Esmaeil Sadeghimeresht, Paria Karimi, Pimin Zhang, Ru Peng, Joel Andersson, Lars Pejryd, Shrikant Joshi

      Abstract

      Oxidation of Alloy 718 manufactured by electron beam melting (EBM) process has been undertaken in ambient air at 650, 700, and 800 °C for up to 168 h. At 800 °C, a continuous external chromia oxide enriched in (Cr, Ti, Mn, Ni) and an internal oxide that was branched structure of alumina formed, whereas at 650 and 700 °C, a continuous, thin and protective chromia layer was detected. The oxidation kinetics of the exposed EBM Alloy 718 followed the parabolic rate law with an effective activation energy of ~248 ± 22 kJ/mol in good agreement with values in the literature for conventionally processed chromia-forming Ni-based superalloys. The oxide scale formed on the surface perpendicular to the build direction was slightly thicker, and more adherent compared to the scale formed on the surface along the build direction, attributed to the varied grain texture in the two directions of the EBM-manufactured specimens. The increased oxygen diffusion and high Cr depletion found on the surface along the build direction were attributed to the fine grains and formation of vacancies/voids along this grain orientation.