Fatigue Crack Growth Mechanisms for Nickel-based Superalloy Haynes 282 at 550-750 °C. PDF Download
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Category :
Languages : en
Pages : 9
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Author:
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Category :
Languages : en
Pages : 9
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Author: Kyle A. Rozman
Publisher:
ISBN:
Category : Heat resistant alloys
Languages : en
Pages : 156
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Electric power needs will only grow over the next decades as more humans ascend from poverty into the middle class. Currently, the majority of electric power is generated by burning fossil fuels. To help mitigate the undesirable effects of burning fossil fuels research is being done to increase the efficiency of power produced. In order to increase the efficiency of power production, the operating temperature of steam turbines must be increased, which presents a materials challenge. Haynes 282 is a nickel based superalloy which has been proposed as a potential rotor alloy for steam turbines operating at high temperature (760°C). Other authors have previously looked at creep, oxidation, low cycle fatigue and other properties of Haynes 282; however, lacking from the literature are studies on the fatigue crack growth mechanisms in Haynes 282. This project investigated Haynes 282 from a fatigue crack growth perspective with an aim to fill this literature gap and assess the utility of Haynes 282 as a steam turbine rotor alloy. This dissertation has specifically evaluated the fatigue crack growth rates of Haynes 282 as functions of temperature and frequency. The testing method utilized was ASTM E647, "Standard test method for measuring fatigue crack growth rates." Temperatures investigated were 550°C, 650°C and 750°C. The loading frequencies were both 25 Hz and 0.25 Hz. In general increasing the test temperature increased fatigue crack growth rate. Thermally activated cross-slip and dislocation annihilation were the primary mechanisms responsible for the increased fatigue crack growth rate. The noted activation energy for dislocation jog migration, which is related to dislocation annihilation, was about 12.5 kJ/mol. This value fit with the measured activation energy. The post-test microstructure showed greatly reduced dislocation density at the highest temperature. Fractography of the crack growth region showed transgranular crack growth at 550°C and 650°C with signs of isolated intergranular features at 750°C. The effect of frequency on the fatigue crack growth rates was minor at 550oC but much more significant at 650°C and 750°C. For the temperatures investigated the effect of decreasing loading frequency was to increase the fatigue crack growth rate. At high loading frequency, the isolated intergranular features were present only at stress intensities below about 11 MPa√m. At 0.25 Hz loading frequency, isolated intergranular features persisted into the high stress intensity range. While the isolated intergranular features are of some concern, the measured activation energy was well below published creep and/or oxidation activation energies. This means the crack path did not have enough energy to sustain an intergranular crack path. In summary, the work discussed in this dissertation investigated the fine details of fatigue crack growth of Haynes 282. This study has closed some of the existing gap in the literature regarding the fatigue crack growth rates of Haynes 282. Previous studies have shown no adverse effects during low cycle fatigue and creep for Haynes 282 at temperatures up to and including 760°C. This study has shown no adverse effects in the fatigue crack growth of Haynes 282 up to 650°C. However, some isolated indications of intergranular crack propagation within the crack growth region were observed at 750°C. These isolated regions were observed to arrest, or to revert back to, transgranular crack propagation. This mechanistic aspect, i.e., change back to transgranular crack propagation, is a promising result.
Author: J. Shingledecker
Publisher: ASM International
ISBN: 1627082727
Category : Technology & Engineering
Languages : en
Pages : 1500
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Proceedings from: EPRI’s 9th International Conference on Advances in Materials Technology for Fossil Power Plants and the 2nd International 123HiMAT Conference on High-Temperature Materials
Author: John A. Newman
Publisher:
ISBN:
Category : Fracture mechanics
Languages : en
Pages : 16
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Author: H. Ghonem
Publisher:
ISBN:
Category :
Languages : en
Pages : 252
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At loading frequencies below that of the transitional frequency level, which is typical of mission cycles of jet engines, the elevated temperature fatigue crack growth process in Alloy 718 is viewed to be fully environment-dependent. Of all the crack growth stages, this process, while is the most critical in high temperature application due to its highly accelerated crack growth rate, is the least studied or understood. The objective of this research program is to focus on the understanding or the mechanism controlling this oxidation-dependent stage in order to develop the ability to predict its associated crack growth performance under different environment conditions. For this purpose, three major studies have been carried out; the first was to provide evidence of the existence of the fully environment-dependent stage in which the crack growth rate would be equal to the oxygen penetration rate at the crack tip. The second study was to establish a crack tip oxidation mechanism on the basis of material, environmental and loading parameters interactions in the crack tip region. The last objective of this program is to establish a micromechanical based quantitative model to predict the environmentally-dominated crack growth stage.
Author: RM. Pelloux
Publisher:
ISBN:
Category : Elevated-temperature fatigue
Languages : en
Pages : 27
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The micromechanisms of high-temperature fatigue crack initiation and crack propagation in solid-solution and precipitation-strengthened nickel-base super-alloys are reviewed. The marked decrease in fatigue strength of a given superalloy with increasing temperature cannot be solely correlated with the temperature dependence of the short-time mechanical properties. The interactions between oxidation rates and fatigue strengths are very complex. The air environment plays a large role in accelerating the initiation and propagation of fatigue cracks at elevated temperatures. Increasing temperature and decreasing frequency lead to a transition from transgranular to intergranular fracture path.
Author: George John Gikas
Publisher:
ISBN:
Category : Alloys
Languages : en
Pages : 0
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Author: Rong Jiang
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Category :
Languages : en
Pages : 0
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Author: Paul Woollin
Publisher:
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Category :
Languages : en
Pages :
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Author: Jonas Saarimäki
Publisher: Linköping University Electronic Press
ISBN: 9176858715
Category :
Languages : en
Pages : 49
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Book Description
Gas turbines are widely used in industry for power generation and as a power source at "hard to reach" locations where other possibilities for electrical supply are insufficient. There is a strong need for greener energy, considering the effect that pollution has had on global warming, and we need to come up with ways of producing cleaner electricity. A way to achieve this is by increasing the combustion temperature in gas turbines. This increases the demand on the high temperature performance of the materials used e.g. superalloys in the turbine. These high combustion temperatures can lead to detrimental degradation of critical components. These components are commonly subjected to cyclic loading of different types e.g. combined with dwell-times and overloads at elevated temperatures, which influence the crack growth. Dwell-times have shown to accelerate crack growth and change the cracking behaviour in both Inconel 718 and Haynes 282. Overloads at the beginning of the dwell-time cycle have shown to retard the dwell time effect on crack growth in Inconel 718. To understand these effects more microstructural investigations are needed. The work presented in this licentiate thesis was conducted under the umbrella of the research program Turbo Power; "High temperature fatigue crack propagation in nickel-based superalloys", concentrating on fatigue crack growth mechanisms in superalloys during dwell-times, which have shown to have a devastating effect on the crack propagation behaviour. Mechanical testing was performed under operation-like conditions in order to achieve representative microstructures and material data for the subsequent microstructural work. The microstructures were microscopically investigated in a scanning electron microscope (SEM) using electron channeling contrast imaging (ECCI) as well as using light optical microscopy. The outcome of this work has shown that there is a significant increase in crack growth rate when dwell-times are introduced at the maximum load (0% overload) in the fatigue cycle. With the introduction of a dwell-time there is also a shift from transgranular to intergranular crack growth for both Inconel 718 and Haynes 282. When an overload is applied prior to the dwell-time, the crack growth rate decreases with increasing overload levels in Inconel 718. At high temperature crack growth in Inconel 718 took place as intergranular crack growth along grain boundaries due to oxidation and the creation of nanometric voids. Another observed growth mechanism was crack advance along phase boundaries with subsequent severe oxidation of the phase. This thesis comprises two parts. The first giving an introduction to the field of superalloys and the acting microstructural mechanisms that influence fatigue during dwell times. The second part consists of two appended papers, which report the work completed so far in the project.
