| 1. |
- Blomstedt, Mats, et al.
(författare)
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Innovative starting procedure of Siemens SGT-600 in cold climate conditions
- 2011
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Ingår i: ASME 2011 Turbo Expo:Turbine Technical Conference and Exposition. - : The American Society of Mechanical Engineers (ASME). - 9780791854648 ; , s. 1021-1026
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Konferensbidrag (refereegranskat)abstract
- A start-up of a gas turbine means that stress and forces are put on the machine. A start-up in cold climate conditions means that the forces are more critical since the material in the machine becomes more brittle. At a certain temperature the material is utilized to its limits (with appropriate margins applied) and for the SGT-600 ambient temperatures below -30°C (-22°F) become critical. In earlier installations in an arctic climate, an electric pre- heater has been utilized to prevent the critical components from becoming too brittle. This additional hardware costs money, is consuming auxiliary power and may contribute to unavailability. Another way to solve this issue may be to install material that is less brittle, but this will also increase the cost of the installation. Siemens is now applying an improved control logic during start-up, solving this issue in the software, without any additional hardware and avoiding unnecessary material changes. This new innovative start-up procedure is performing an automatic check of the stress levels before loading the machine, resulting in a safe and reliable start at temperatures below -30°C (-22°F).
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| 2. |
- Moverare, Johan, 1973-, et al.
(författare)
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A New Single Crystal Superalloy for Power Generation Applications :
- 2011
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Ingår i: Proceeding of the 8th International Charles Parsons TurbineConference 2011. ; , s. G2-2-
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Konferensbidrag (refereegranskat)abstract
- A newly developed single crystal superalloy (known as STAL15) is described which is suitable for use in first stage blades of highly efficient industrial gas turbines (IGTs). With 15 wt% Cr and 4.55 wt% Al, the alloy combines good corrosion and oxidation resistance with sufficient creep and fatigue performance. Thus a novel and useful balance of environmental and mechanical properties is displayed. In this paper, the details of the development project are described. The new alloy is shown to be an alumina (Al2O3) former; the mechanisms behind the Al2O3-formation process are studied and the effects arising from changes in the chemical composition have been modelled. In addition, the mechanical properties in terms of creep and fatigue resistance are evaluated together with the alloy stability during long term (up to 10,000 hours) exposure. For such applications, the new alloy is superior to existing nickel-based single crystal superalloys designed for aero engine applications and which are optimized for very high creep resistance; experience has shown that they are not ideal for use in IGTs of the type used for power generation or mechanical drive, due to their low corrosion resistance. The main reason for this is that IGTs are exposed to more harsh conditions in terms of air quality through the turbine and significantly longer time between overhaul/inspection. Traditionally this has been handled by use of polycrystalline alloys such as IN792 and IN738LC or the single crystal alloy PWA1484; unfortunately these do not form a protective Al2O3 layer and hence display limited resistance to environmental degradation. The new alloy does not display this weakness and is therefore highly optimised for IGT applications.
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| 3. |
- Moverare, Johan, 1973-, et al.
(författare)
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Damage mechanisms of a high-Cr single crystal superalloy during thermomechanical fatigue
- 2010
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Ingår i: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 527:3, s. 553-558
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Tidskriftsartikel (refereegranskat)abstract
- The deformation and damage mechanisms arising during thermomechanical fatigue of a high-Cr single crystal superalloy have been investigated and a completely new failure mechanism involving recrystallization and oxidation has been discovered. The primary deformation mechanism is slip along the {1 1 1} planes. The deformation is highly localized to a number of bands, where recrystallization eventually occurs during the thermomechanical fatigue process. When the final failure occurs along these recrystallized bands it is accompanied by the formation of voids due to the presence of grain boundaries. The damage process is further enhanced by oxidation, since recrystallization occurs more easily in the γ′ depleted zone under the oxide layer.
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| 4. |
- Moverare, Johan, 1973-, et al.
(författare)
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Effect of SO2 and water vapour on the low-cycle fatigue properties of nickel-base superalloys at elevated temperature
- 2013
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 564, s. 107-115
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Tidskriftsartikel (refereegranskat)abstract
- In this study the effect of SO2+water vapour on strain controlled low cycle fatigue resistance of three different nickel based superalloys has been studied at 450°C and 550°C. A negative effect was found on both the crack initiation and crack propagation process. The effect increases with increasing temperature and is likely to be influenced by both the chemical composition and the grain size of the material. In general the negative effect decreases with decreasing strain range even if this means that the total exposure time increases. This is explained by the importance of the protective oxide scale on the specimen surface, which is more likely to crack when the strain range increases. When the oxide scale cracks, preferably at the grain boundaries, oxidation can proceed into the material, causing preferable crack initiation sites and reduced fatigue resistance. © 2012 Elsevier B.V.
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| 5. |
- Moverare, Johan, 1973-, et al.
(författare)
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Environmental aspects on LCF-life of Ni-base superalloys during long term operation
- 2012
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Rapport (refereegranskat)abstract
- The applicability of fine grain polycrystalline nickel base superalloys in gas or steam turbine applications is often limited by their susceptibility to fast intergranular cracking during fatigue in combination with extended hold times at high temperatures and high tensile stresses. This effect is further enhanced in corrosive environments even at moderate temperatures such as 400-600°C. In this study the negative effect of SO2 + water vapour on the low cycle fatigue resistance of three different nickel based superalloys (Nimonic 901, Inconel 718 and 718plus) has been studied at 450°C and 550°C. A negative effect was found on both the crack initiation and crack propagation process. The effect increases with increasing temperature and is likely to be influenced by both the chemical composition and the grain size of the material. The negative effect of water vapour + SO2 is manifested by a decreased resistance to cyclic plastic deformation and a transition from transcrystalline to intercrystalline fracture behaviour. In Nimonic 901 this negative effect increases with the degree of plastic deformation. For lower mechanical strain amplitudes where the number of cycles and the total exposure time is increased the environmental impact is reduced. Similar trends can also be noticed for the other alloys. The surface corrosion in air and in SO2 + water vapour are found to be rather similar in this study and it is therefore concluded that the surface scale can remain adherent and protective if the strains on the oxide scale are low. However, for LCF tests with higher strain ranges, the oxide scale will rupture preferably at the grain boundaries and intergranular microcracks will initiate which promotes inward diffusion of embritteling elements such as oxygen and sulphur. Inconel 718 seems to be more sensitive to SO2 + water vapour than Nimonic 901 when tested at 450°C. The overall resistance to LCF is however still higher in Inconel 718 for the test conditions investigated in this study, this is due to the exceptionally good resistance to low cycle fatigue that this alloy has from the start (as seen for the tests in air).
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| 6. |
- Moverare, Johan, 1973-, et al.
(författare)
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Hold-time effect on the thermo-mechanical fatigue crack growth behaviour of Inconel 718
- 2011
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Ingår i: Materials Science & Engineering. - : Elsevier. - 0921-5093 .- 1873-4936. ; 528:29-30, s. 8660-8670
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Tidskriftsartikel (refereegranskat)abstract
- In-phase TMF crack growth testing with different lengths of the hold time at the maximum temperature of 550 °C has been conducted on Inconel 718 specimens. Focus has been on establishing a method for TMF crack growth testing and investigating the effect of high temperature hold times on the TMF crack growth of the material. The tests are compared to isothermal crack propagation tests and show good correlation. It is concluded that the controlling effect of the crack growth is an embrittlement of the material. This embrittlement is related to the concept of a damaged zone active in front of the crack tip. The size of this damaged zone will control the crack propagation rate and therefore it does not matter if the load is cycled under isothermal or TMF conditions.
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| 7. |
- Movérare-Skrtic, Sofia, et al.
(författare)
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Leukocyte Telomere Length (LTL) is reduced in stable mild cognitive impairment but low LTL is not associated with conversion to Alzheimer's Disease: A pilot study
- 2012
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Ingår i: Experimental Gerontology. - : Elsevier BV. - 1873-6815 .- 0531-5565. ; 47:2, s. 179-182
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Tidskriftsartikel (refereegranskat)abstract
- Leukocyte telomere length (LTL) is associated with the aging process and may be related to cognitive aging. Previous studies have shown conflicting results whether LTL is affected in patients with Alzheimer's disease (AD). In this pilot study, we investigated LTL in a well-defined homogeneous mono-center population. Sixty consecutive patients admitted for cognitive impairment to a memory clinic were recruited. The participants included patients with AD or mild cognitive impairment (MCI) diagnosed with AD upon follow-up (n=32), patients with stable MCI (n=13), patients with other dementias diagnosed at primary evaluation or upon follow-up (n=15), and healthy controls (n=20). LTL was determined using a quantitative PCR assay. Patients with AD had similar LTL as healthy controls. Patients with stable MCI had reduced LTL both compared to AD patients (p=0.02) and controls (p=0.008). Subanalyses within the AD group showed that patients with MCI that later converted to AD had similar LTL as patients with clinical diagnosis of AD at primary evaluation and healthy controls whereas the LTL was longer compared to the stable MCI group (p=0.02). There were no correlations between LTL and the core AD biomarkers A beta(1-42), T-tau and P-tau. In conclusion, in this pilot study, patients with AD or MCI that later converted to AD had similar LTL as healthy controls. Patients with stable MCI that did not progress to dementia had reduced LTL compared to controls, which might suggest a more marked biological aging as a cause of the cognitive symptoms in this group. (C) 2011 Elsevier Inc. All rights reserved.
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| 8. |
- Sato, Atsushi, et al.
(författare)
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On the Mechanical Behavior of a New Single-Crystal Superalloy for Industrial Gas Turbine Applications
- 2012
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Ingår i: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 43A:7, s. 2302-2315
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Tidskriftsartikel (refereegranskat)abstract
- The mechanical behavior of a new single-crystal nickel-based superalloy for industrial gas turbine (IGT) applications is studied under creep and out-of-phase (OP) thermomechanical fatigue (TMF) conditions. Neutron diffraction methods and thermodynamic modeling are used to quantify the variation of the gamma prime (γ′) strengthening phase around the γ′ solvus temperature; these aid the design of primary aging heat treatments to develop either uniform or bimodal microstructures of the γ′ phase. Under creep conditions in the temperature range 1023 K to 1123 K (750 °C to 850 °C), with stresses between 235 to 520 MPa, the creep performance is best with a finer and uniform γ′ microstructure. On the other hand, the OP TMF performance improves when the γ′ precipitate size is larger. Thus, the micromechanical degradation mechanisms occurring during creep and TMF are distinct. During TMF, localized shear banding occurs with the γ′ phase penetrated by dislocations; however, during creep, the dislocation activity is restricted to the matrix phase. The factors controlling TMF resistance are rationalized.
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| 9. |
- Skoglund, Peter, et al.
(författare)
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Fatigue and strength of new grey iron alloys for break discs
- 2013
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Ingår i: Seventh International Conference on Low Cycle Fatigue. - Berlin : Deutscher Verband für Materialforschung und -Prüfung E.V.. - 9783981451627 ; , s. 45-50
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Konferensbidrag (refereegranskat)abstract
- The static and low cycle fatigue properties of four pearlitic grey iron alloys intended for brake discs are investigated. The effect of alloying a base composition with 0.1 wt% and 0.3 wt% niobium is compared to a 0.3 wt% addition of the more conventional alloying element molybdenum. The results show that the static properties remain unaffected for an Nb content of 0.1 wt% while an addition of 0.3 wt% Nb or 0.3 wt% Mo increases the strength compared to the base alloy without Nb and Mo. The fatigue life on the other hand remains more or less unaffected by the alloying, although the stress levels are substantially higher for the compositions with 0.3 wt% Mo or 0.3 wt% Nb. Thus, the experiments show that graphite is very important for the low cycle fatigue life. It is also seen that crack propagation controls more than 90% of the life. Full scale brake dynamometer crack tests also gave a similar fatigue life of the different brake disc alloys independent of the composition.
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