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  • 1
    ISSN: 1573-4889
    Keywords: internal oxidation ; carburization ; computer simulation ; finite-difference technique
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract A computer-based method is used to simulate the process of penetration of a foreign element (e.g., oxygen) into an alloy connected with internal precipitation of a compound of this element with a solute in the alloy. The method employed is based on the solution of the differential equations corresponding to this high-temperature corrosion phenomenon with the help of the finite-difference technique. To demonstrate the possibilities of the calculation method, various cases were treated which differ especially regarding the thermodynamic stability of the internal precipitates. The calculated concentration profiles and penetration depths obtained are reported and discussed. In the case of internal precipitates of high stability, the results are compared with the approaches derived in the literature for internal oxidation and are used to determine the parameter ranges in which the analytical equations apply. The results obtained for the case of a less-stable compound is used to illustrate the influence of the solubility product on the concentration profiles.
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  • 2
    ISSN: 1573-4889
    Keywords: Ni-base alloy IN 617 ; reactor helium ; oxidation ; carburization ; decarburization ; kinetics ; mechanisms
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Application of the technical nickel-base alloy IN 617 in the primary circuit of the high-temperature gas-cooled reactor is limited essentially by the chemical reactions with the impurities (CO, CO2, H2, CH4, H2O) in the helium coolant. The interactions of the alloy with the reactive impurities in different helium-base gases were investigated by thermogravimetry in a gas-tight microbalance and by simultaneous measurement of the changes in gas composition by a continuous sensitive mass spectrometric analysis. The results demonstrate that the set of six reaction equations deduced in part 1 can be applied to describe the corrosion of the alloy. The occurrence of the various reactions is determined essentially by temperature. For the case of a standardized helium gas (HHT-He), three temperature regions can be distinguished. Below a critical temperature (about 1105 K), the presence of CO can cause simultaneous oxidation and carburization. Above this temperature, this reaction does not reverse itself. Rather, oxidation by CO2 and H2O takes place that shows, after a transient period, the same kinetics observed in undiluted oxygen-containing gases. At temperatures above about 1205 K, decarburization of the alloy accompanied by the production of CO takes place, leading to severe destruction of the carbide microstructure and, therefore, limiting the applicability of the material.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Oxidation of metals 26 (1986), S. 45-76 
    ISSN: 1573-4889
    Keywords: Ni-base alloys ; oxidation ; influence of oxygen partial pressure at high temperatures ; kinetics and mechanisms of high temperature oxidation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The oxidation behavior of the Ni-base alloys IN 617, IN 713 LC, Ni20Cr, and Ni20Cr+Si has been investigated in the temperature range from 850°C to 1000°C in air and at low-oxygen partial pressure p(O2) (10−19 to 10−16 bar). With the exception of alloy IN 713 LC, the materials show no influence of p(O2) on the oxidation mechanisms and the kinetics. This result can be explained by the formation of a dense Cr2O3 layer, the growth rate of which is controlled by the Cr ion interstitial concentration in Cr2O3 at the phase boundary oxide/alloy and the mobility of Cr ions in Cr2O3. For the alloy IN 713 LC which develops a dense Al2O3 layer in air, a modified transition mechanism at low p(O2) leads to the formation of Cr2O3 at the surface and a strong internal oxidation of Al.
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  • 4
    ISSN: 1573-4889
    Keywords: Reactor helium ; carbon activity ; oxygen activity ; flow conditions ; reactions with Pt, Cr, Cr3C2, Cr2O3
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract Helium used as a coolant in high-temperature gas-cooled reactors contains gaseous impurities that cause various corrosion effects. To determine the mechanisms of the reactions that occur in this complex gas phase, a theoretical and experimental characterization of the gas is given in part 1. To obtain estimates of the reactions that are possible in principle and to derive the basis for the interpretation of the experimental results, thermodynamic considerations on gas equilibrium and partial equilibria are presented. Because of the extremely low concentrations of the impurities, diffusion processes within the gas may become rate-controlling. This requires a consideration of the flow conditions. The reactions of Pt, Cr, Cr2O3 and Cr3C2 with helium-based atmospheres containing impurities are investigated by means of a mass spectrometric gas analysis with respect to temperature and flow rate. A set of six reactions is derived from the results and discussed in detail. The application of these equations to describe the high-temperature corrosion phenomena of a technical nickel-base alloy is dealt with in part 2.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Journal of materials science 19 (2000), S. 437-438 
    ISSN: 1573-4811
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 6
    Electronic Resource
    Electronic Resource
    Weinheim [u.a.] : Wiley-Blackwell
    Materials and Corrosion/Werkstoffe und Korrosion 37 (1986), S. 145-149 
    ISSN: 0947-5117
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Influence of creep deformation on the formation of the oxide layer on the high temperature alloy Ni20CrThe formation of the Cr2O3-layer on Ni20Cr has been investigated at 850°C in H2/H2O (p(O2) = 10-19 bar) under simultaneous creep deformation with flat samples. The damage of the protecting oxide layer by cracks has been observed in dependence on deformation rate and strain. For additional information about the influence of the plastic deformation of the oxide layer and the healing of the cracks, preoxidized samples have been deformed in pure Ar-atmosphere.At strain rates below 10-9s-1 cracks cannot be observed. When strain rates \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document} 〈 about 3 × 10-8s-1 are applied, cracks occur only above grain boundaries of the alloy, at higher strain rates they also lie in regions above the grains of the alloy. For \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document} 〉 about 3 × 10-8s-1 the crack density depends no more on \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document} but only on strain \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document}. The different damages of the oxide layers in the two atmospheres allow the conclusion, that at \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document} from 10-9s-1 to 10-7s-1 beside the plasticity of the oxide layer in particular the crack healing influences the sum of the crack openings measurably. With increasing strain rates the contribution of plasticity can be neglected.
    Notes: Die Ausbildung der Cr2O3-Schicht auf Ni20Cr ist bei 850°C in H2/H2O (p(O2) = 10-19 bar) unter gleichzeitiger Kriechverformung an Flachproben untersucht worden. Die Schädigung der schützenden Oxidschicht durch Risse wurde in Abhängigkeit von der Verformungs-geschwindigkeit und der Dehnung des Grundwerkstoffes beobachtet. Zur Trennung des Einflusses der plastischen Verformung der Oxidschicht und der Rißheilung wurden ergänzend voroxidierte Proben in reiner Ar-Atmosphäre verformt.Unterhalb der Kriechgeschwindigkeit \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document} von 10-9s-1 werden keine Risse mehr festgestellt. Bei \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document} 〈 etwa 3 × 10-8s-1 treten Risse nur oberhalb Legierungskorngrenzen auf, bei höheren \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document} liegen sie auch in Bereichen über den Legierungskörnern. Für \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document} 〉 etwa 3 × 10-8s-1 hängt die Rißdichte nicht mehr von \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document} ab. Aus der unterschiedlichen Schädigung der Oxidschicht in den beiden Gasatmosphären kann geschlossen werden, daß bei \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document} zwischen 10-9s-1 und 10-7s-1 neben einer Plastizität der Oxidschicht insbesondere die Rißheilung meßbare Einflüsse bei der Summe der Rißöffnungen hervorruft, mit steigendem \documentclass{article}\pagestyle{empty}\begin{document}$$ \dot{\rm \varepsilon} $$\end{document} wird der Beitrag der Plastizität vernachlässigbar klein.
    Additional Material: 5 Ill.
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  • 7
    ISSN: 0947-5117
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Carburization of high-temperature materials - Part I: Mathematical model description of the penetration and simultaneous precipitation of a compound of the diffusing elementMetallic materials often exhibit a change in structure in high temperature applications caused by the penetration of a foreign element and the precipitation of compounds of this element. This change in structure influences the mechanical properties and may be the life-time-limiting factor. The kinetics of this process can be described by a simple time law only in the case, where certain restrictive conditions are given - for instance an extremely small solubility product for the forming compound. In this work a computation method is presented, which allows a calculation of the reaction progress under more general conditions. The method is based on the solution of differential equations by the use of the finite difference technique. The basic relations are derived from considerations of the processes of diffusion, precipitation and transformation for the case of carburization. The difference between the numerical result and the analytical solution is demonstrated in an example with restrictive conditions, and the possibilities for applications of the method are discussed.
    Notes: Metallische Werkstoffe erfahren häufig beim Hochtemperatureinsatz eine Gefügeänderung durch die Eindiffusion eines Fremdelementes mit einer Ausscheidung von Verbindungen dieses Elementes mit einem Legierungsbestandteil (z.B. innere Oxidation, innere Nitrid-, innere Sulfidbildung). Diese Gefügeänderung beeinflußt die mechanischen Eigenschaften und stellt unter Umständen den lebensdauerbegrenzenden Faktor dar. Die Kinetik des Vorganges kann nur dann durch ein einfaches Zeitgesetz beschrieben werden, wenn relativ spezielle Voraussetzungen erfüllt sind, beispielsweise, wenn das Löslichkeitsproudukt der sich bildenden Verbindungen sehr klein ist. In dieser Arbeit wird ein Rechenverfahren vorgestellt, welches eine Berechnung des Reaktionsablaufes unter allgemeinen Bedingungen ermöglicht. Die Methode beruht auf der Lösung von Differentialgleichungen mittels finiter Differenzen. Die zugrundeliegenden Beziehungen werden aus der Betrachtung der Vorgänge der Diffusion, Ausscheidung und Umwandlung für den Fall der Aufkohlung abgeleitet. Anhand eines speziellen Beispieles wir der Unterschied zur analytischen Beschreibung für den Fall mit eingeschränkten Voraussetzungen demonstriert und Anwendungsmöglichkeiten des Verfahrens diskutiert.
    Additional Material: 5 Ill.
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  • 8
    Electronic Resource
    Electronic Resource
    Weinheim [u.a.] : Wiley-Blackwell
    Materials and Corrosion/Werkstoffe und Korrosion 37 (1986), S. 437-443 
    ISSN: 0947-5117
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Carburization of high-temperature materials - Part II: Experimental testing of the computer aided description of inward diffusion and precipitationIn part I of this study a calculation method was presented which permits the computation of carburization processes which involve simultaneous precipitation of carbides. In the present study, this model is confirmed by measurements of carburization kinetics of two model alloys Ni 10 Cr and Ni 25 Cr with the help of thermogravimetric methods. The experiments were carried out in the temperature range from 850°C to 950°C in an atmosphere of H2 with 1.14 Vol.% CH4. The dependence of the solubility and the carbon diffusion coefficient on temperature in the above alloys were determined and the stabilities of the forming carbides adapted. Satisfactory agreement between calculation and experiment is found on the basis of metallographically determined volume fraction, the progress of the precipitation front of the carbides, the remaining chrome concentration of the alloy matrix and the weight gain per unit surface area.
    Notes: Mit Hilfe thermogravimetrischer Untersuchungen der Aufkohlungskinetik der beiden Modellegierungen Ni 10 Cr und Ni 25 Cr wird die Anwendbarkeit der in Teil I entwickelten Rechenmethode zur Beschreibung der Aufkohlung bei gleichzeitiger Ausscheidung von Karbiden bestätigt. Die Versuche erfolgen im Temperaturbereich von 850°C bis 950°C in H2 + 1,14 Vol.-% CH4. Die Temperaturabhängigkeit der maximalen Löslichkeit und des Diffusionskoeffizienten des Kohlenstoffs in diesen Legierungen werden ermittelt, die Stabilitäten der auftretenden Karbide angepaßt. Die Übereinstimmung von Rechnung und Experiment wird anhand des metallographisch ermittelten Volumenbruchteils, der Lage der Ausscheidungsfronten der Karbide, des Restgehaltes an Chrom in der Legierungsmatrix und der flächennormierten Gewichtszunahme überprüft.
    Additional Material: 11 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    Weinheim [u.a.] : Wiley-Blackwell
    Materials and Corrosion/Werkstoffe und Korrosion 39 (1988), S. 287-292 
    ISSN: 0947-5117
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: The influence of corrosive atmospheres on low voltage insulationsThe insulation behaviour of electrotechnical equipment is deteriorated, if conductive corrosion products get between the electrodes under voltage stress. For accelerated simulation of this process, standardized test methods with corrosive atmospheres, containing SO2 and H2S, are used. The effect of these tests was investigated by using test specimens, consisting of Al2O3-ceramic, carrying dc-stressed electrode couples of copper, brass and lead-tin solder. By means of SIMS-analysis, it was found out, that the influence of H2S on copper and brass results in sulfid-containing corrosion products, which get on to the insulation between the electrodes. SO2-influence on brass or lead-tin solder electrodes only sporadically results in conductive connections, which are probably caused by metal dendrite growth.
    Notes: Das Isolationsverhalten elektrotechnischer Erzeugnisse wird beeinträchtigt, wenn leitfähige Korrosionsprodukte zwischen spannungsführende Elektroden gelangen. Zur zeitraffenden Simulation dienen genormte Prüfverfahren mit SO2- bzw. H2S-haltigen korrosiven Atmosphären. Die Wirkung derartiger Prüfungen wurde an Prüflingen aus Al2O3-Keramikplättchen mit gleichspannungsbeanspruchten Elektrodenpaaren aus Kupfer, Messing oder Lötzinn untersucht. Mit Hilfe von SIMS-Analysen wurde festgestellt, daß bei H2S-Einwirkung auf Kupfer und Messing sulfidhaltige Korrosionsprodukte in den Elektrodenzwischenraum gelangten. Vereinzelt unter SO2-Einwirkung entstandene leitende Verbindungen zwischen Messing- und Lötzinnelektrodenpaaren sind demgegenüber wahrscheinlich auf vorgewachsene Metalldendriten zurückzuführen.
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Acta mechanica 1 (1965), S. 75-80 
    ISSN: 1619-6937
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
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