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| Home > Publications database > Prediction of Oxidation Induced Life Time for FCC Materials at High Temperature Operation |
| Book/Dissertation / PhD Thesis | FZJ-2017-03768 |
2017
Forschungszentrum Jülich GmbH Zentralbibliotek
Jülich
ISBN: 978-3-95806-230-6
Please use a persistent id in citations: http://hdl.handle.net/2128/14898 urn:nbn:de:0001-2017071203
Abstract: With an increasing application of high temperature alloys, especially Ni-based superalloys in automobile and other industrial fields, the ability to predict components‟ lifetime becomes a predominant demand from both safety and energy consumption aspects. In the present investigation, an attempt was made to develop a generalized oxidation lifetime model for chromia-forming FCC alloys that can be incorporated into alloy data sheets and easily understood and employed by component designers. The model captures the most important damaging oxidation effects relevant for component design: wall thickness loss, scale spallation and the occurrence of “breakaway” oxidation. The material used for development of the concept was the wrought NiCrW base alloy 230. For deriving modeling input parameters and for verification of the model approach, specimens of this alloy with different thicknesses were exposed cyclically for different times at temperatures in the range 950 - 1050°C in static laboratory air. The studies concentrated on thin specimens (thickness 0.2 - 0.5 mm) to obtain data for critical subscale depletion processes resulting in “breakaway” oxidation within reasonably achievable test times up to 3000 h. The oxidation kinetics and oxidation induced subscale microstructural changes from the long term tests were combined with results from thermogravimetric analyses (TGA), scanning electron microscopy (SEM) with energy dispersive x-ray (EDX) spectroscopy and electron backscatter diffraction (EBSD), as well as glow discharge optical emission spectroscopy (GDOES). Oxidation induced microstructural changes as function of specimen thickness, time and temperature were estimated and modeled using the software packages Thermocalc and DICTRA. The modeling of the oxide scale spalling and re-formation was based on the cyclic oxidation spallation program (COSP), which was published previously. The program was modified to adapt the approach to the present experimental observations. A new model was developed to describe accelerated oxidation occurring after longer exposure times in case of the thinnest specimens. The calculated oxidation kinetics was correlated with the Cr reservoir equation, by means of which the relation between the consumption and the remained concentration of the scale forming element (Cr) in the alloys is established as a function of temperature and specimen thickness. The results obtained by the reservoir approach were compared with calculations of Cr concentration profiles using a finite difference model. Based on this approach, a generalized lifetime diagram is proposed in which wall thickness loss as function of time, specimen thickness and temperature as well as times to reaching a critical chromium subscale depletion are presented. The same approach was subsequently applied to the nickel base alloys X and NiCr 8020 as well as the austenitic steel Nicrofer 2020. Both Ni base alloys showed shorter times to critical subscale depletion than alloy 230; alloy X mainly due to higher scale growth rates, alloy NiCr 8020 due to poorer scale adherence and a lower initial Cr content. The austenitic steel showed the shortest lifetime mainly due to Mn and Ti induced high growth rates of the chromia scale and resulting low Cr interface concentrations.
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