The microstructural development of oxide scales on low carbon steels
thesisposted on 25.01.2010 by Soran Birosca
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The microstructures of the oxide scale on steels are very complex and their development depends on many factors including time, temperature, oxidation conditions and alloying elements. Classically, steel oxide scales are characterised by a three-layer model: The innermost layer, closest to the steel substrate, with the lowest oxygen content is wüstite (FeO); there is an intermediate magnetite (Fe3O4) layer and a final oxygen rich hematite (Fe2O3) layer. This classical model is more complicated in reality and its properties change with the factors that affect their development. Microstructure characterisation of the oxide scale helps to obtain a better understanding of the oxidation mechanism. Furthermore, knowledge of the oxide microstructure is critical in understanding how the oxide behaves during the high temperature deformation of steels and more importantly how it can be removed following processing. An understanding of the oxide scale formation and its properties can only be achieved by careful examination of the scale microstructure. The oxide scale microstructure may be difficult to characterise by conventional techniques such as optical or standard scanning electron microscopy. An unambiguous characterisation of the scale and the correct identification of the phases within the scale are difficult unless the crystallographic structure for each phase in the scale is considered and a microstructure-microtexture analysis is carried out. In the current study Electron Backscatter Diffraction (EBSD) has been used to investigate the microstructure and microtexture of iron oxide layers grown on low carbon steels at different times and temperatures. EBSD has proved to be a powerful technique for identifying the individual phases in the oxide scale accurately. The results show that EBSD can be used to give a complete characterisation of the oxide scale. The microstructural features such as grain size, shape and grain or phase boundaries characteristics has been successfully determined and analysed. The results show that different grain shapes and sizes develop for each phase in the scale depending on time and temperature. Also, a complex relationship between the microstructures and the crystallographic texture of the different scales components has been observed. In the light of new approach of understanding the steel oxidation phenomena from the current study, previous steel oxidation hypothesis and postulations have been reviewed and examined.
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