S.Birosca PhD Thesis.pdf (61.36 MB)
The microstructural development of oxide scales on low carbon steels
thesis
posted on 2010-01-25, 12:08 authored by Soran BiroscaThe 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.
History
School
- Aeronautical, Automotive, Chemical and Materials Engineering
Department
- Materials
Publication date
2006Notes
A doctoral thesis submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy.EThOS Persistent ID
uk.bl.ethos.429829Language
- en