Iron-based alloys containing 15% chromium, 2-3% molybdenum and 0.02-1.7% carbon, consisting of M23C6 and M6C carbides in an austenitic matrix were oxidised at 8500C to study their oxidation resistance and a precipitate-free zone formation. Alloy design was carried out using a thermodynamic software Thermo-Calc. Carbides in these alloys were expected to dissolve during oxidation, releasing chromium required for the protective oxide formation. Decarburisation of the matrix was expected to trigger the carbide dissolution, and form a precipitate-free zone. Transformation of the austenitic into ferritic matrix in the precipitate-free zone was expected be essential for providing a fast chromium supply to the oxide/alloy interface. Upon exposure to pure oxygen, most of the alloys oxidised non-protectively due to the fast oxidation attack and low chromium content in the matrix, while carbide dissolution was too slow. The alloys were then pre-oxidised in H2+10%H2O to grow a purely chromia scale. In this low oxygen partial pressure environment, carbides in the alloy's sub-surface dissolved and formed a ferritic precipitate-free zone. The precipitate dissolution model developed by previous investigators was then tested and proven to be valid in this iron-based alloy system. The endurance of the pre-formed chromia scale with its underlying precipitate-free zone was then tested in pure oxygen environment. All of the alloys that had successfully developed a ferritic precipitate-free zone in the pre-oxidation stage, survived the subsequent oxidation in pure oxygen up until 3 weeks observation. Although x-ray diffraction found some minor iron oxides, the oxide consisted of mainly Cr2O3. Since iron activity had increased and iron oxides had become stable after the pure oxygen gas was introduced, the growth of the precipitate-free zone had to compete with the rate at which it was consumed by oxidation. It was concluded that the transformation from austenite to ferrite at the subsurface region of the alloy could be achieved provided that the volume fraction of the carbides did not exceed 0.2. Evidence indicated that the chromia scale grew by chromium provided by the dissolving carbides. Pre-oxidation led to a promising use of the alloys at atmospheric oxygen pressure.
| Identifer | oai:union.ndltd.org:ADTP/187887 |
| Date | January 2004 |
| Creators | Susanto, Benny Laurensius, Materials Science & Engineering, Faculty of Science, UNSW |
| Publisher | Awarded by:University of New South Wales. Materials Science and Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Benny Laurensius Susanto, http://unsworks.unsw.edu.au/copyright |
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