| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 93 === Stainless steels (SSs) could induce metal dusting damage and even lead the occurrence of the localized thinning/pitting phenomenon when they are exposed in high carbon-activity atmospheres at elevated temperatures. The relationship between the susceptibility to metal dusting of material and the surface to form the protective passive film is rather close. The surface state (eg. surface working, surface oxidation, and surface carburizing) of material on the susceptibility to metal dusting becomes the meaningful topic. In this investigation, the effect of surface pre-treatment (grinding. sand-blasting, and pre-oxidation) on the susceptibility to metal dusting of 304L SS was examined. Moremore, the pre-carburization treatment is expected to help the progress of carburizing on 304L SS. The process of metal dusting occurrence and nano-sized carbon filaments formation on 304L SS was investigated. The effect of carburizing for 304L SS on metal dusting was also examined. Consequently, the effect of surface pre-treatment on metal dusting susceptibility for 304L SS exposed in CO-CO2 atmosphere was investigated in this study.
For the as-ground, sand-blasted, and pre-oxidized 304L SS, the results showed that no localized pit was formed on specimen surface after TGA tests. A linear relationship of weight gain versus time was obtained for all specimens, and the weight gain increased with increasing temperature. More specifically, the decomposition of CO gas to form carbon was mainly responsible for the weight gain, and the contribution to the weight gain due to oxidation could not be ruled out. The as-ground and sand-blasted surfaces enhance the formation of oxide scale after long-term exposure. A thicker Cr-rich oxide layer was found and showed a significant resistance to metal dusting on 304L SS with the sand-blasted surface. The existence of an initial non-protective oxide on the pre-oxidized 304L SS surface, however, gave rise to the most susceptibility to metal dusting associated with localized attack. The Fe/Ni-rich metallic phase beneath the site of oxide breakdown was responsible to metal dusting attack. The direct carbon ingress could more easily induce the occurrence of metal dusting. The internal reaction layer of localized pitting site existed three different structure zones consisting of an outer oxide/metal composite layer, an intermediate graphite/oxide/metallic particle mixture, and an inner internal-oxidation layer. The number, average maximum width, and maximum depth of the localized pitting site observed on the pre-oxidized 304L SS were increased with increasing temperature.
The pre-carburization was performed using the pack-cementation process which gave rise to the formation of Cr-rich oxide scale on the 304L SS surface. On top of the metal surface, a carbon-rich metallic layer was produced (chromium depletion) beneath the oxide scale. The TGA experimental results showed a linear relationship of weight gain versus time was obtained for the pre-carburized 304L SS (reserved and descaled), and the weight gain increased as the temperature increased. Metal dusting associated with alloying element redistribution and Fe/Ni-rich particle formation could occur on the pre-carburized 304L SS. The results showed that the Fe/Ni-rich metal particles disintegrated from the substrate could promote the formation of nano-sized carbon filaments in the high carbon-activity atmosphere. It indicated that the formation of carbon filaments resulting from the initial stage of metal dusting was occurred. Pre-carburization treatment on SS could accelerate the damage from metal-dusting attack. After long-term exposure, the pitting with localized damage was formed distinctly on the pre-carburized 304 SS surface due to the metal dusting attack. The total number, average maximum width, and maximum depth of the attacked localized pit increased with increasing temperature. Moreover, uniform attack due to metal dusting occurrence was seen if the oxide scale formed on the pre-carburized 304 SS surface was removed by grinding.
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