| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 107 === In this study, we use two kinds of fillers, such as T23 and 309L, to weld T23/T91 and 304H/T91 by TIG (Tungsten Inert Gas) separately. The quality of the weldment is examined by using NDT (Nondestructive Testing). Heat treatments are performed after specimens selected from wire cutting of the qualified weldments. In this study, during the aging test of dissimilar weldments under high temperature and long time, carbon migration will occur at the fusion line. Because there is the most obvious difference of chromium concentrations at the fusion line, it causes different activities of carbon atoms diffuse from low-alloy to high-alloy. To balance the solubility of carbon, the carbides dissolve into the base at low-alloy. This is the mechanism of formation of decarburization zone. Besides, decarburization zone can be confirmed by optical microscope and the variation of micro-hardness values.
In the T23/T23/T91 part, after PWHT and simulated aging test for 100,000 hours, the continuous and coarse decarburization zone forms at the interface of T23 weldment and T91 HAZ. The micro-hardness of decarburization zone decreases significantly. In 304H/309L/T91 part, after PWHT and simulated aging test for 200,000 hours, the continuous decarburization zone forms at the interface of 309L weldment and T91 HAZ. The micro-hardness of decarburization zone decreases. The change in the values of micro-hardness is not obvious compared with the former. With the longer time period in the aging test, the size of decarburization zone becomes larger. The formation of decarburization zone deteriorates the mechanical properties and reliability of the T91 weldment applied in ultra-supercritical boiler.
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