| Summary: | 碩士 === 國立臺北科技大學 === 材料科學與工程研究所 === 104 === Boron is an effective alloying element for promoting liquid phase sintering of iron-based powder metallurgy (PM) steels. Boron can react with iron to form eutectic liquid, which provide good wettability and help the particle arrangement. Numerous studies have indicated that the alloy system and element can obviously affect liquid phase sintering, and the sintering behavior and microstructure in various PM steels could be different. This research was to study the densification of PM stainless steels including 304L and 410L by liquid phase sintering. The effects of boron content (0.0 wt%, 0.3 wt%, and 0.6 wt%) and sintering atmospheres (Ar, H2, and Ar-H2) on the liquid phase sintering and microstructure were investigated.
The results showed that carbon plays a key role in determining the martix of 410L+B steel. When the 410L+B steel were sintered in argon, carbon atom could tend to diffuse to grain boundary, and martensite was formed near grain boundary after sintering. However, sintering in hydrogen, no preferred segregation of carbon atom could be found. In contrast, the sintering atmosphere does not affect the matrix of 304L+B steel. The results of thermal analysis showed that both chromium and nickel can much raise the temperature for eutectic liquid formation. The ranges for liquid generations in 410L+B and 304L+B systems were 1236 oC~1248 oC and 1244 oC~1277 oC, respectively. The borides in the two stainless steels are rich in boron, iron, and chromium, as demonstrated by elemental mappings.
The highest sintered density of 410L+0.6B steel is 7.48 g/cm3 when the steel is sintered at 1250oC in hydrogen. The further increase in the sintering temperature to 1300oC does not apparently increase the sintered density. However, in the 304L+0.6wt%B steel, superior densification can be only achieved after sintering at 1300oC, and the highest sintered density is 7.75g/cm3. On the other hand, the 0.6 wt% B additive could improve the corrosion resistances of 304L and 410L steels.
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