| Summary: | 碩士 === 國立臺北科技大學 === 材料科學與工程研究所 === 104 === Powder metallurgy (PM) steels have been extensively used in the parts for the mechanical, automobile, and optoelectronic industries. However, raw powders of PM steels highly rely on foreign suppliers, which lead to a disadvantage in the overall development. Therefore, the objective of this study was to develop a hydrogen-reduced spray-dried iron powder for PM steels. To fabricate the spray-dried granules with solid morphology and high flowability, a sub-micron Fe2O3 powder was used as the raw powder to formulate the ceramic slurry for spray drying. The reducing parameters must be precisely controlled to optimize the combinations of reduction completeness and powder compressibility.
The results showed that increasing the hydrogen reduction temperature of spray-dried Fe2O3 granule from 600 ˚C to 930 ˚C decrease the sintered density of Fe from 7.42 g/cm3 to 6.86 g/cm3, due to the particle coarsening in the spray-dried granules and the decrement in the surface area. In the second part, the spray dried Fe2O3-NiO-Mo-C granules were produced and were hydrogen-reduced. After 600 ˚C reducing, the residual oxygen content was as high as 1.18 wt%. The oxygen content could be decreased with increasing the reducing temperature. After reducing at 650˚C and 700˚C, the oxygen content was 0.58 wt% and 0.26 wt%, respectively.
Based on the previous findings, the ultrahigh strength Fe-6Ni-0.8Cr-0.8Mo-0.4C steel fabricated by metal injection molding (MIM), reported in the literature, have been fabricated in this study. The result indicated that the sintered density of Fe-6Ni-0.8Cr-0.8Mo-0.4C steel sintered at 1250˚C in vacuum is 7.49 g/cm3. This high sintered density is superior to those of most PM steels and is close to those of MIM steels. The hardness, tensile strength, and elongation of the Fe-6Ni-0.8Cr-0.8Mo-0.4C steel in this study are 42 HRC, 1628MPa, and 2 %, respectively.
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