| Summary: | 碩士 === 國立臺北科技大學 === 材料科學與工程研究所 === 106 === Liquid phase sintering is an effective method to improve the densification of powder metallurgy products and increase the sintering rate. Boron is the most commonly added element in iron-based materials to promote liquid phase sintering. This study will explore 304L stainless steel (Fe-18.45Cr-11.04Ni) powder with different sizes of pure boron powder and iron boride powder. The boron content was controlled at 0.6 wt%. The effects of sintering temperature (1250°C, 1275°C, 1300°C) on the microstructure and sintering density and the influences of heat treatment on the mechanical properties and corrosion characteristics were studied.
The results show that the 3.2 μm pure boron size in 304L can attain the highest sintered density and promote liquid phase sintering among the different sizes of pure boron powder (1.7 μm, 2.9 μm, and 3.2 μm). The results of thermal analysis indicate the liquid phase can be found in a temperature range between 1245°C and 1277°C when pure boron powder is added. While iron boride powder is used, the liquid phase is formed in a temperature range between 1239°C and 1268°C. Moreover, a large amount of eutectic boride at grain boundaries can be observed according to the microstructural observation. After 1300°C sintering, adding B and Fe2B powders can sufficiently increase the sintered density of 304L from 6.91 g/cm3 to 7.65 g/cm3 and 7.48 g/cm3, respectively. The porosity is decreased from 14 vol% to 2 vol% and 3 vol%, respectively.
The 304L+B steel exhibits better performances than 304L+Fe2B by comparing the sintering density and porosity. Therefore, the tensile properties of 304L+B steel was carried out. The results show that the addition of boron in 304L increases the tensile strength from 264 MPa to 451 MPa, and the elongation can be as high as 13%.After heat treatment of 304L+B at 600°C, the tensile strength can be obviously increased to 472 MPa, and the impact energy can be increased from 43 J to 47 J. These findings show the positive influence of heat treatment on the mechanical properties of 304L+B steel. However, when the heat treatment temperature is too high, more eutectic boride at grain boundary could dissolve into the matrix and impair the mechanical properties. In conclusion, pure boron powder can assist in the liquid phase sintering of 304L stainless steel, and increases the sintering density and the mechanical properties. After heat treatment at 600°C, the mechanical properties of 304L+B steel can be slightly increased.
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