| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 100 === Powder Injection Molded (PIM) 316L stainless steels are widely applied in the industry for making small components with complicated shapes, but its austenitic structure and poor strength limit its applications. The methods of grain refinement, solid-solution strengthing by nitrogen, carburizing treatments, and alloying were used in this study to improve the mechanical properties of 316L.
In order to attain high sintered density, 316L is generally sintered at high temperatures, which causes significant grain growth. To circumvent this drawback, fine (D50 = 4.1 μm) 316L powder was sintered at a low temperature of 1120℃ for 2 hrs in dissociated ammonia. A high sintered density of 7.70 g/cm3 was obtained, the nitrogen content reached 0.3 wt%, and the hardness and tensile strength achieved were 95 HRB and 740 MPa, respectively. Sintering regular (D50 = 12.0 μm) 316L powder at 1370℃ for 2 hrs in vacuum can reach only 7.50 g/cm3 density, 52 HRB hardness, and 450 MPa tensile strength. With hot isostatic pressing in nitrogen, the nitrogen content in the fine 316L specimen that was sintered in dissociated ammonia for 2 hours can be further increased from 0.3 wt% to 0.8 wt% and the hardness and tensile strength of fine 316L specimen were improved to 110 HRB and 820 MPa, respectively. After being low-temperature carburized (LTC) at 500℃ for 24 hrs, the surface hardness of 316L reached 810 HV. The corrosion resistance remained the same because no Cr23C6 formed during carburization. The hardness and tensile strength of powder metallurgy 316L and 304L can also be increased by LTC obviously, since carbon can diffuse into the center of the specimen through interconnected pores.
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