9~12Cr 耐熱鋼의 機械的 性質 및 耐酸化性에 미치는 製造工程의 影響

Abstract

Many interests on environment problems and energy saving induced exhaustion of resources have been paid attention. These problems are especially related to industry of power plant. Accordingly, constant efforts and conversion of power generation system utilizing atomic power have been tried to overcome those in this field. Besides, materials utilized at very high temperature, pressure and capacity for improvement of generation efficiency should be investigated with precise conditions of use. Therefore, in the present study, the effects of manufacturing process on micrographs and mechanical properties in 9Cr-1Mo and 12Cr heat resisting steel centrifugal-casted, forged and ring milled after centrifugal casting were investigated. Moreover, the effects of forging rate on each properties in specimen centrifugal-casted were examined. In order to evaluate the optimum conditions of tempering treatment, various conditions were considered. Finally, the effects of aluminizing and diffusion treatment on the formation of alloy layer in 12Cr heat resisting steel forged after centrifugal casting were examined and then oxidation properties were investigated. The conclusions are as follows: Every specimen centrifugal-casted, forged and ring-milled after the casting identically have martensite phase. Furthermore, in case of specimens under different forging conditions, the phases were all martensite. When two specimens are forged and ring-milled after centrifugal casting, martensite phase showed a tendency to fine off. While tensile strength and hardness in two specimens centrifugal-casted were higher than those in other, there was no difference in elongation and impact value properties. However, strength and elongation in specimens forged and ring-milled after centrifugal casting had hardly big difference. With increasing the forging starting temperature, the martensite phase was fined. In contrast of slight increase of tensile strength and hardness, elongation and impact value were decreased. With increasing the forging rate, the martensite phase was fined with formation at random. Additionally, there existed defects such as dislocation induced in martensite phase. Tensile strength and hardness sharply increased with increasing the forging rate until it was 30%. While those in condition of over 30% increased slowly, elongation and impact value rapidly decreased. Both specimens indicated to martensite phase regardless of tempering temperature and time. However, with increasing tempering temperature and time, amount of precipitated carbide increased. With identical mechanism in two specimens, in contrast of slight decrease in tensile strength and hardness and slight increase in elongation while impact value sharply increased. While tensile strength and hardness steadily decreased with increasing the tempering time, elongation and impact value sharply increased at the beginning stage and then slowly increased after 300 minutes of tempering. Without regard to tempering temperature and time, Cr23C6 and Mo2C carbide were precipitated in 9Cr-1Mo heat resisting steel while precipitation in 12Cr heat resisting steel was Cr23C6. Damping capacity was sharply increased with increasing the tempering temperature and slowly increased over 640℃. Besides, it rapidly increased at the beginning stage with increase of tempering time and showed a slight increase after 300 minutes of tempering. With increase of aluminizing treatment temperature and time, thickness of alloy layer increased. When diffusion treatment was carried out after aluminizing treatment, thickness of alloy layer was increased. Moreover, it was increased as well with increasing the diffusion temperature and time. When either aluminizing treatment or diffusion treatment after aluminizing treatment is carried out, alloy layer with over Hv 880 which was consisted of Al13Fe4 chemical compound was obtained. As substituted Al for Cr in accordance of aluminizing and diffusion treatment, Al concentration increased in alloy layer while Cr concentration decreased. Al concentration in alloy layer of aluminized specimen increased as location goes inner part. In contrast, Al concentration decreased in alloy layer of aluminized and diffusion-treated specimen. A degree to increase the thickness of alloy layer induced aluminizing treatment time and diffusion treatment time after aluminizing increased with increasing the temperature of aluminizing and diffusion treatment. When diffusion treatment after aluminizing treatment is carried out, according to the boundary between alloy layer and matrix cracks layer were formed in alloy. Besides, more cracks were formed with increasing the diffusion temperature and time. When alloy layer is formed due to either aluminizing treatment or diffusion treatment after aluminizing treatment, oxidation resistance showed the increase of over 35% in air atmosphere and over 30% in water vapor compared with untreated specimen. However there was no difference in oxidation resistance of both specimens.