오스테나이트계 스테인리스강의 입계부식에 미치는 탄소와 용체화 열처리의 영향

Abstract

Austenitic stainless steels with two types of carbon contents of 0.074 wt.% C and 0.064 wt.% C were tested to evaluate the effect of carbon content on intergranular corrosion of HAZ. Investigation of characteristics such as mechanical properties, nondestructive properties, structural observation, and chemical composition including delta ferrite content showed that all requirements were met, which were not directly related to carbon content. In the ASTM A262 corrosion test for detecting intergranular corrosion by the sensitization of the continuous precipitation of chromium carbide at the grain boundaries, HAZ of stainless steel with 0.064 wt.% C showed great resistance to intergranular corrosion, but the intergranular corrosion such as ditch structure and cracks in HAZ 2 ~ 3 mm apart from the fusion line between weldment and HAZ was observed in the stainless steel with 0.074 wt% C. As a result of temperature distribution analysis of the intergranular corrosion characteristics, HAZ 2 ~ 3 mm apart from the fusion line was analyzed to be exposed to the sensitized temperature range at 650 ~ 870 ℃ for about 3 ~ 6 sec. These results numerically support the ASTM A262 corrosion test results for the stainless steel with 0.074 wt.% C. This study demonstrates that the maximum carbon content of 0.065 wt.% C specified in the safety analysis report, which is a regulatory requirement for austenitic stainless steel welding, contributes sufficiently to corrosion resistance in the operating environment of nuclear power equipment, and in industrial field welding, the need to limit the carbon content of austenitic stainless steels to a maximum of 0.065 wt.% C has been identified. The holding time during solution heat treatment of unstabilized austenitic stainless steels as specified in the nuclear regulatory requirements was investigated using stainless steels containing 0.74 wt.% C. The sensitized 2.54 cm thick specimens at 675 ℃ for 1 h were rejected by ASTM A262 corrosion test due to the large amount of chromium carbide precipitated in the form of 50 ~ 300 nm particles at grain boundary and showed about 10.8 % of DOS in DL-EPR test. However, as result of the complete dissolution of chromium carbide into the grains after solution heat treatment of the sensitized specimens at 1,038 ℃ and 1,121 ℃ for at least 1 min, they were passed ASTM A262 corrosion test and showed less than 0.01 % of DOS in DL-EPR test. As a result of solution heat treatment at 1,038 ℃ for 5 h of the sensitized 25.4 cm thick specimen at 675℃ for 10 h, it was passed ASTM A262 corrosion test and DL-EPR test at any position in the specimen thickness. While the surface of this specimen showed step structure without precipitation of chromium carbide and DOS less than 0.01 %, towards the center, dual structure was observed and showed about 0.6 % of DOS due to the longer exposure time than the surface to the sensitization range of 427 ~ 816 ℃ during the cooling process. However, considering that the surface is directly affected by intergranular corrosion, it is judged that nuclear power components are not affected by the corrosive operating environment. Considering that at least 1 min when the chromium carbide precipitated at the grain boundary at 1,038 ℃ was completely dissolved into the grain, and analysis of the holding time when the center of 25.4 cm, the maximum use thickness of nuclear power industry reached the solution heat treatment temperature of 1,038 ℃ showed that the center reached 18.3 min later than the surface, the holding time for complete solution heat treatment to the center was found to be up to 2 min, per 2.54 cm of material thickness. According to these results, the solution heat treatment for 0.5 ~ 1.0 h per 2.54 cm of material thickness at 1,038 ~ 1,121 ℃, which is applied in the nuclear power industry, has been proved to prevent grain boundary corrosion by inhibiting the sensitization of austenitic stainless steels.