Abstract
Austenitic stainless steels with superior corrosion resistance such as R-SUS304ULC (Ultra Low Carbon) have been used in corrosive environments of Rokkasho Reprocessing Plant. However, austenitic stainless steels suffer an intergranular corrosion in boiling nitric acid solutions. The intergranular corrosion is caused by the segregation of impurities to the grain boundaries. R-SUS310ULC EHP^[○!R] (Extra High Purity), hereafter referred to as 310EHP, has been developed in order to suppress the intergranular corrosion by reducing contents of the harmful impurities, such as carbon, phosphorus, sulfur and boron. Slight boron in the austenitic stainless steels was regarded as the causes for the intergranular corrosion in particular. The intergranular corrosion behavior of 310EHP, which was melted and cast with a small scale melter, was examined in boiling HNO_3 solution with highly oxidizing ions to clarify the correlation between the intergranular corrosion behavior and boron content. As a result, it was confirmed that 310EHP with lower boron content, less than 50 ppm, has the superior intergranular corrosion resistance compared with that of R-SUS304ULC. And then some over 1000 kg ingots of 310EHP were melted and cast with a large scale melter, and it was confirmed that the chemical composition was controlled within target ranges and the corrosion resistance were the same as that with a small scale melter. And the mechanical properties of 310EHP were obtained and these data were added to HPIS (High Pressure Institute Standard) C108:2011 AMD1:2014 in Japan.
