C-O relations of the extremely low carbon austenitic stainless steels and nickel base high alloys in vacuum induction melting

It is well known that in vacuum-melted austenitic stainless steel and nickel base alloy, the impurities of minute amounts affect adversely the corrosion resistance and high temperature strength. Therefore the materials of high quality, such as those in extremely low carbon range below 0.01%, are required in nuclear and chemical plants. In this study, austenitic stainless steel such as SUS 308, 309 and 316 and nickel base alloy such as Ni-20 Cr-2.6 Nb and Ni-20 Mo-3W were melted in a 200 kg vacuum induction furnace, and the behaviors of C and O during the refining were investigated, also the thermodynamical analysis was performed. For comparison, pure iron was studied at the same time. The amounts of C and O were reduced from the beginning of melting through intensive boiling period, and when quiescent period was reached, the equilibrium relation of C and O was able to be applied also to the case of austenitic stainless steel. In case of the nickel base alloy, it was presumed that the relation of C and O in quiescent period of molten alloy was near the equilibrium state. The partial pressure of CO in the stainless steel was low as compared with the pure iron, because the effect of refractory material to the oxygen potential of molten steel is different according to the steel composition. (auth.)