Oxidation and carburization behaviour of two type 316H stainless steel casts in simulated AGR gas environment at 550 and 600 °C

The time-dependent oxidation and carburization behaviour of two type 316H austenitic stainless steels with varying Mn content and differing average austenite grain size of 142 mu m in 'Low Mn' (0.98 wt.% Mn) and 81 mu m in 'High Mn' (1.52 wt.% Mn), were assessed at 550 degrees C and 600 degrees C in the presence of simulated reactor primary gas coolant containing 500 vppm H2O, 100 vppm H-2, 300 vppm CH4 and 1 vol.% CO, balanced with CO 2. Rupture and spallation of the initial protective chromia layer occurs in the 'High Mn' steel after 2000 h at 550 degrees C, and leads to the formation of a magnetite/spinel oxide layer that reaches 75% surface coverage only after 800 0 h. In contrast, 'Low Mn' steel reaches similar to 85% coverage after only 100 0 h at the same temperature. The development of an inner carburized layer occurs gradually in both steels once the duplex oxide layer is forming. The differences in steel behaviour are reduced at 600 degrees C, where both oxidation and carburization are significantly accelerated. Only 0.02 wt.% carbon remains in solid solution in the austenite lattice in the carburized layer, the excess carbon atoms precipitating out in the form of Cr-rich M23C6 particles, present both inter- and intragranularly. The experimental values of the activation energy for carburization suggest that diffusion of substitutional solutes as the rate-limiting mechanism of the process. Previously observed carburization depths of >= 200 mu m in ex-service austenitic stainless steels, may be achieved by pre-conditioning 316H steel in simulated reactor gas coolant for >= 60 00 h at 600 degrees C. (C) 2021 Elsevier B.V. All rights reserved.