Abstract
The UK’s advanced gas-cooled (AGR) nuclear reactors have operated over the last 30 years, with the austenitic stainless steel sections primarily operating at temperatures ranging from 470 0C up to 650 0C. The coolant gases used in this type of nuclear system contain a mixture of carbon dioxide, carbon monoxide, hydrogen, methane and water vapor. A number of cracks have been reported in superheater boiler components made from 4 mm thick austenitic stainless steel tubes. The mechanism underlying the initiation of cracks is believed to be creep-fatigue which may be exacerbated by carburization of the metal surface, associated with the presence of a duplex oxide layer. In this paper, complementary microstructural characterization techniques have been used to investigate the oxidation behavior of Type UNS S31609 stainless steel in the simulated AGR environments. A primary focus was given to the effects of surface finish and the water vapor content on oxidation. The experimental results show that surface deformation promotes the formation of a thin oxide layer, whereas a deformation-free surface leads to formation of thick duplex oxide layers. Furthermore, the presence of water vapor in the mixed gas environment accelerated the growth of the oxides.
