CREEP-RUPTURE PROPERTIES OF OXIDIZED 20-PERCENT CR AUSTENITIC STAINLESS-STEELS

Under operational conditions in advanced gas cooled reactors (AGRs), fuel cladding alloys are exposed to an oxidising environment for prolonged periods approaching 850°C. It is essential, from safety considerations, that the mechanical integrity of this thin section material is not threatened by any potential oxidation effects. This aspect is addressed in the present work. Sheet specimens of 20Cr–25Ni niobium stabilised steel and nitrided 20Cr–25Ni–Ti stainless steel, both used as fuel cladding materials in AGRs, have been oxidised in simulated reactor gas [CO2– (1–2% CO)] for periods up to 7•0×106 s (1900 h) at 850°C, including intermediate thermal cycles to room temperature. These exposures have been carried out in thermobalances which provided a full gravimetric analysis for each specimen. The oxidised specimens have been creep tested subsequently at 750°C, under conditions of constant stress, together with non-oxidised and thermally aged specimens, which formed the ‘baseline’ mechanical property data. After testing, the specimens have been examined using optical metallography and transmission electron microscopy to determine the microstructural changes caused by thermal aging and by oxidation. The paper compares and contrasts the response in mechanical behaviour to oxidation effects for both of the fuel cladding alloys. The creep rupture properties are affected differently for the two materials, but it is shown that this can be interpreted in terms of the observed changes in microstructure and the nature of the oxidation attack. © 1990 Maney Publishing.