Creep rupture properties of oxidised 20%Cr austenitic steels

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 x 106 s (1900 h) at 8500C, 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 7500C, 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. (author)