Effect of high carbon activity sodium upon the mechanical properties of AISI 316 stainless steel

Many mechanical properties tests have been performed upon specimens immersed in or pre-exposed to liquid sodium, but in general the sodium involved has been relatively poorly characterised. Experiments are described which apply recent advances in the determination of carbon activity in sodium to the mechanical properties field. The influence of pre-exposure to liquid sodium of controlled, known, and high carbon activity upon the tensile, creep, and fatigue behaviour of AISI 316 stainless steel in subsequent air tests is described. Thin section specimens and highly carburizing conditions were chosen to examine extreme effects. In order to determine general trends in behaviour, tensile and fatigue properties were measured at room temperature and creep rates at 6000C. All specimens had a large ratio of carburized case/uncarburized core, which was found to cause a dramatic decrease in tensile ductility and a small reduction in ultimate tensile strength. Unusual behaviour was observed in creep tests at 6000C. Creep rates were reduced relative to controls and periods of negative creep associated with carbide redistribution were observed. Ambient temperature fatigue tests demonstrated that carburization enhanced endurances at low stresses and reduced endurances at high stresses. Further work is required to permit extrapolation of the results to thicker section components, and to less severe conditions of exposure and mechanical loading. (author)