MICROSTRUCTURAL AND MECHANICAL PROPERTY CHANGES IN 316-STAINLESS-STEEL EXPOSED TO HIGH-TEMPERATURE FLOWING SODIUM

Austenitic stainless steel of type AISI 316 was exposed to flowing sod ium in a loop at 823 K for 16000 hours. These specimens were examined for microstructural and mechanical property changes. A specimen, subje cted to thermal-ageing, was also analysed to determine the deleterious effects of sodium-exposure from mere thermal effects. Carbide precipi tation and a modified surface layer containing ferrite was noticed on the specimen exposed to sodium. Thermal ageing caused only the precipi tation of carbide at the grain boundaries. Analysis by Energy Dispersi ve X-ray method (EDAX) indicated leaching of elements from the sodium- exposed surface. The concentration profile of carbon was determined us ing SIMS from which the effective diffusion coefficient of carbon in s tainless steel exposed to sodium was calculated. The carburization pro file of stainless steel for different durations of exposure to sodium (up to 40 years) was predicted. Tensile tests on sodium-exposed and th ermally-aged material revealed an increase in yield strength (YS) and ultimate tensile strength (UTS) and reduction in ductility. This behav iour was attributed to carbide precipitation and microstructural chang es. A reduction in impact energy was indicated by both types of specim ens which was the consequence of weakening of the grain boundaries by carbide precipitation. Microhardness profile indicated a hardened surf ace layer on the sodium-exposed specimen.