Grain Boundary Self-Diffusion of Alloy 800 as Affected by Sulphur, Phosphorous and Carbon

Using a radioactive tracer method the bulk and grain boundary diffusion of exp 59 Fe was determined in industrial Alloy 800 and melts of Alloy 800 with additional P and S in the temperature range 800 to 1000 deg C. The use of the approximation of Suzuoka was confirmed by autoradiographs. In Alloy 800H the activation energy of grain boundary diffusion of exp 59 Fe is (209plus /minus17) kJ/mol. Dissolved elements especially P increase the activation energy of the grain boundary diffusion of Fe by their segregation to the grain boundaries. In addition the influence of the grain boundary diffusion on the growth of creep cavities was investigated in the same materials, and the chemical composition of the creep cavities and grain boundaries were analysed by Auger electron spectroscopy (AES). For Alloy 800+0.088 wt-% P an enrichment of about 14 at-% P was observed at the grain boundaries. The addition of P clearly enhances the creep strength of Alloy 800; this can probably be explained by precipitation of Cr sub 23 C sub 6 on the grain boundaries during the creep process.