The effects of impurities on creep and creep cavitation characteristics in Type 304 stainless steels with and without addition of sulfur and/or phosphorus have been studied using four experimental heats. Test condition was mainly 1000 K $(~0.6T_m)$ and 75~150 MPa (applied stress normalized with elastic modulus, $σ/E = 5×10^{-4}~1×10^{-3}$). High temperature intergranular embrittlement pheneomena have been analyzed through the investigation of the change in creep characteristics, in size distribution of creep cavities and carbide precipitates, and in the level of impurity segregation with the amount of addition. Possible significance of the impurities was discussed briefly, in the stages of the nucleation and growth of creep cavities, respectively.
It was found that phosphorus ratards the steady state creep rate, decreases the rupture elongation, and increases the fraction of intergranular fracture. This is attributable to the fine carbides formed in matrix due to phosphorus addition. However, sulfur had little effect on creep and creep fracture behavior.
The AES results of crept specimen provide direct evidence for impurity segregation effects on creep cavitation. Phosphorus and sulfur segregation to grain boundaries and cavity surfaces has been observed in the crept samples. Segregation of phosphorus was more extensive to grain boundaries than to cavity surfaces, whereas, sulfur was vise versa. Phosphorus segregation in P-added steels was more intensive than in P-free steels.
Impurity effects on cavity nucleation could be assessed through the investigation of the variation in the cavity size distributions due to impurity. It has been found that phosphorus accelerates the nucleation of the creep cavities, but retards the growth of them. However, sulfur had little effect on the nucleation and the growth of creep cavities as in creep deformation characteristics.
It was also found that aging prior to testing improves the creep and creep fracture properties, and inhibits the nucleation and the growth of creep cavities.
From the experimental results, it may be concluded that high temperature intergranular embrittlement phenomena are strongly influenced by the species and the level of segregated impurities and heat treatment condition.
