Numerical simulation of long-term precipitate evolution in austenitic heat-resistant steels

Numerical simulation of the long-term precipitate evolution in five different austenitic heat-resistant steels-NF709, Super304H, Sanicro25, CF8C-PLUS and HTUPS-has been carried out. MX and M23C6 are predicted to remain as major precipitates during long-term aging in these steels. While the average size of MX is maintained below several ten nanometers during the aging, that of M23C6 exceeds 100 nm after 100,000 h of aging at 700 ∘C. The addition of 3 wt% Cu produces very fine Cu-rich precipitates during aging in Super304H and Sanicro25. It is found that the amount of Z phase starts to increase remarkably between 1000 and 10,000 h of aging at the expense of MX precipitates in the steels containing a high nitrogen content. However, the growth rate of Z phase is relatively slow and its average size reaches at most a few ten nanometers after 100,000 h of aging at 700 ∘C, compared with 9%-12% Cr ferritic heat-resistant steels. The simulated precipitation sequence and precipitate size during aging are in general agreement with experimental observations.