Precipitation and phase transformation behavior during high-temperature aging of a cobalt modified Fe-24Cr-(22-x)Ni-7Mo-xCo superaustenitic stainless steel

In this paper, the chemical composition of 7Mo-alloyed super austenitic stainless steel was optimized and the principle of composition designing of Fe-24Cr-(22-x)Ni-7Mo-xCo (x = 0, 2, 4) alloys was discussed. Thermodynamic calculation, as well as microstructure observation, was carried out to investigate the precipitation behavior of Fe-24Cr-(22-x)Ni-7Mo-xCo alloys aged at 700-1100 degrees C. The results demonstrated that none of second phase was detected at 700 degrees C aging and at least three different types of precipitates, sigma phase, chi phase and M23C6 were identified after aging at 800-1100 degrees C for 2 h. At all aging temperature, the intermetallic sigma was determined as the dominant second phase. For alloys of 22Ni, 20Ni2Co and 18Ni4Co, the area fraction of precipitates increased first and then decreased with temperature increasing from 700 to 1100 degrees C. The 1000 degrees C was determined as the most sensitive temperature for this alloy. Furthermore, the area fractions of second phase of 20Ni2Co (x = 2) at all temperatures were lower than that of 22Ni (x = 0) and 18Ni4Co (x = 4). In addition, the characteristics of cellular precipitates growing have been disclosed in this paper. Compared with nickel-based alloy, the addition of cobalt did not affect the misfit between sigma phase and austenite matrix, which is probably due to the high content of nitrogen in Fe-24Cr-(22-x)Ni-7Mo-xCo alloys.

Automated summary

This paper optimized the chemical composition of 7Mo-alloyed super austenitic stainless steel and discussed the composition designing principle of Fe-24Cr-(22-x)Ni-7Mo-xCo (x = 0, 2, 4) alloys. The precipitation behavior of these alloys aged at different temperatures was investigated through thermodynamic calculation and microstructure observation. The results showed the presence of sigma phase, chi phase, and M23C6 precipitates in the aged alloys, with sigma phase being the dominant second phase. The area fraction of precipitates increased and then decreased with increasing temperature, with 1000 degrees C identified as the most sensitive temperature. Furthermore, the addition of cobalt did not affect the misfit between sigma phase and austenite matrix.