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 °C. The results demonstrated that none of second phase was detected at 700 °C aging and at least three different types of precipitates, σ phase, χ phase and M23C6 were identified after aging at 800–1100 °C for 2 h. At all aging temperature, the intermetallic σ 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 °C. The 1000 °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 σ phase and austenite matrix, which is probably due to the high content of nitrogen in Fe-24Cr-(22-x)Ni-7Mo-xCo alloys. [ABSTRACT FROM AUTHOR]