Alloy 709, a type of 20Cr-25Ni austenitic stainless steel is studied after thermal aging at 550°C and 650°C. The microstructure is characterized using variety of techniques. The alloy shows excellent thermal stability at its working temperature 550°C, while its microstructure evolves rapidly at 650°C with complex precipitation behaviors. NbX particles are of large amount in as-received material but partially dissolve during thermal aging, showing less stability compared to M23C6 at 650°C. A type of M6C’s variant is found after long-term aging and strongly correlated with M23C6, which could either be the result of a possible transition from M23C6 to M6C, or the dissolution of Nb(C, N). Small amount of Z phase are found to precipitate densely on dislocations and modify the dislocation structure after 3000 hours aging at 650◦C. The results are in good agreement with those of NF709 and further support the idea that the η structure can be stabilized by nitrogen.
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