Evolution and its stability of M23(C,N)6 carbonitride in martensite ferritic steel during long-term thermal aging

Highlights: • Results show that nitrogen goes to M₂₃C₆ and forms M₂₃(C,N)₆ after thermal aged at 650 °C for 27,000 h. • Nitrogen could reduce the coarsening rate of M₂₃C₆ once it is soluble in M₂₃C₆. • Hardness remains stable from 27,000 h to 36,700 h attributed to high stability of M₂₃(C,N)₆. • Having discussed the mechanism for high stability of M₂₃(C,N)₆·carbonitrides. -- Abstract: Evolution of M₂₃C₆ carbide during isothermal aging of the 10% Cr martensite ferritic steel at 650 °C has been investigated by mainly using SEM-BSE and TEM-EELS. It was found that the M₂₃(C,N)₆ carbonitride formed after aging for 27,000 h since nitrogen atoms replacing part of carbon atoms dissolved into the M₂₃C₆ carbide. Such M₂₃(C,N)₆ carbonitride exhibited high stability as compared with previous M₂₃C₆ carbide. The size of M₂₃(C,N)₆ hardly changed even if the duration of aging exceeded 36,700 h, keeping at the level of ~200 nm. This suggests that the nitrogen could suppress the coarsening of M₂₃C₆ carbide once it dissolved into the M₂₃C₆ carbides. Moreover, the hardness of the10% Cr steel remained stable during aging from 27,000 h to 36,700 h, attributing to the high stability of the M₂₃(C,N)₆.