Precipitates evolution during tempering of 9CrMoCoB (CB2) ferritic heat-resistant steel

In this paper, the evolution of precipitate phases in 9CrMoCoB (CB2) steel after tempering at 650 degrees C for different times has been investigated using transmission electron microscopy. As the tempering time prolonged from 0 to 100 h, four types of secondary phases consisting of M3C, M2X, M7C3 and M23C6 were identified in the experimental steel. It was found that the needle-like M3C carbides could be precipitated during both the air cooling process after austenization and the early stage of tempering. After tempering for 15 to 120 min, fine M2X particles were observed. The precipitation and growth of M2X were derived from the partially consumption of M3C carbides. After tempering for 50 h, M2X disappeared, while the rod-like M7C3 carbides were observed to be precipitated independent of the early formed M3C precipitates. After tempering for 100 h, only M23C6 carbides persisted, of which the number density and size kept increasing throughout 30 min to 100 h owing to their high thermostability. In addition, the precipitation mechanisms of the four types of precipitates were explained, and the relationship between the measured hardness values and microstructure was discussed.