Effect of N Content on Precipitated Phase and Long-term Aging Stability of 9CrMoCoB Steel

Abstract

Microstructure observation and hardness analysis of 9CrMoCoB steel with different N content during long-term aging at 620°C were carried out by Thermo-Calc thermodynamic calculation, scanning electron microscope, transmission electron microscope and hardness test. The results show that the average size of M₂₃C₆ phase increases first with the extension of aging time, then stabilizes after aging for 2000 h, and decreases gradually with the increase of N content. The Laves phase precipitated after aging for 1000 h and coarsened continuously with the aging time prolonging. The coarsening rate tended to be flat after 2000 h. The average size of Laves phase decreased gradually with the increase of N content. With the increase of N content, the average diameter and unit number of BN inclusions increase gradually, and the unit number of BN inclusions larger than 2 µm increases obviously when N content is 0.025% and 0.030%. The hardness of 9CrMoCoB steel decreases gradually with the aging time and tends to be stable after aging for 2000 h. With the increase of N content, the hardness and stability of 9CrMoCoB steel first increase and then decrease, and both reach the maximum when N content is 0.020%. Considering comprehensively, in order to give full play to the strengthening effect of B and N in steel and improve the properties and stability of 9CrMoCoB steel, the optimal control range of N content in steel is around 0.020%.