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

抄録

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 2000h, and decreases gradually with the increase of N content. The Laves phase precipitated after aging for 1000h and coarsened continuously with the aging time prolonging. The coarsening rate tended to be flat after 2000h. 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 μ m 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%.