Influence of B and N on the microstructural characteristics and high-temperature strength of 9Cr-2W steel during an aging treatment

The evolution of the microstructure and strength of sodium-cooled fast reactor (SFR) fuel cladding steel with different B and N contents based on 9Cr-2W steel after aged at 650 °C is experimentally investigated in this study. The strength and hardness of a cladding tube fabricated containing relatively more B (0.013 wt% B, 0.02 wt% N) remained mostly stable with an increase in the aging time. This demonstrated the cause of the suppression of the growth of M23C6; the depletion of carbon in the matrix was prevented when B diffused to the C position in M23C6, thereby stabilizing the microstructures. A cladding tube fabricated containing more N (0.004 wt% B, 0.0767 wt% N) formed fine MX precipitates which were twice as fine as the cladding tube containing more B in the as-received state. Thus, the initial strength and hardness in the sample with more N are greater than when more B is used. Although the initial strength and hardness with more N are higher those with more B, the strength and hardness after 7000 h of aging were low due to the rapid growth of the M23C6 in the main precipitates.