Effect of B on microstructural change during creep deformation in high Cr ferritic heat resistant steel

In order to investigate the effect of the boron on microstructural change during the creep deformation of the high Cr ferritic heat resistant steel, the change of precipitate. microstructure and hardness during creep deformation was evaluated. Creep strength increases by suppressing creep deformation with increasing the boron content. The type of the precipitate doesn't change by the boron addition. M23C6 and MX precipitate at quality heat treatment, and then in addition, the Laves phase also precipitates during creep deformation, It is confirmed that the coarsening of M23C6 and Laves phase is suppressed and the refinement of MX is promoted by the addition of boron. In the boron free steel, the recovery of lath martensite structure such as the increase of the martensite lath width and the change into subgrain structure. progresses during creep deformation. and then homogeneous subgrain structure is observed in crept specimen. On the other hand, in 140 ppm boron containing steel, the lath martensite structure is kept within the grain, though subgrain is observed along prior austenite grain boundary in crept specimen. In accelerated creep region, the reducing of hardness of 140 ppm boron containing steel is smaller than that of boron free steel by the heterogeneous recovery at the gage portion. From the evaluation of the creep interrupted test material. it was confirmed that the addition of boron contributed to the creep strengthening by stabilizing the precipitate and suppressing the recovery of the lath martensite structure in the high temperature and long term condition.