With increasing boron content, the coarsening rate of M23C6 carbidesbecomes reduced near grain boundaries in tempered martensitic 9Cr-3W-3Co-VNb steel during creep at 650 °C. This retards the recovery ofmartensitic microstructure up to long times and increases long-termcreep strength. However, excess addition of boron and nitrogen causesthe formation of large amount of massive boron-nitrides during normalizingat 1050-1150 °C, which significantly decreases the boronconcentration available for microstructure stabilization and degradesthe creep strength.