The effect of boron on creep rupture properties of steels has been investigated in order to develop new 12% Cr heat resisting steel with improved strength at elevated temperatures.
1) The creep rupture strength of a 12% Cr-Mo-V-Nb steel is improved by boron addition. Especially, the creep rupture life of the steel containing 0.037% boron is five to six times as long as that of the steel free from boron in long term creep rupture tests at 650°C and above.
2) Boron forms boro-carbide M₂₃ (C, B) ₆, substituting some amounts of carbon in carbide, type M₂₃C₆. It is observed under electron microscope that M₂₃ (C, B) ₆ is finer than M₂₃C₆ at elevated temperature. This is one of the reason for that the 12% Cr-Mo-V-Nb-B steel has superior creep rupture strength to the 12% Cr-Mo-V-Nb steel.