Creep rupture strength of 12 wt%Cr steel for boiler drastically changes due to chemical compositions and heat treatments. The relationship between microstructure, affected by alloying elements V, Nb and N and tempering conditions, and creep rupture strength was investigated by means of transmission electron microscopy. The results are summarized as follows.
(1) Finely dispersed precipitate of vanadium nitride (VN) increased linearly according with nitrogen addition for 0.25 wt%V steels. The VN which did not change in size during creep, strongly improved long-term creep rupture strength.
(2) The low niobium steels with 0.05 and 0.1 wt%Nb had high and stable creep rupture strength due to precipitation strengthening of VN and Nb (C, N) effectively. However, in case of higher niobium content, the amount of insoluble Nb (C, N) significantly increased and coarse Nb (C, N) coexisted with VN, which resulted in the deterioration of creep rupture strength.
(3) Long-term creep rupture strength at 600 and 650°C significantly dropped for the steel tempered at 750°C because of softening and recovery due to high dislocation density in tempered martensite. In case of 800°C tempering, the long-term creep rupture strength was improved.