EFFECTS OF CHEMICAL-COMPOSITIONS AND HEAT-TREATMENTS ON CREEP-RUPTURE STRENGTH OF 12WT-PERCENT-CR HEAT-RESISTANT STEELS FOR BOILER

Creep rupture strength of 12 wt%Cr steel for boiler drastically change 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. 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. 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. Long-term creep rupture strength at 600 and 650°C significantly droped 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.