In order to understand the cause of heat-to-heat variation in creep strength of Grade T91 steels, influence of initial microstructure on creep strength has been investigated. Distribution of chromium concentration considered to be a remained segregation was observed, corresponding to a lamellar contrast parallel to longitudinal direction of boiler tube. Standard deviation (SD) of ΔCr was employed as an indicator of degree of segregation, and good correlation was recognized between SD of ΔCr and creep rupture life at 650°C. Remained segregation was reduced by renormalizing heat treatment at not 1250°C, but 1200°C. Creep rupture life of the steel subjected to renormalizing heat treatment at 1200°C and tempering at 760°C, followed by normalizing and tempering with a standard heat treatment condition for Grade T91 steel, was prolonged for 2.3 to 2.8 times. Strengthening effect of renormalizing at 1200°C to reduce a remained segregation was confirmed by creep tests for up to about 10,000h at 600 and 650°C. Decreases in number density of M23C6 carbide particles, length of high angle boundaries and average KAM value during creep exposure have been promoted by a presence of remained segregation. Since diffusion is enhanced by concentration gradient of elements, degradation due to microstructural change is promoted by a presence of remained segregation. Segregation should be reduced to obtain high creep strength with homogenized concentration of chemical composition.
