Effect of Initial Microstructure on Creep Strength of ASME Grade T91 Steel

To clarify the cause of heat-to-heat variation in the creep strength of Grade T91 steels, the influence of the initial microstructure on creep strength was investigated. The distribution of chromium concentration considered to be remaining segregation was observed as corresponding to lamellar contrasts parallel to the longitudinal direction of the boiler tube. Standard deviation (SD) of ΔCr was employed as an indicator of the degree of segregation, and a good correlation was found between the SD of ΔCr and the creep rupture life at 650°C. Remaining segregation was reduced by renormalizing heat treatment at 1200°C instead of 1250°C. The creep rupture life of steel subjected to renormalizing heat treatment at 1200°C and tempering at 760°C, followed by normalizing and tempering under standard heat treatment conditions for Grade T91 steel, was prolonged by a factor of 2.3–2.8. The strengthening effect of renormalizing at 1200°C to reduce the remaining segregation was confirmed by creep tests up to about 10000 h at 600°C and 650°C. Decreases in the number density of M23C6 carbide particles, length of high-angle boundaries and average KAM values during creep exposure are promoted by the presence of remaining segregation. Since diffusion is enhanced by the concentration gradient of elements, degradation due to microstructural change is promoted by the presence of remaining segregation. Segregation should be reduced to obtain high creep strength with homogenized concentration of chemical composition.