2010 Volume 96 Issue 11 Pages 665-672
Microstructure and creep strength at 650°C of two high-Cr ferritic heat resisting steels were investigated. 11Cr steel containing relatively high carbon, aluminum and nickel showed a significant drop in creep strength after 1000 h while 9Cr steel containing relatively low carbon, aluminum and nickel showed a recovery of creep strength after a slight drop. The 11Cr steel contained M23(CB)6, Z phase and relatively coarsened Laves phase as precipitates after 7000 h at 650°C, while the 9Cr steel contained fine MX and ultrafine M6C in addition to M23(CB)6 and Laves phase. Z phase was not found and the growth of Laves phase was retarded in the 9Cr steel. It was concluded that the drop of creep strength in the 11Cr steel was attributed to the coarsened Laves and Z phases and the recovery of the strength in the 9Cr steel to fine MX and M6C. Experimental results on the effects of steel compositions on the stability of precipitates in the steels were thermodynamically consistent with Thermo-Calc calculation results using an existing database. It was suggested that chromium and aluminum increased the kinetics of the formation and coarsening of the precipitates.