Effect of chromium, aluminum and nickel on microstructure and reverse-S type creep rupture strength of high Cr ferritic heat resistant steels

Abstract

Microstructure and creep strength of two high-Cr ferritic heat resistant steels -11Cr and 9Cr - were investigated at a temperature of 650°C. 11Cr steel containing relatively higher carbon, aluminum and nickel than 9Cr steel showed a significant drop in creep strength after 1 000 h, while 9Cr showed a recovery in creep strength after a slight drop. 11Cr steel contained M 23(CB)6, Z phase and a relatively coarsened Laves phase as precipitates after 7 000 h at a temperature of 650°C, while the 9Cr steel contained fine MX, ultrafine M6C, as well as M23(CB) 6 and Laves phase. Z phase was not found and the growth of Laves phase was retarded in the 9Cr steel. Hence, it was concluded that the drop in creep strength of 11Cr steel was attributed to the coarsened Laves and Z phases, and the recovery of creep strength in the 9Cr steel to the fine MX and M 6C precipitates. The experimental results on the effect of steel composition on the stability of precipitates in steels were thermodynamically consistent with the Thermo-Calc calculation results, which used an existing database. We concluded that chromium and aluminum increased the kinetics of the formation and coarsening of the precipitates. © 2012 ISIJ.