Alloy Design of Nano-sized Precipitates Bearing High-strength Ferritic Heat-resistant Steels

Precipitation site, morphology and high temperature stability of the fine carbonitride particles were precisely analyzed for the high strength ferritic heat resistant steels, both the 9%Cr-W containing ASME Gr. 92 and the 2.25%Cr-1%Mo-V ASME Gr. 22V. TEM observation, EDX analyses identified the fine (Nb,V) (C, N) precipitation inside of the lath structure of Gr. 92 steel and dense precipitation on the lath boundary of Gr. 22V steel. Thermodynamic calculation and the void strengthening estimation inferred that the nano-scale size particles inside of the lath structure effectively delayed the sub-grain microstructure degradation in Gr. 92 steel, and that the nano-scale size VC on the lath boundary possibly determined the creep rupture strength of Gr. 22V steel.