DISPERSION HARDENING EFFECTS OF NB-V PRECIPITATES IN MOD.9CR-1MO STEELS

The mechanism of high creep strength of high nitrogen Mod.9Cr-1Mo steels was metallurgically investigated by using an analytical high resolution TEM. The threshold stress in the constituent equation, epsilon = A(sigma(a)-sigma(th))n exp(Q/RT), is strongly dependent on dispersion strengthening due to peculiar-shaped niobium-and-vanadium-precipitates, i.e. wing-like vanadium-nitrides ''V-wings'' adhering to spherical niobium-carbonitrides. Key factors of the strength are size and shape of the precipitates. Increase of nitrogen addition is effective to growth of V-wings leading to large threshold stress. Calculated stresses based on a dispersion strengthening showed a good fit with experimentally-measured stresses.