In order to develop a low alloy heat resistant steel which has a higher creep-rupture strength than that of 2 1/4 Cr-I Mo heat resistant steel, the effects of simultaneous addition of Ta, V, Ti, Co, Nb, and B on creep-rupture strength and oxidation resistance of 21/4 Cr-1 Mo steel were investigated.
The creep-rupture strength of the steel at 600°C was improved by simultaneous addition of these elements, which was mainly due to precipitation hardening by V4C3 and NbC. The oxidation resistance of the steel at 600°-700°C was slightly improved by simultaneous addition of Co, V, Nb, and Ta, but the addition of B was not effective for the improvement of the oxidation resistance.
As the result, the steel with nominal composition of 0.15% C, 2% Cr, 1% Mo, 0.4% V, 2% Co, 0.12% Ti, 0.02% Nb, with or without 0.03% B was selected as the best. 105 hr creep-rupture strength at 600°C obtained from the master rupture curve of this steel was 11.3kg/mm2, which was about 2.6 times higher than that of 2 1/4 Cr-1 Mo steel. It was concluded that this favourable creep-rupture strength for long time is mainly due to the excellent structural stability of the steel.