THE EFFECT OF NIOBIUM ADDITION ON STEAM OXIDATION BEHAVIOR OF FERRITIC HEAT RESISTANT STEELS AT 923 K

High Cr ferritic steels have been developed for the large components of fossil power plants due to their excellent creep resistance, low thermal expansion, and good oxidation resistance. Development works to improve the operating temperature of these steels mainly focused on the high mechanical properties such as solid solution strengthening and precipitation hardening. However, the knowledge of the correlation between laves phase precipitation and oxidation behavior has not clarified yet on 9Cr ferritic steels. This research will be focused on the effect of precipitation of laves phase on steam oxidation behavior of Fe-9Cr alloy at 923 K. Niobium was chosen as the third element to the Fe-9Cr binary system. Steam oxidation test of Fe-9Cr (mass%) alloy and Fe-9Cr-2Nb (mass%) alloy were carried out at 923 K in Ar-15%H2O mixture for up to 172.8 ks. X-ray diffraction confirms the oxide mainly consist of wiistite on the Fe-9Cr in the initial stage while on Nb added samples magnetite was dominated. The results show that the Fe-9Cr-2Nb alloy has a slower oxidation rate than the Fe-9Cr alloy after oxidized for 172.8 ks