Effect of Silicon on the Steam Oxidation Resistance of a 9%Cr Heat Resistant Steel

Abstract

The steam oxidation of 9Cr–0.5Mo–1.8W steels containing 0.06 to 0.49%Si was investigated at 500°, 550°, 600°, 650° and 700°C. The steam oxidation rate of the steel decreased with increasing silicon content. The effect of silicon was most remarkable at 700°C. At 500°, 550° and 600°C, the effect was almost the same, and was smaller than that at 700°C. At 700°C, the formation of a protective amorphous-SiO2 film reduced the oxidation rate considerably. On the other hand, at 600°C or less, silicon dissolved in the Fe–Cr spinel lattice with no evidence of SiO2. At 650°C, although amorphous SiO2 was observed, as at 700°C, at the scale–metal interface, the effect of silicon was the least within the test-temperature range. Thus, 650°C was a peculiar temperature for the effect of silicon on the steam oxidation of 9%Cr steels. The relatively small effect of silicon at 650°C is attributed to the formation of metastable FeO.