Abstract
Fe-Al, Fe-Cr and Fe-Si alloys were internally oxidized at 800°∼1050°C in a H2/H2O mixture which was adjusted not to oxidize Fe into FeO. The character and the depth of the subscale formed were investigated with an optical microscope, and the distribution of alloying elements with E.P.M.A.
The results are summarized as follows:
(1) The character of the subscale can be classified into 5 types. The types depend on the heating temperatures, kinds and concentrations of alloying elements, and not on the surface oxygen partial pressure.
(2) Generally, the depth of the subscale is proportional to the biquadratic root of the surface oxygen partial pressure and is inversely proportional to the square root of the concentration of alloying element.
(3) Alloying elements are enriched in the subscale formed at low oxygen partial pressure.
(4) No subscale was found in Fe-Si alloys heated at 850°C in H2 of a fairly low dew point. That is probably because, as a result of the enrichment, the precipitated oxide particles have become a compact layer to hinder the diffusion of oxygen.
(5) The diffusion coefficient of oxygen through internally oxidized iron at 850°C was found to be 2∼3×10−8 cm2 sec−1.
