FACTORS AFFECTING CHROMIUM CARBIDE PRECIPITATE DISSOLUTION DURING ALLOY OXIDATION

Ferrous alloys containing significant volume fractions of chromium car bides were formulated so as to contain an overall chromium level of 15 % (by weight) but a nominal metal matrix chromium concentration of onl y 11%. Their oxidation at 850 degrees C in pure oxygen led to either p rotective Cr2O3 scale formation accompanied by subsurface carbide diss olution or rapid growth of iron-rich oxide scales associated with rapi d alloy surface recession, which engulfed the carbides before they cou ld dissolve. Carbide size was important in austenitic alloys: an as-ca st Fe-15Cr-0.5C alloy contained relatively coarse carbides and failed to form a Cr2O3 scale, whereas the same alloy when hot-forged to produ ce very fine carbides oxidized protectively. In ferritic alloys, howev er, even coarse carbides dissolved sufficiently rapidly to provide the chromium flux necessary to form and maintain the growth of a Cr2O3 sc ale, a result attributed to the high diffusivity of the ferrite phase, Small additions of silicon to the as-cast Fe-15Cr-0.5C alloy rendered it ferritic and led to protective Cr2O3 growth. However, when the sil icon-containing alloy was made austenitic (by the addition of nickel), it still formed a protective Cr2O3 scale, showing that the principal function of silicon was in modifying the scale-alloy interface.