Factors Affecting Oxide Growth Rates and Lifetime of FeCrAl Alloys

摘要

A number of factors must be taken into account when considering the lifetime oxidation performance of commercial FeCrAl-alloys. For achieving the best oxidation resistance with respect to oxide growth and adhesion it seems to be necessary that the minor alloy composition includes a combination of various reactive elements. For example, in yttria containing ODS alloys titanium addition appears to be of vital importance for maintaining optimum scale adhesion during cyclic oxidation. At the same time, the impurity elements, such as carbon and nitrogen should be kept at minimum levels in order to prevent enhanced oxidation due to incorporation of matrix carbo-nitride precipitates into the alumina scale. During cyclic oxidation the mechanical properties of the substrate material may significantly affect the scale spallation resistance and consequently the lifetime. It is not only the scale thickness for spall initiation, which seems to be influenced by the mechanical properties of the metallic substrate but also the critical Al-concentration for the occurrence of breakaway (C_B), i.e. C_B decreases with decreasing alloy or component strength. For real FeCrAl-components the lifetime assessment based on the results of short term oxidation testing of laboratory specimens at high temperatures must be made with great care. If the FeCrAl-alloy is used as construction material in a constrained component, higher overall oxidation rates may occur. This results in shorter lifetimes than those predicted from the data obtained for free-hanging specimens in laboratory tests. At lower temperatures (≈ 900℃) a further enhancement of Al-depletion and a shorter lifetime than expected from extrapolation of high temperature (1000℃-1200℃) data can occur if the alloy is prone to formation of me-tastable aluminas.