Alloys
3.4 Lifetime of thin-walled FeCrAl components at temperatures below 1000 °C
In oxidation of FeCrAl-alloys, especially when used as thin foils for car catalyst bodies, the service temperature is a crucial parameter determining the lifetime since it directly affects the scale growth rate. Considering the Arrenius temperature dependence of the scale growth rate [16] a dramatic life-time extension for the thin foil FeCrAl-specimens is expected when the temperature is decreased from 1200 °C to 900 °C [17]. However, extrapolating the scaling kinetics obtained at high oxidation temperatures (1000 °C–1200 °C) to lower temperatures (800-900°C) can only be made if no change in the scale growth mechanism occurs within the temperature range considered.
Isothermal testing of two commercial wrought foil materials at 900 °C revealed that one of them (batch HKF) exhibited initially higher oxidation rate than the other one (batch HKL, Figure 10). The mass change data in Figure 10 is in good agreement with the scale morpholo- gies of HKF and HKL after long term oxidation presented in Figure 11. The thick outward growing scale on HKF contrasts with that on HKL, where a generally flat scale featured only a few areas with outward growing nodules.
Figure 10: Mass change data for two 50 µm thick FeCrAl foils during isothermal oxidation at 900 °C in air. The values for ,mB indicate the mass changes which will result in total Al-depletion and thus occurrence of breaka- way oxidation.
0 0,2 0,4 0,6 0,8
0 10 20 30 40 50 60 70
Exposure time / h
Mass change / mg.cm-2
HKF
HKL ,mB for 50 µm thick foil
,mB for 20 µm thick foil
The reason for this rapid initial scale growth at temperatures of around 900 °C is the forma- tion of metastable modifications of alumina such as q. This results in an oxide growth rate, which is much higher than that of a-alumina [18]. Formation of the metastable alumina is apparently responsible for the enhanced oxidation kinetics of material HKF. Why alloy HKF is susceptible to the effect and HKL not has not been fully investigated and is subject of an on- going study. First results indicate that it might be related to differences in formation of tran- sient oxides from the other alloying elements. Of practical importance is the Al-depletion caused by the high oxidation rate of batch HKF. In Figure 10 the mass changes corresponding to the complete Al-consumption, i.e. breakaway oxidation are shown. Although, for 50 µm thick HKF-foils such a high initial oxidation rate might have no significant effect on the life- time, for 20 µm thick foils it could lead to breakaway oxidation after just 40 h of exposure at 900 °C.
Figure 11: Scale cross sections of FeCrAl alloys HKF (a) and HKL (b) after 2000 h oxidation at 900°C in air
4 Conclusions
A number of factors must be taken into account when considering the lifetime oxidation per- formance of commercial FeCrAl-alloys. For achieving the best oxidation resistance with re- spect 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 adhe- sion 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 in- corporation 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 (CB), i.e. CB decreases with decreasing alloy or component strength.
For real FeCrAl-components the lifetime assessment based on the results of short term oxi- dation 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 °C) a further enhancement of Al-depletion and a shorter lifetime than expected from extrapolation of high temperature (1000 °C–1200°C) data can occur if the alloy is prone to formation of me- tastable aluminas.
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