EFFECT OF ZIRCONIUM AND YTTRIUM ALLOYING ON HIGH TEMPERATURE OXIDATION OF Fe—15 wt % Cr—4 wt % Al

Abstract

Effect of Zirconium on high temperature oxidation behaviour cf Fe-15 wt% Cr-4 wt% Al alloy was investigated. Zr concentration was varied as 0.2, 0.55 and 1 wt% respectively. These alloys were subjected to isothermal oxidation in the temperature range of 1000-1200°C in different atmospheres} air, 02 and wet oxygen and cyclic oxidation in the range of 1150°C - 1300 C in air. For comparison purpose Fe-15 Cr-4A1-1Y alloy was also investigated. Analysis of wt. gain vs time data for iso thermal oxidation revealed that the linear rate law is observed for a short period in the initial stages and then it is followed by parabolic rate law which is observed for most of the oxida tion period. Yttrium addition decreased the oxidation rate appreciably at 1000°C in 02-H20 atmosphere whereas it did not effect oxidation rate in 02 but it induced slightly higher wt. gain in air as compared to that of the base alloy. Under similar conditions 1% Zr addition increased the weight gain both in 0 and air but in 02-H20 wt. gain was much less as compared to that of the base alloy. The effect of Zr concentration on oxidation of Fe-15Cr-4Al was investigated only at 1200°C in air. The increase in Zr ' concentration increases the oxidation rate. Lowest oxidation rate was observed in case of Fe-15Cr-4Al-0.2% Zr alloy. In cyclic oxidation each cycle consisted of one hour heating at required temperature follov/ed by half an hour cooling -Vin air. Fe-15Cr-4A1 alloy showed spalling which increases with increase in number of cycles. Spalling occurred during cooling leg of the thermal cycling. Zirconium addition prevented spalling but effect of Zr concentration on wt. gain was pronouns ed. 0.2% Zr and 1% Y addition caused minimum wt. gain even at 1300°C whereas in other alloys, 0.55% Zr, 1% Zr and base alloy wt. gain increased very appreciably after 1200°C. At 1300°C wart formation along edges of the specimen appeared in some alloys after three cycles. This was not observed in 0.2% Zr and 1% Y alloys even after being subjected to 18 and 8 cycles respectively. Surface topography of the scale was studied by SEM. X-ray diffraction analysis and EPMA were used to identify the phases present in scale and distribution of various elements in it respectively. Extensive spalling was observed in case of Fe-15Cr-4Al alloy. Spailed region showed higher Fe and Cr contents and little Al , whereas the unspalled region had mainly aluminium. All zirconium containing alloys showed irregularly shaped grannules which were observed to have much higher amount of zirconium and iron as compared to other regions of the scale and the extent of grannular structure increases with increase in zirconium concentration. X-ray diffraction revealed the scale to be mainly a-Al 0 2 3 with little Cr203 in case of Fe-15-Cr-4A1 alloy. Zr02 and YCrO, were incorporated in the scale with a-AlpO, in 1% Zr and 1% Y containing alloys respectively.