Influence of minor additions on the emissivity of the oxide scale of FeCrAlY alloys during resistance heating

Abstract

Chromium aluminum yttrium (FeCrAlY) alloys owe their low oxidation rate to the formation of a slow growing α‐alumina scale. For material used for heating elements not only the life time and the behavior of the resistance during the life time is of relevance, but also the emission coefficient of the oxide scale. The power density JS produced by resistance heating of strip with 50 µm thickness and about 5–6 mm width at 1050 °C is approximately equal to the radiant flux density, which is according to Stefan–Boltzmann's law proportional to the total emission coefficient εg. Resistance heating tests were performed on samples made from FeCrAlY alloys with different zirconium and carbon content. The “high zirconium” containing FeCrAlY alloys (zirconium > about 0.10%) have a higher power density/emissivity than the “low zirconium” alloys. In parallel with this, all samples with higher power density/emissivity have internal oxidation and therefore a “rough” metal–oxide interface. Thus, one cause for the increase of the emissivity of the scale could be this rough metal–oxide interface; other causes could be a higher amount of zirconium incorporated into the scale, more pores and/or different grain structure in the scale. Additionally the carbon content influences the appearance of a higher emissivity and the internal oxidation.

Effect of Zr Content on the Morphology and Emissivity of Surface Oxide Scales on FeCrAlY Alloys

“…are in qualitative agreement with findings reported by Hattendorf [10] and with data presented by Gopalakrishnan et al [13] obtained with the hemispherical reflectance method.…”

“…The subsequent discussion will concentrate on the results at 1 050 C, because this temperature is actually the closest to the real operating temperature of a typical heating element produced from FeCrAlY foils [10] and equals the envisaged temperature during the resistance heating tests (as explained in the section "Experimental").…”

“…Commonly, a-alumina scales formed on a FeCrAlY alloy show a light gray appearance and exhibit an emissivity in the range of 0.65-0.7. [10,12] Increasing the emissivity due to formation of a "dark" alumina scale would, thus, allow reducing the exposure temperature of the heating element without loss of power output. [13] This reduction in temperature would result in a significant reduction in oxidation rate and thus an extension of the component life time.…”

“…[13] This reduction in temperature would result in a significant reduction in oxidation rate and thus an extension of the component life time. [8] In previous studies, [10,13] it was shown that minor additions of Zr may influence the oxide scale appearance and the emissivity of oxide scales on FeCrAlY alloys. Investigations using alloys with a relatively low-Zr concentration of 0.03 wt % showed that an increased emissivity of the alumina scales as a result of a change in oxide morphology (i.e., wavy scalealloy interface and incorporation of Zr-oxide precipitates into the oxide scale) could be obtained compared to non-doped alloys.…”

“…[13] Pre-treatment of the alloys in a H 2 -H 2 O containing atmosphere (low pO 2 ) resulted in an increased emissivity, probably due to an increased Fe (and Cr) content in the outer part of the alumina scale. [13] Hattendorf [10] performed resistance heating tests at 1 050 C on 50 mm thick foils made of FeCrAlY alloys with different zirconium and carbon contents. The author found that in heating element tests, alloys with a higher Zr concentration (>0.1 wt%) exhibited a higher power density/emissivity than alloys with lower Zr concentrations.…”