Evaluation of Si Content in FeSiCrNi Alloys Containing Carbon on Cyclic Oxidation Resistance at 950 °C

Abstract

Applications at elevated temperature require alloys that can form a protective oxide layer, and Cr and Si are known to form these layers, allowing FeCrSi alloys to be oxidation resistant. However, few studies have considered Si content higher than 2 wt.%. The objective here was to analyze the cyclic oxidation resistance of innovative FeSiCrNi alloys with different Si and Cr contents (around 5 wt.% of both). Cyclic oxidation tests were performed on three alloys at 950 °C in still air. The microstructure of the cast and heat-treated alloys and the oxide scales formed were analyzed by scanning electron microscope and energy-dispersive X-ray spectroscopy. Oxide layers were also characterized by X-ray diffraction. Comparing silicon contents of 4.65 and 5.9 wt.% in two different alloys containing around 4 wt.% chromium and 0.8 wt.% carbon, it was demonstrated that the lower silicon content was not enough to avoid iron oxides formation and continuous spallation. On the other hand, the higher silicon alloy exhibited low mass gains and a protective oxide layer composed of Cr2O3 and MnCr2O4, with some enrichment of silicon at metal/oxide interface. A third alloy, containing 2.7 wt.% carbon and, thus, graphite in its microstructure suffered decarburization in some regions, although in other regions it formed a protective oxide even in an alloy containing 3.25 wt% Cr and 6.6 wt.% Si. These results demonstrate that it is possible to have a high-temperature resistant alloy with low Cr content relative to conventional alloys by using high Si additions.