Effect of Silicon and Graphite Degeneration on High-Temperature Oxidation of Ductile Cast Irons in Open Air

The use of high silicon ductile irons is increasing as they offer some advantages with respect to conventional pearlitic–ferritic grades such as high elongation at rupture for a given tensile strength value and a fully ferritic matrix. Besides addressing mechanical requirements, some castings must fulfil corrosion and high-temperature oxidation requisites. Two different ductile cast irons with silicon contents of 2.04 and 5.21 wt% were used so as to comparatively study their mechanical properties and high-temperature oxidation responses. The structure of both alloys contains nodular graphite and a fully ferritic matrix. Some samples of the alloy with 5.21 wt% silicon also contain abundant degenerated graphite which was identified as Chunky graphite. Oxidation resistance of both materials was evaluated by exposures to air at 650 °C for 720 h using a tubular furnace. The alloy with 5.21 wt% silicon showed an oxidation resistance about three times higher than the low silicon alloy. Although both alloys showed similar oxidation mechanisms, the oxidation scale formed on the high-silicon alloy stands out for having lower thickness, higher silicon content in the metal/oxide interface and more compact and adherent layers. Samples with Chunky graphite showed a similar evolution to those with graphite nodules, so no negative effect from this graphite degeneration on oxidation process was observed. The analyses performed by XRD revealed the presence of fayalite in the 5.21 wt% Si alloy, which is responsible for the better oxidation resistance.