Improvement of High-Temperature Performance of High Si SGI by Al Alloying and Optimizing Microstructural Dispersity.

High alloyed by Si cast iron with spherical graphite (SGI) has a unique combination of mechanical properties at room temperature and high oxidation resistance at elevated temperature. The performance of this cast material can be improved by precise alloying and for the development of desired microstructural dispersity, using an effective melt inoculation in combination with an optimal cooling rate during solidification. There is consensus in the metalcasting community that high microstructural dispersity is desirable for good mechanical performance of high Si SGI at room temperature. However, it could not be the case for high-temperature applications. In this study, static high-temperature oxidation of heavy (100 mm), medium (18 mm), and thin (5 mm) wall castings from two SGI (base, referred to as SiMo and alloyed by 3% Al (SiMoAl) were investigated. Microstructural dispersity was characterized in 2D sections and in 3D space by µCT. Surface degradation was quantified by SEM and TEM analysis in cross sections after 100 h static oxidation in air at 800 °C. Carbon analysis was used to decouple the scale formation and decarburization (deC) processes which occurred simultaneously. Scale topology was determined by microcomputed tomography (µCT) analysis. Additional thermo-cycling tests of constrained specimens were performed to determine resistance to SGI failure during transient thermomechanical loading. These methods provided the quantification of the effects of microstructural dispersity parameters on high-temperature performance in SGI. The different trends were observed in the base SiMo and alloyed SiMoAl SGI. Methods for improving high-temperature performance by optimizing microstructural dispersity are suggested based on trends in surface degradation. [ABSTRACT FROM AUTHOR]