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The prevention of microporosity in ductile iron castings improves fatigue life; however, there is no standard foundry practice which will promote or eliminate the sporadic appearance of microporosity. While the [...]

Thin-walled castings made from Cr–Ni austenitic steels offer a combination of light weight of a near-net component with significant high temperature corrosion protection by forming a surface oxide layer. However, above critical service conditions (temperature, atmosphere, thermal cycling), oxidized surface can result in intensive surface degradation due to scale spallation. Scale spallation can decrease the wall thickness which could be detrimental to the in-service life of thin-walled castings. Experiments and stochastic simulations of spallation assisted oxidation were performed with three cast austenitic heat-resistant steels having different Cr–Ni concentrations at temperatures between 900 and 1000 ℃ on air and water vapor containing combustion atmosphere. The recorded specimen and spalled scale weight together with SEM and TEM analysis were used to predict the oxidation constant to form adherent layer and spallation intensity. Three oxidation modes, including oxidation controlled by diffusion with forming a strongly adherent to steel surface multi-layered scale, spallation assisted oxidation, and oxidation with additional partial vaporization of scale components in the water vapor environment were distinguished. It was revealed that the Cr and Ni concentrations moved temperature boundaries between these surface degradation mechanisms depending on the exposed oxidation environment. Our approach is aimed to alleviate an appropriate alloy selection for service conditions.

High alloyed by Sicast 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 SiSGI 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 SiMoand 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 SiMoand alloyed SiMoAlSGI. Methods for improving high-temperature performance by optimizing microstructural dispersity are suggested based on trends in surface degradation.

Reliable and realistic thermal properties data for investment casting shell molds are required to correctly simulate the metal solidification and predict the shrinkage. Investment casting shells exhibit several phase transformations [...]