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Influence of aluminium on the formation of pinholes in cast irons

Elbel T., Hampl J., Vladik R., Koukal R.

Archives of Foundry Engineering

2008

Vol. 8, iss. 2

27-32

Purpose: This study concerns the formation of pinholes in castings by reaction between cast iron with compacted graphite and green sand mould. Methodology: Experiments were done on bar castings moulded in green bentonite mixtures without carbonaceous matters. Moulding sand moisture and aluminium content in the melt were purposely changed. Metal was melted in a 100 kg furnace by remelting the uniform charge of return material. Metal was inoculated all at once in a ladle and modified in a reaction chamber in a mould or as sandwich method in a pouring ladle. Findings: The formation of pinholes was not caused by high moisture of the mould but this defect was sensible to aluminium content in metal. In castings with high aluminium content > 0.2 % the pinholes occurred in high numbers, and namely under low and also high moistures [> 4 %]. In previous melts with aluminium contents < 0.02 % defects occurred in small range. Practical implications: Thus the conclusions known from literature about the influence of Al on pinholes occurrence in cast irons were confirmed. Oxygen activity in metal during its flowing and cooling in the mould was also measured but this quantity was not changed too in dependence on pinholes content. Morphological analyses near the defect have indicated that it is a physical type of pinholes caused by hydrogen. Originality: A number of works were aimed at explanation of causes of pinholes formation in lamellar graphite iron castings and spheroidal graphite ones. But this defect is formed in castings from compacted graphite cast iron too and the research was aimed just to this material. The contribution is in such a way an exceptional one.

State of the art of metal reoxidation study of iron castings

Elbel T., Senberger J., Zadera A., Vladik R.

Archives of Materials Science and Engineering

2007

Vol. 28, nr 11

649-652

Purpose: Metal reoxidation causes a number of iron castings defects, particularly when pouring them in greensand moulds. One of them is pinholes the occurrence of which is explained by several hypotheses. One of possible causes is reoxidation processes in the foundry mould cavity. During its flowing in the gating system and in the mould cavity the liquid metal gets into contact with oxygen from air and water vapour. Secondary oxidation of elements takes place in sequence of their affinity to oxygen. Design/methodology/approach: Therefore the authors were aimed at cast irons. Besides indirect methods the reoxidation was researched mainly by direct measurement of oxygen activity during filling of a mould up to solidus temperature. Findings: The use of oxygen activity measurement for direct study of changes caused by reoxidation processes is limited and that it is more suitable for study of the influence of different factors on cast iron crystallization and graphite morphology. Practical implications: Obtained results have confirmed that oxygen activity measurement can serve to the casting quality control. But it is little sensitive for explanation of processes running in a cavity during metal casting. Originality/value: Continuous monitoring of oxygen activity changes in a foundry mould is an original solution since other authors have done their measurements only in a furnace with disposable sensors.