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Study of pinholes genesis in iron castings

Elbel T., Hampl J., Senkypl M.

Archives of Foundry Engineering

2011

Vol. 11, iss. 1

21-26

Purpose: The study concerns the formation of pinholes in castings formed by reaction between a green foundry mould and lamellar graphite cast iron. Great numbers of works have been aimed at clarifying the causes of pinholes formation in iron castings. In spite of this there exists no united opinion on the pinholes formation (genesis) and the authors of this contribution having studied this phenomenon in compacted graphite and spheroidal graphite iron castings were also aimed at lamellar graphite cast iron and they applied for it their knowledge gained in study of reoxidation processes during casting of ferrous alloys. Methodology: Experiments were done on castings of stepped bars moulded in green bentonite mixtures with s graduated moisture and with use of two types of carbonaceous matters. Metal was melted in a 100 kg induction furnace from the same charge. Inoculation was done in a ladle after pouring out from the furnace. Aluminium was dosed in the ladle in some cases and the inoculator kind was changed too. Results: Pinholes were present on castings as small flat pits; on horizontal surfaces sooner singly, on casting edges in clusters. The formation of pinholes wasn't caused by high moisture of moulds but the defect was sensitive to aluminium content in metal. In castings with high aluminium content > 0.01 % the pinholes were present in great numbers, and namely both under low, and also high moistures of moulding mixtures. In melts with low content of Al < 0.01 % the pinholes occurred less extensively only. Practical implications: Conclusions from literature about influence of Al on pinholes occurrence were confirmed in such a way. Study of the defect morphology has shown that the question is a oxidation reaction type of pinholes caused by oxidation of the residual melt between dendrites with formation of CO.

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.