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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.