Factors Related to the Formation of Gas Porosity in Grey Cast Iron: Investigation of Core Gas Evolution and Gas Concentrations in Molten Iron

Licentiatavhandling, 2008

Since gas porosity is a major casting problem, it is of greatest importance to increase the knowledge about the foundry process in order to minimise the presence of gas porosity and thereby improving the casting quality. Gas porosity is generally considered to originate from gases precipitated during solidification of iron and from entrained gas picked up by the metal during the mould filling. Entrained gas can arise from air present in the mould cavity prior to casting, from steam produced by water in the green sand, from gases produced by metal-mould-core reactions, and from gases produced by the decomposition of coatings, binders, or sand additives in mould and cores. In the present project the levels of gas (hydrogen, nitrogen, and oxygen) in molten iron and how the concentrations are affected by the melting process and the mould filling have been investigated. In addition, it was examined how the gas evolution rate and gas volume vary for different mould and core materials. It is concluded that the hydrogen concentration and the total amount of nitrogen and oxygen are mainly affected by the melting method. The hydrogen concentration is also sensitive to the dwell-time in ladles and furnaces and at least indirectly affected by temperature. The total amount of oxygen is insignificantly affected by temperature, while the dissolved amount of oxygen shows a strong relationship with temperature. The concentration of all three gases in the molten iron was found to be below the solubility limit, but the investigation revealed that hydrogen and nitrogen gas are absorbed during mould filling and in some cases raised to levels close to the maximal solubility concentration. It has been shown that the absorption of hydrogen and nitrogen during mould filling is favoured by turbulent mould filling, low permeability, and larger gas evolution from the mould and cores. Green sand evolves much more gas than chemical bonded sand, and the organic binders produce more gas than the inorganic binders. A large amount of the gas comes from the sand, and the type of sand affects the gas evolution. It has also been found that additives, coatings, iron temperature, and core geometry can affect both gas evolution rate and gas volume.