What are non-metallic inclusions and how do you avoid them?

Sand inclusions are exogenous inclusions. Formed when particles of the moulding sand dislodges into the melt, they can account for between 30% and 40% of rejected castings in application that utilize sand moulds (e.g., in ductile cast iron and cast steel) and. Moulding sand can also act as a carrier of other contaminants that may form inclusions. Sand inclusions are also problematic because they occur at the final stage of casting: as the melt is poured and solidifies in the mould. This makes it essential to adopt good mould making practices.

Slag inclusions are caused by contact between the melt surface and ambient air. They are particularly common in ductile cast iron, since slag formation is an inevitable result of nodularisation. It is therefore important to follow best practices to control slag formation and separate it from the melt, to ensure slag and dross defects are not present in the final casting. This might include the use of slag coagulants or melt filtration.

Slag inclusions also occur in cast steel, where they are caused by the reaction of elements in the melt with high affinity to oxygen with the oxygen in the ambient air. Inclusions may also be formed by reaction products of refractory lining and metallurgical slag.

Endogenous non-metallic inclusions mainly comprise compounds of oxygen (in cast steel and aluminium), sulphur and nitrogen (in cast steel). It is not possible to avoid the chemical reactions that produce these compounds; the focus is therefore on measures to minimise the quantity of these compounds that end up in the final casting as inclusions. This includes controlling the formation of these compounds (e.g., clean input materials, melt treatment, pouring shrouds), while ensuring the removal of any that have formed in the melt before pouring (e.g., filtration, slag coagulation).