Use of first-bubble technique for predicting blowhole formation in steels

It has been shown that the degree of porosity in experimental steel ingots can be predicted from measurements of the first-bubble pressure on samples of liquid metal taken immediately before casting. The critical first-bubble pressure at which the ingots first show signs of porosity increases with carbon content. This is attributed to the fact that a higher nitrogen content is required to produce blowholes as the carbon content is increased. A detailed assessment of the technique has shown that, the higher the carbon content, the smaller the crevices in the refractory which act as heterogeneous nucleation sites. At carbon contents of 0·3 wt-% and above, cavities of 300 μm are filled by metal. Therefore, it is necessary to have some knowledge of the alloy composition before applying the technique.