Unmixing phenomenon (macro segregation) occurring in large, thick-walled castings. The main cause for this is unmixing of the still liquid residual melt (solubility of individual elements or impurities is lower than in a solidified body). Therefore, the zone to solidify last is oversaturated with such elements and possibly also their oxides, sulfides, nitrides, etc.
This segregation phenomenon often occurs in unkilled cast steel in such a manner that the steel crystallites push the impurities contained in steel alloys, such as sulfur, phosphorous, and carbon, towards the center so that they accumulate in the inside of the ingot. The cooling conditions also have significant influence on this process; The degree of segregation increases with larger ingot cross sections. Thereby it is possible for sulfur to reach concentration levels of around 300 %, phosphorous of around 200 %, carbon of around 100 %, and manganese of around 50%. In unkilled steel, segregation takes place in the entire ingot in contrast to killed steel, in which it is generally restricted to the ingot head, i.e. the upper third of the ingot.