Abstract
The occurrence of white iron in the interior of otherwise gray casting and a gray surface rim in a nominally white iron casting is contrary to general expectations. These phenomena, called inverse chill and inverse grayness respectively, affect cast metal quality and have important practical consequences. Hence, the sometimes conflicting published data was examined and a number of experiments were carried out to evaluate the apparent anomalies, using vacuum-refined, highly pure synthetic and commercial (Sorel F-10, S-125) Fe–C–Si alloys. The effects of various foreign elements on the formation of inverse chill were also studied by adding them singly in a highly pure form to the vacuum-refined alloys. Measurements of the cooling rates were made at the centre and top and bottom surfaces of the solidifying ingot to verify the relationship between the differential cooling rate and inverse chill.
Argon, oxygen, hydrogen and sulphur were found to be critical to the formation of inverse chill (occurrence of carbides). This study indicated that inverse chill can result from: (a) slow growth of nodules; (b) segregation of carbide-stabilizing elements towards the centre; (c) reduction of the carbon equivalent value, mainly by a reduction in silicon content; (d) the presence of oxygen or argon in the melt.
