INTERNAL CRACKS IN STRAND-CAST BILLETS.

An examination of the incidence of half-way cracks in strand-cast billets from four different casting machines has been conducted. Two factors appear necessary for appreciable crack formation: the existence of a columnar structure, and considerable reheating of the surface of the strand as it leaves the spray-cooling region. The extent of the columnar zone is shown to vary with the casting temperature; at a low-casting temperature columnar growth, and hence cracking, are suppressed. On the basis of published high-temperature mechanical properties and a two-dimensional, finite-difference, heat-flow analysis of the solidifying billet, it is shown that surface reheating can account for both the location and extent of internal cracks. It is suggested that reheating can be minimized by employing a properly designed secondary cooling system.