Air Gap Measurement During Steel-Ingot Casting and Its Effect on Interfacial Heat Transfer

Air gap formation during solidification greatly affects interfacial heat transfer and both must be understood quantitatively for accurate numerical simulation of casting processes, which are needed for fundamental understanding to enable quality improvements. Displacement and temperature in a 23 kg steel ingot and mold were measured simultaneously during solidification using a new experimental system. Interfacial heat transfer coefficients were extracted from the measurement results using an inverse heat conduction model. The evolution of temperature, air gap thickness, and interfacial heat transfer coefficients (IHTC) were quantified during this ingot casting process. The air gap forms earlier and grows larger on the narrow side than on the width side of the ingot. Interfacial heat transfer can be divided into four stages. In the first stage, there is good contact between the steel shell and the mold, so there is no air gap, and IHTC is high: 2700 to 3000 W m−2 °C−1. In the second stage, an air gap starts to form, so heat transfer decreases sharply. In the third stage, as the air gap thickness continues to grow, its effect weakens. In the fourth stage, even as the air gap continues to grow, IHTC remains almost constant at about 600 W m−2 °C−1. The IHTC can be predicted reasonably well with a simple equation based on conduction across the measured gap thickness, radiation, and contact resistance based on the measured roughness of the cast surface. Conduction is more important than radiation across the gap, accounting for about seventy percent of the effective IHTC at later times when the gap is large.