SIALON was chosen as a ceramic horn for transmitting ultrasonic vibration to molten metals, because SIALON has high strength at high temperatures, high thermal shock resistance, small change in elastic modulus in a wide range of temperature, and low reactivity with molten metals. Ultrasonic vibration through the SIALON horn and effects of ultrasonic vibration on graphite structures of cast irons solidified were investigated. At room temperature, the SIALON horn vibrated at a frequency of 19 kHz and at the amplitude of 20 μm. When one end was immersed in molten cast iron, the SIALON horn vibrated at a slightly reduced frequency and at a reduced amplitude of 11∼16 μm. Graphite structures of cast irons solidified with ultrasonic vibration were fine and straight in flake graphite cast irons, and fine and spherical in C/V cast irons and in spheroidal graphite cast irons. The diameter of the fine spheroidal graphite was about 13 μm.
