Effects of ultrasonics on the kinetics of phase transformations in steels

Abstract

The effect of ultrasonic vibrations on the physical properties of materials has been extensively investigated by many research investigators. However, no previous attempts have been made to understand the effect of ultrasonic energy on the mechanisms and kinetics on phase transformations in steels. In this present investigation, attempts have been made to determine the effects of ultrasonic vibrations on the kinetics of phase transformations, carbon diffusion, hardenability, martensitic transformation temperature (Ms), tempering and aging phenomena of various grades of steel. Several significant effects have been observed when steels are quenched under ultrasonic vibrations. A definite increase in hardenability is a result of cavitation that is produced by the imposition of ultrasonic energy in a liquid quenching media. This produces an increase in the cooling capacity of the liquid by an increase in the rate of heat transfer through the vapor transport stage of cooling. Furthermore, there is an enhancement of carbon diffusion; either by room temperature ultrasonic quenching, or ultrasonic treatment at ambient temperature. An examination of the microstructure of the resultant transformation product showed a significant increase in the precipitation of new carbides. Further investigation indicates that only short-range diffusion is enhanced, since the growth of existing carbides is not induced. When specimens were quenched under ultrasonic energy above room temperature and tempered at 1000°F, the resultant martensitic morphology had been significantly affected. This is due to strain-induced carbon diffusion, thereby relieving the supersaturation of the martensite matrix.