Ultrasonic assessment of microstructures of steel casting, and hardness (HRC)

Abstract

Ultrasonic parameters such as velocity and attenuation, microstructure, and hardness (HRC) in steel casting (as-cast, normalized and tempered state) have been studied in order to find the relationship between those. The carbon segregation was severe in the upper region of steel casting (Gr-C, AAR Spec.) in the as-cast state where the riser was attached. This macro-segregation was not removed by the normalization and tempering due to diffusion of carbon elements, and the size, shape, and distribution of ferritic grains were ununiform and irregular, also after heat treatment. However, the size of ferritic grain was reduced about one-tenth as small as that in the as-cast state. The functional dependence of attenuation on the frequency ($\alpha=af^n$ (db/mm) a=0.1123, n=1.12) in the normalized and tempered state showed that the attenuation was mainly dependent on the hysteresis loss due to dislocation damping, atomic relaxation, or magnetic effects, rather than Rayleigh scattering. Ultrasonic velocity was observed to increase with percentage of ferrite increase the degree of equi-axed state of ferritic grain was viewed to increase with the amount of ferrite, and so caused to increase the elastic modulus. At the fixed frequency (14.1MHz, 15.7 MHz), the ultrasonic velocity was observed to increase with the attenuation of ultrasound. It may be thought that the hysteresis loss affects the elastic modulus, so does the ultrasonic velocity. However, no such trends have been observed between hardness(HRC) and percentage of ferrite, and between HRC and ultrasonic parameters.