A study on characterization of cast iron microstructure and mechanical properties through ultrasonic nondestructive evaluation.

摘要

The goal of this research was to develop correlations between the microstructure and mechanical properties of gray and ductile irons using nondestructive evaluation (NDE) techniques. The research focused on developing procedures for measuring density, ultrasonic velocity, ultrasonic attenuation, graphite volume fraction, and microcarbide contents in iron and relating these measurements to the microstructure, certain mechanical properties, and machinability.; Density procedures were developed to determine bulk and apparent and true densities. Expressions were found to relate density to the silicon concentration and free graphite weight percent. Ultrasonic procedures, including through transmission and contact method, were developed for measuring ultrasonic velocity and attenuation. Ultrasonic attenuations were based on measurements of signal amplitude reduction, specimen surface roughness, and thickness.; A procedure was developed for determining graphite volume fraction from density and the silicon concentration. The results were in agreement with microstructural measurements but were more precise. Expressions were developed for the microcarbide content based on density, silicon concentration, and the total carbon content, and these results agreed with values obtained from measurements obtained from combined carbon measurements.; Higher ultrasonic velocities were associated with lower graphite volume, more rounded graphite nodules, and pearlite in the matrix. These conditions were associated with higher yield strength, higher hardness, and lower machinability. Ultrasonic attenuation was found to be related to the volume of graphite and pearlite in the matrix. Graphite and pearlite absorbed and scattered ultrasound energy. Higher ultrasonic attenuation was associated with higher graphite volume in gray iron but higher pearlite content in ductile iron.; Ultrasonic velocity and attenuation measurements were found valuable in evaluating gray iron microstructures, mechanical properties, and machinability. The trends made from NDE measurements in ductile iron were correct, but the correlation coefficients with machinability and tensile properties were not as high as in gray iron, probably because the NDE measurements do not reflect strain hardening involved in tensile testing and machining.