Nodularity characterization of spheroidal graphite cast irons by nondestructive ultrasonic method

Spheroidal graphite cast iron has become one of the most used members of the cast iron family in recent years. KGDDs have come into play when conventional cast irons such as gray and white cast irons cannot meet the required specifications. While the annual production amount has increased, other needs have also emerged. One of these is the determination of the production quality of spheroidal graphite cast iron. One of the parameters in the production quality of spheroidal graphite cast iron is the nodularity rate which is also the subject of this study. High rates of nodularity, which is an important feature in the production of KGDDs, is the most important criterion for product acceptance. In foundries, microstructure of KGDDs is examined by microscope and nodularity ratios are determined as a percentage. Samples below a certain value are rejected. In this study, as an alternative to this quality control method, it was aimed to characterize KGDD nodularity by using non-destructive ultrasonic inspection method. Sound velocity, which is one of the spesific metallurgical properties of the materials, were investigated to determine the nodularity ratios. This velocity is the speed at which ultrasonic sound waves travel through the material and depends on the material microstructure. One of the ultrasonic ispection methods, pulse-echo method, was used to determine the sound velocities in this study. Samples of spheroidal graphite cast iron were obtained from the cast iron already in mass production process. Therefore, measurements were taken on samples of different thicknesses. Briefly, the thickness of KGDD samples was measured with an ultrasonic measuring device calibrated in a calibration block made of 1018 carbon steel. The thickness values measured by the ultrasonic device were different from the actual thickness of the samples. The sound velocity of the cast iron samples was calculated using the sound velocity of the calibration block, measured thickness and actual thickness values. The larger the difference between the measured thickness value and the actual thickness value, the more the nodularity rate reduced. The measured sound velocities were compared with the nodularity ratios obtained from the manufacturer of the samples, as well as the nodularity ratios determined by computer-aided image processing softwares. In addition, sound velocity measurement was taken from a non-spheroidal graphite cast iron sample. The sound velocities of the 1018 carbon steel calibration block, KGDD samples and non-spherical cast iron were also compared. In these comparisons, the relationship between the sound velocities of the samples and the sphericity ratios was evaluated and the differences in the velocity of sound between the extreme nodularity ratios (those with high and very low percentages) were observed. Because of the study, it was concluded that samples with high sound velocity had high nodularity ratios. In addition, scanning electron microscopy (SEM) was used to closely examine the microstructures of the samples and energy distribution spectroscopy (EDS) method was used to find the percentage weights of the elements on the sample surface. Keywords: Spheroidal graphite cast iron, ultrasonic inspection, sound velocity, pulse - echo method, nodularity.