Non-Destructive Evaluation of High Chromium Cast Iron ‎Mill Balls with Eddy Current Test

In this paper, the eddy current nondestructive testing method was presented to evaluate the residual ‎austenite in the microstructure and hardness of high chromium cast iron mill balls. For this purpose, some ‎‎40 mm high alloy cast iron balls with 30 wt.% Cr and 3 wt.% C nominal composition were selected after ‎sand casting process. To achieve the different hardness and different residual austenite contents in the ‎microstructure, the balls were heat treated at different austenitzing and tempering temperatures. The ‎microstructural studies using by optical and scanning electron microscopies, XRD and Vickers hardness ‎were performed on the samples. Then, the microstructures of the samples were evaluated using an eddy ‎current device at 50 to 200 kHz frequencies. The result showed that, using the eddy current method, ‎different microstructures of the balls with different values of residual austenite and hardness can be ‎identified and separated, rapidly and nondestructively. To the purpose, the eddy current inspection at 150 ‎kHz frequency is selected using the impedance index as a non-destructive evaluation method. This index ‎decreases with increasing residual austenite content.‎