Evaluation of Microstructure and Hardness of Heat Treated Cast Steel by Eddy Current Nondestructive Method

In this paper, the nondestructive eddy current method was used for evaluation of microstructure and hardness of the heat treated cast steel. For this purpose, eight reference samples in same sizes were prepared from the cast block of hypereutectoid low alloy steel called FMU-226 and then the samples were heat treated under different cycles such as full annealing, quenching and quench-tempering. The microstructural studies were performed using scanning electron microscope (SEM), optical microscope and Vickers hardness (HV) instrument as destructive tests. Then, the microstructure and hardness of the samples were evaluated nondestructively by an eddy current test device at different frequencies. The capacities of different eddy current characters such as resistivity, inductive reactance, impedance and resistivity to impedance ratio were examined for developing a mathematical model to hardness measuring and microstructural recognitions. The results showed that the eddy current method can be nondestructively recognized and separated the steel parts according to their microstructure and hardness, rapidly. It was recognized that the eddy current characters were related to the current frequency, variation of retained austenite content in matrix, type of matrix microstructure and carbides morphologies. By using of impedance plane in the eddy current test at 20 kHz, it can be visually detected the microstructure of the heat treated steel. In addition, the impedance (Z) at this frequency was recognized as proper eddy current character for hardness measuring and the special equation for quality control of the heat treated steel was presented.