Abstract
This paper presents a frequency sweeping excitation and spectrogram method (FSES method) by magnetic sensor for non-destructive testing of hardened carbon steel. High frequency induction heating can make changes in the microstructure and residual stress. As a result, the magnetic properties such as magnetic permeability, magnetic coercivity and remanent magnetization in materials are changed. This makes it possible to evaluate the hardening conditions by using the FSES method. We tried to examine the degree of yield strength which varies with hardened conditions. Controlling and evaluating of yield strength are very important for actual industrial products.