Usual heat treatments of steels like austenitization are generally conducted in air. In such atmosphere, a part of the atoms of carbon could be removed from the superficial zone of steel. Indeed, those atoms of carbon, combined with oxygen present in atmosphere, can take gaseous form of carbon monoxide due to the great attraction between atoms of carbon and oxygen. This well-known phenomenon is called decarburizing. It can change microstructure to a large extent and, as a result, bring dramatic modifications of mechanical properties of steel, like decrease of fatigue lifetime. To characterize the extent of decarburizing phenomenon, observations by optical microscopy and/or hardness profiles measurements must be performed in a cross section, as it is advised by international standard. Until today, the eddy current technique is used to detect superficial defects. In practice, the control consists to create two groups of available samples or not, from a large sampling. In this study, we propose to estimate the decarburizing extent. In the present work, different durations of austenitizing at 920 °C temperature before oil quenching (50 °C) were performed on the SAE 92V45 steel in order to obtain various morphologies and different total depths of the decarburized zone. We will show, in the continuation, how eddy currents control is used to assess the level of decarburizing after a Fourier transformation performed on the output signal. This analysis allows us to link the harmonic decomposition of the signal to the duration of the heat treatment and/or to the total decarburizing depth and, consequently, to the mechanical properties.