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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.

In the present study, gears made of 21NiCrMo2 (AISI 8620) steel were subjected to heat treatments including carburizing, decarburizing, boriding and hardening. Carburizing, decarburizing and boriding were performed in a gaseous atmosphere, a salt bath consisting of 60% NaCl and 40% NaCO3, and a liquid medium containing borax and silicon carbide, respectively. Borocarburazing was carried out in two steps: carburizing and then boriding. Some specimens were treated to obtain different carbon concentrations and to observe the effect of carbon content on the boriding process. This process, consisting of carburizing followed by decarburizing and finally boriding, is named borodecarburizing. The microstructures and phase compositions of the diffusion layers were examined by means of X-ray diffractometry, scanning electron microscopy and optical microscopy. The microhardness profiles of these layers were studied by a Vickers indenter. The hardness value obtained by the borodecarburizing process is 10% higher than a borocarburized specimen's hardness. One-phase iron boride zone (Fe2B) was observed in the layers. This phase is preferred due to its mechanical properties.

In the present study, gears made of 21NiCrMo2 (AISI 8620) steel were subjected to heat treatments including carburizing, decarburizing, boriding and hardening. Carburizing, decarburizing and boriding were performed in a gaseous atmosphere, a salt bath consisting of 60% NaCl and 40% NaCO3, and a liquid medium containing borax and silicon carbide, respectively. Borocarburazing was carried out in two steps: carburizing and then boriding. Some specimens were treated to obtain different carbon concentrations and to observe the effect of carbon content on the boriding process. This process, consisting of carburizing followed by decarburizing and finally boriding, is named borodecarburizing. The microstructures and phase compositions of the diffusion layers were examined by means of X-ray diffractometry, scanning electron microscopy and optical microscopy. The microhardness profiles of these layers were studied by a Vickers indenter. The hardness value obtained by the borodecarburizing process is 10% higher than a borocarburized specimen’s hardness. One-phase iron boride zone (Fe2B) was observed in the layers. This phase is preferred due to its mechanical properties.

Carbon is the most important element going in the steels composition since it largely contributes to their mechanical performance. During the heat treatment of steels, a part of atoms of carbon could be removed from the superficial zone of the steel, which is the result of the atmospheric oxygen attraction by forming gaseous carbon monoxide. This phenomenon, called decarburizing, can change microstructure in a large extent and, as a result, mechanical properties of the steel. This could lead to unsuitable properties for the service life of mechanical parts. In order to prevent such part from breaking in service, it is necessary to know the affected zone by the decarburizing process. For this purpose, the most used technique is a visual observation by optical microscopy of a cross section of the material. This technique is often associated to the hardness profile obtained by Vickers or Knoop indentations in the same section. Depending on the material and on the specimen preparation, some discrepancies are often observed between the two informations. On the other hand, these techniques require a long preparation and cautious interpretation. Then we propose here to use eddy current testing, which is widely employed to detect fatigue cracks, to give some information about changes in microstructure and to examine carburizing of steel from both mechanical and microstructural point of view. For carburizing, eddy current has been successfully connected to the increase of micro-hardness near the surface. This last result has particularly motivated us to use eddy current to analyze decarburizing. In the present work, different durations in the furnace at a temperature of 920°C before oil quenching were performed on 54SiCrV6 (SAE 92V45) steel in order to obtain various morphologies of the decarburized zone. We show that eddy current test may be used to qualify the level of decarburizing since, after a convenient Fourier transformation, three components of the amplitude of the signal are directly linked to the duration of the heat treatment. Associated to optical and hardness measurements, it is also shown that the eddy current signal is well related to the decarburizing depth.