Combined Micro-Hardness and Eddy Currents Applied to the Study of Steel Decarburizing

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.