Mesure d'hystérésis magnétique volumique de l'acier 4340 en fonction de la température

The field of induction heating relies on the electrical and magnetic properties of a material to transfer electromagnetic power inside the work piece by means of eddy currents. With complex geometries, finite element simulations are commonly used to determine the magnetic field distribution, and thus the power distribution. Better predictions of the transferred power accelerate the prototyping scheme used to obtain the optimal heating profile. Considering the advances in numerical techniques, measurement bias in magnetic properties can have a relatively large impact on the precision of the results. In the present work, we propose an approach to magnetic property measurements at high temperature. A two windings quasistatic approach is used to monitor the evolution of the properties up to the Curie temperature, i.e. a thermomagnetic analysis (TMA). The developed setup can acquire approximately 50 hysteresis curves spread between 20°C and 850°C within 20 minutes. This rapid TMA is crucial to avoid any microstructural transformation in the steel over the course of the heating. An exposure of five minutes to high temperatures is suspected to be enough to alter the magnetic properties in some sensitive steels. Furthermore, the sample's machining, prior to the measurements, most often implies cutting techniques that create residual stress, plastic deformation and high temperatures. These three phenomenon can alter the microstructure and therefore permanently change the initial material's magnetic properties. The chosen magnetic measurement method favours samples with a large volume to surface ratio to reduce the impact of material alteration at the surfaces. A contribution of this work is the development of a high temperature winding (900°C) at low cost. This expertise applied to our method can increase the precision in the magnetic measurement of hysteresis curves by allowing the windings to be part of the sample and therefore inside the oven. This enables the use of closed samples (toroids) with a diameter of approximately 5 cm and a 5 mm cross-section diameters, which is impractical with traditional windings.