Measurement of decarburisation of steel rods with an electromagnetic sensor using an analytical model

This paper presents the measurement of decarburization of steel rods with an electromagnetic sensor using an analytical model both in online and offline scenarios. Loss of carbon (decarburization) at the steel surface can have a significantly detrimental effect on mechanical properties of products, since hardness, fatigue, strength and wear properties are strongly dependent on carbon content. Currently, measurement of decarburization is by destructive methods, such as metallographic observation or hardness tests on a sample cross-section after processing. A non-destructive EM method is proposed in this paper. An analytical solution for the inductance of a circular air-cored coil encircling a steel rod is deduced and further a simplified model is obtained which shows that for both online and offline cases, the thickness of the decarburisation layer is proportional to the inductance difference with and without decarburisation. Using this simplified model, decarburisation thickness can be obtained. The results have been verified by both finite element models and metallographic observations.