Cross-numerical simulations and characterisations of magnetic properties of steels for non-destructive evaluation purposes

Online magnetic non-destructive assessment of mechanical properties is of growing interest. As there is a need for better understanding of the magnetic behaviour of steels in relation to their microstructural features, the consortium undertook the following theoretical and experimental work. - To write and execute finite element simulations to predict the magnetic behaviour of a selection of BCC steels. With respect to this target, the consortium managed to outline the limitations of 2D simulations, which feature realistic distributions of magnetic domains but fail to predict the interaction between microstructure and magnetisation processes. Therefore, 3D computations are required, but cannot be executed with reasonable computing power. - To check applicability with online measurements (AlK BH applications, other microstructural features for IF steels, bearing grades). Results allowed grading of the relevances of microstructural features on magnetic properties. First order effects come from the distribution of precipitates and secondary phase islets exceeding 150 nm in diameter, and the presence of uniformly distributed dislocation cells. Texture and slight non-recrystallised fractions are of less importance. Temper-rolling-induced elongation is sensed only through high-frequency magnetic techniques, which investigate depths comparable to the surface layers featuring the dislocation tangles. For long products, magnetic techniques are clearly able to assess microstructural changes induced by thermal treatments, such as pearlite spheroidisation.