Characterization on multiphase microstructures of carbon steels using multi-frequency electromagnetic measurements

Phase composition is dominant in determining mechanical properties of carbon steels therefore is one of important microstructural elements that needs to be characterized and monitored in the steel production process. However, the characterization method usually used in steel production is off-line with destructive inspection. Microstructure characterization using electromagnetic signals can be applied in real time with in-line measurement that can meet requirements of steel continuous productions. The key is to establish the accurate electromagnetic responses to steel microstructure variations. This paper studied responses of electromagnetic signals on carbon steels with different phase compositions using a U-shaped and a cylindrical electromagnetic sensor. Relationships between steel microstructures and electromagnetic signals were established using the multi frequency electromagnetic system for steel samples with low to high carbon grades. Influences of phases, phase fractions, grain size and grain shapes on the relative permeability values were investigated. Results show that the low frequency inductance of electromagnetic signals can be used to distinguish the phase composition and the phase fraction of carbon steels. Effects of sensor lift-off distance and sample edge effects are also studied as requirements of industrial application.