Evaluation of induction hardened case depth through microstructural characterisation using magnetic Barkhausen emission technique

Abstract

The influence on the hysteresis loop and the magnetic Barkhausen emission (MBE) of microstructure within the case of different induction hardened carbon steel shafts has been studied. The hysteresis loop shows a distortion with a sudden reduction in the rate of magnetisation (dB/dH ) before approaching the maximum magnetic flux density indicating surface hardening. The systematic changes in the MBE profile for different voltages applied during induction heating indicate the microstructural variations within the case. A single peak MBE profile for a fully martensitic structure gradually changes into two peaks on reducing the induction hardening voltage indicating the formation of an additional soft ferrite phase within the case. The systematic changes in the two MBE peak heights indicate the synergistic decrease in the volume fraction of martensite and the increase in the volume fraction of ferrite phase within the case due to reduction in the induction hardening voltage. The changes in the MBE profile for different case depth specimens are more prominent than the hysteresis loop. This study shows that the MBE alone gives better insight in evaluating the induction hardened components (having case depth ≲1·5 mm), since the height and position of the two MBE peaks are directly influenced by the volume fraction and composition of hard and soft phases within the case. In general, this study reveals the high sensitivity of the MBE technique to the finer microstructural changes due to surface heat treatment in ferritic steels.