Magnetic Non-Destructive Evaluation of a Ductile Cast Iron with a Retained Proeutectoid Cementite

This work studies the potential of magnetic hysteresis and Barkhausen noise measurements for detection of an inimical cementite/chill fraction in a ductile cast iron. The samples are cast into a specially shaped mold with four steps of different thickness to vary a cooling rate during the solidification. Six castings are performed with different chill acceleration components: 0.1, 0.2 and 0.3% of Cr; 0.7, 1.2 and 1.7% of Mn. This results in a wide variation of the cast iron composition, which is evaluated by a standard optical microscopy procedure and Vickers hardness techniques. Magnetic measurements are performed using a unique laboratory system with a direct determination of the surface tangential magnetic field. The presented results suggest that the magnetic response is mainly determined by the ferrite content. Increase of the ferrite content expectedly leads to a linear decrease of the magnetic coercive field. The magnetically/mechanically harder cementite increases the coercive field, but the complex cast iron structure with differently mixed fractions of ferrite and pearlite disturbs the linear correlation.