130 mm OD and 9 mm thick tube, serviced exposed to carburization was cut into a 100mm long rectangular pieces. After surface cleaning, the samples were subjected to a cyclic magnetic field with a period of 15 s/cycle in an electromagnetic yoke. An in- house developed MBN system has been used for this study. The applied magnetic field at the center of the yoke was varied between ±12000A/m and this corresponds to a current of 0.7 A. The applied magnetic field was calibrated with respect to current supplied to the yoke.

The MBN signal was acquired using a 1.5 mm diameter ferrite cored surface probe having 4000 turns and is amplified to 68 dB. The Root Mean Square (RMS) voltage of the MBN signal and the applied current were acquired from negative maximum of the field to positive maximum of the field using a PC- AT compatible 12 bit resolution A/D converter card. The MBN signal were acquired at ID, OD and 0.75, 2.25, 3.75, 5.25, 6.75 mm average distance from ID on the thick side (cross section) of the sample. The RMS voltage of the MBN signal was plotted against the magnetic field. The Vicker's hardness value were also measured at these positions. The carburized ID side of the sample shows the lowest MBN peak activity corresponding to the maximum hardness and there is gradual increase in the MBN peak and a decrease in VHN as the distance from ID increases. It is observed that upto 4 mm from ID, the MBN peak height increases and the hardness value decreases linearly with distance from ID and beyond that, both MBN and hardness values remain more or less constant. Metallographic examination also shows the carburization only upto 4 mm from ID. The MBN peak position also gradually shifts from highest field at ID to lowest field at OD side. This indicates the gradual decrease in the carbon content away from ID. A quantitative linear relationship has been obtained between MBN peak height and hardness value within the carburized layer.
This study shows that it is possible to quantitatively correlate the carbon content and/or the hardness for a given carburized sample using MBN peak height values. Even though, the cross section of the tube would not be available for MBN measurement in practice, the MBN measurement on the carburized surface (ID) would give the carbon content and/or hardness values of the carburized surface which would help in theoretically predicting the approximate depth of the carburized layer. It is possible to evolve a criterion based on the minimum MBN peak height on the carburized (ID) surface, below which the depth of carburization would be beyond the acceptable limit.