Testing of case depth in case carburized gear steels using magnetic barkhausen emission technique

Magnetic barkhausen emission measurements have been made on G86200 and G33106 steel specimens which have been case carburized to different case depths ranging from 0.5 to 1.5 mm (0.02 to 0.06 in.). The profile of root mean square voltage of the magnetic barkhausen emission signal plotted as a function of the voltage applied to the electromagnet has been used for analysis. It has been observed that, in both steels, the specimens with a case depth of less than 1 mm (0.04 in.) show distinct double peak magnetic barkhausen emission profiles, whereas in specimens with a higher case depth, the barkhausen profile shows only a single peak. It has been found that the first barkhausen peak at a lower voltage (magnetic field) gradually decreases with an increase in case depth while the second barkhausen peak at a higher field remains more or less the same up to a certain case depth beyond which it decreases sharply. The first magnetic barkhausen emission peak at a lower magnetic field is attributed to the domain wall activity in the subsurface region (greater than 300 mum [0.01 in.]) and the second peak at a higher magnetic field is attributed to the domain wall activity near the surface region (less than 300 mum [0.01 in.]). In lower case depth samples, the surface removal by electropolishing gradually increases the first magnetic barkhausen emission peak, clearly indicating the decreasing case depth and hence the enhanced effect of the subsurface with lower hardness. Also, in higher case depth specimens, the surface removal by grinding tip to 300 mum (0.01 in.) increases the barkhausen peak but still shows only a single peak, indicating that the remaining case depth would be greater than 1 mm (0.04 in.). This study suggests that the skin depth of the barkhausen signal would not be more than approximately 600 to 700 mum (0.023 to 0,028 in.) from the surface. The ratio of the two barkhausen peaks has been correlated to the case depth. This study suggests that the magnetic barkhausen emission profile can be successfully used to test the case depth in components with a shallow case depth (less than 1 mm [0.04 in.]) and to estimate the minimum remaining case depth in components with a deeper case depth after grinding operations.