Surface and subsurface stress evaluation in case-carburised steel using high and low frequency magnetic barkhausen emission measurements

The effect of near-surface and subsurface stresses on the magnetic Barkhausen emission (MBE) profile has been studied in case-carburised and tempered En36 steel. The high- and low-frequency MBE measurements were made, on both tensile and compressive sides of the rectangular bar specimens with case-depth of 0.95 mm loaded in cantilever beam, under different stress levels as well as after unloading from different higher stress levels. The high-frequency MBE profile showed a single peak while the low-frequency MBE profile showed two peaks. Under applied elastic stresses, both types of measurement showed increase in MBE under tension and decrease in MBE under compression. But, the MBE profiles measured after unloading from higher stresses showed different behaviour. On the tensile side, the high-frequency MBE profile did not change significantly due to pre-stress. But, in the low-frequency MBE profile, the first peak increases and the second peak decreases with increase in pre-stress. On the compressive side, the peak height of high-frequency MBE profile decreased gradually with increase in pre-stress. The first peak of the low-frequency MBE profile also decreased gradually with increase in pres-stress level. But, the second peak of the low-frequency MBE profile decreased by about 10% at a pre-stress level of −1094 MPa and remained more or less the same even after unloading from −1783 MPa. The MBE behaviour has been correlated to the residual stress (RS)-depth profile measured using X-ray diffraction method. This correlation clearly indicates that the high-frequency MBE reflects only the changes in surface RS level. It does not indicate RS changes occurring at depths >20 μm below the surface. The low-frequency MBE profile reflects the changes in the RS distribution occurring in the near-surface as well as deeper subsurface layers.