# In-Process Measurement of Barkhausen Noise for Detection of Surface Integrity during Grinding
## 3 Results and Discussion
H\({}_{\text{CM}}\) slightly decreases with initial tempering and increases again with heavier tempering. This widely agrees with the observations in [18; 19]. DH\({}_{\text{25M}}\), DH\({}_{\text{50M}}\) and DH\({}_{\text{75M}}\) determined after grinding decreased with increasing influence on the surface area, while the equivalent FWHM in [18] decreased only in the area of light tempering zones slightly and increased afterwards. During grinding, the behavior of DH\({}_{\text{25M}}\) and DH\({}_{\text{50M}}\) was remarkable especially because of strong scattering. Figure 4: Depth profiles of the tangential residual stresses after grinding of heat treatment states 0.8/1/150 (**a**) and 0.8/2/210 (**b**). Figure 3: Micrographs and surface hardness of the ground workpieces with different heat treatment states and specific material removal rates. Based on the ratio of signal differences between the damage-free state and the different tempering zones and scattering, M\({}_{\text{MAX}}\) and M\({}_{\text{MEAN}}\) seem to be most suitable for the detection of damages during grinding. In the following, a more detailed consideration of the characteristics of these values is made. To understand the characteristics observed and described before, Figure 6a shows exemplarily some BN envelopes measured during and after grinding with different Q\({}^{\prime}\)w (directly before and after sparking out). The values M\({}_{\text{MAX}}\) and M\({}_{\text{MEAN}}\) derived from the envelopes are shown in the bar chart (b). For the condition ground with Q\({}^{\prime}\)w = 1 mm3/(mm-s), peaks during and after grinding have the same height. This corresponds to Figures 2 and 5. During grinding with Q\({}^{\prime}\)w = 14 mm3/(mm-s), M\({}_{\text{MAX}}\) is around 0.17 V and increases to 0.22 V after sparking out. The reason for this change is not yet clear but might be an effect of worse contact, and a larger air gap between the sensor and the workpiece due to vibrations during grinding with high Q\({}^{\prime}\)w. As shown in [23], BN decreases with increasing distance between the sensor and the workpiece surface. Furthermore, it is noticeable that, during grinding, the BN in the saturation area (near \(\pm\)100 A/cm) of the hysteresis curve increases. This effect is already visible at Q\({}^{\prime}\)w = 1 mm3/(mm-s) but increases with higher Q\({}^{\prime}\)w. Reasonably, a superposition of the magnetization by a mechanical excitation, as described in [24], can be assumed. In [29], [PERSON] and [PERSON] described that the superposition of a magnetic excitation with a mechanical excitation of higher frequency especially increases the BN signal at higher and lower field strengths but not in the area of coercivity. This is explained by the different processes during magnetization. Near coercivity, a change in magnetization takes place due to shifts of the 180\({}^{\circ}\) Bloch walls [6] that are not magnetostrictive and therefore not affected by mechanical excitation [29]. Figure 5: Characteristic values of the BN during (**a**,**b**) and after (**c**,**d**) grinding with different specific material removal rates (heat treatment state 0.8/1/150). For a better comparison of the suitability for the detection of thermo-mechanical surface influences, Figure 7 shows the BN values M\({}_{\text{MAX}}\) and M\({}_{\text{MEAN}}\) as a ratio with the values measured with Q\({}^{\prime}\)\({}_{\text{w}}\) = 1 mm\({}^{3}\)/(mm-s), which is defined as a damage-free reference. Ratios during (a and c) and after (b and d) grinding are plotted over Q\({}^{\prime}\)\({}_{\text{w}}\) for the four heat treatment states. M\({}_{\text{MEAN}}\) during grinding and M\({}_{\text{MAX}}\) allow for a clear distinction of the reference state from the other ground states. M\({}_{\text{MEAN}}\) after grinding shows a similar course, but the ratios are smaller. Even the heat treatment state 0.8/2/210 ground with Q\({}^{\prime}\)\({}_{\text{w}}\) = 6 mm\({}^{3}\)/(mm-s) can be distinguished from the reference by BN signals.