# In-Process Measurement of Barkhausen Noise for Detection of Surface Integrity during Grinding
## 3 Results and Discussion  
By grinding with varying specific material removal rates Q\({}^{\prime}\)w, different surface area states were achieved, which influence the BN signals during and after grinding. Figure 2 gives an example for the signal course of maximum BN amplitude M\({}_{\text{MAX}}\) (a) and averaged BN M\({}_{\text{MEAN}}\) (b) during grinding with different Q\({}^{\prime}\)w. Since the duration of the grinding processes differs significantly, the time axis for a better overview is shown as a percent of the machining time (beginning of contact between grinding wheel and workpiece to the beginning of spark-out). Both parameters increased with the beginning of contact with the grinding wheel, and the maximum signal level increased with higher Q\({}^{\prime}\)w. M\({}_{\text{MEAN}}\) decreased from the beginning of spark-out, and after grinding, the signal level of the grooves with different Q\({}^{\prime}\)w differed only slightly. M\({}_{\text{MAX}}\) scatters more than M\({}_{\text{MEAN}}\) and stays either at nearly the same level after the end of grinding wheel contact or even increases after the end of grinding, with Q\({}^{\prime}\)w = 14 mm\({}^{3}\)/(mm-s). Possible reasons for this behavior and consequences for the detection of thermo-mechanical damages are discussed later. Figure 3 shows the micrographs of all ground states, the example discussed above (0.8/1/150) is revealed in the second row. The first micrograph (Q\({}^{\prime}\)w = 1 mm\({}^{3}\)/(mm-s)) shows a surface state without any tempering effects while light tempering starts to occur for the second grinding process (Q\({}^{\prime}\)w = 6 mm\({}^{3}\)/(mm-s)). With increasing Q\({}^{\prime}\)w, tempering zones (dark etching zones) become more pronounced and reach larger depths. The surface hardness given in the figure shows a trend of decreasing hardness with increasing tempering zones, with some scattering in the values. For all heat treatment states, damage-free surface states and increasingly pronounced tempering zones were generated, while no clearly identifiable rehrending was observed at the surface. However, the thickness of the tempering zones and the limit at which Q\({}^{\prime}\)w leads to visible damages differ between conditions. Especially for the higher tempered material state (0.8/2/210), tempering zones appear later. The influence of the heat treatment state on the grindability is not further discussed at this point, as it is already well documented in [15; 16]. Figure 2: Maximum BN amplitude M\({}_{\text{MAX}}\) (**a**) and averaged BN M\({}_{\text{MEAN}}\) (**b**) during grinding processes with different specific material removal rates Q\({}^{\prime}\)w on a workpiece of the heat treatment state 0.8/1/150. The residual stress depth profiles shown in Figure 4 were determined by the method described in Section 2 and confirm the increasing influence on the surface state with higher Q\({}^{\prime}\)w. After grinding with Q\({}^{\prime}\)w = 1 mm\({}^{3}\)/(mm\(\cdot\)s), the residual stresses are clearly in the compressive range. Due to grinding with Q\({}^{\prime}\)w = 6 mm\({}^{3}\)/(mm\(\cdot\)s), the residual stresses are shifted in the direction of tension but are still in the compressive range. With further increases in the specific material removal rate, the maximum tensile stresses as well as the depth of influence increase. Figure 5 shows the characteristic BN values introduced in Figure 1 during (a and b) and after (c and d) grinding. Average values and standard deviations are calculated from seven or ten measurements, respectively, before the beginning (in-process) and after the end (post-process) of sparking out on the rotating workpiece. In general, MMAX, MMEAN and M\({}_{\text{R}}\) (a and c) initially increase with thermo-mechanical surface changes and decrease again for the strongest damage. This corresponds well with the behavior known for post-process measurements from the literature [11; 12; 13]. For the values derived from peak position and width (b and d), this is less unambiguous.