# In-Process Measurement of Barkhausen Noise for Detection of Surface Integrity during Grinding
###### Abstract  
The Barkhausen noise (BN) analysis is a method increasingly used for the post-process assessment of thermo-mechanical surface damages from grinding and has several advantages compared with the established natal etching method. In-process measurement of the BN has not been used industrially yet, but the basics have already been developed and promise time savings by avoiding time spent on inspections after grinding. Furthermore, it bears potential for the optimization of grinding processes and, in perspective, a process control. In the present work, the suitability of in-process BN analysis for the detection of thermo-mechanically influenced near-surface regions was assessed. Case-hardened workpieces were ground, and BN signals were related to the properties of the surface and subsurface area, in particular residual stresses, microstructure and surface hardness after grinding. The results show a clear dependency of BN on surface layer properties that allows for an in-process detection of detrimental changes in the surface state. Special attention was paid to the differences between in-process and post-process measured signals, and the suitability of the different measurement parameters for in-process detection was investigated. the manufacturer of the measurement technology, various characteristic values are derived from the curve. Figure 1 shows the measurement quantities of the 3 MA-II technique from Fraunhofer IZFP. Other systems use, e.g., the root mean square of Barkhausen noise pulses [9]. Furthermore, wavelets [10; 11] or Fourier Transformation [12; 13] can be used to decompose the BN signal into frequency spectra. This allows for the extraction of useful components from the raw signal while eliminating disturbances [14]. The magnetic BN analysis is increasingly used industrially as a post-process method for the inspection of ground components. It is able to detect slight influences of the surface properties even before the typical tempering zones become visible through nital etching as well as sub-surface damages [1]. For some commonly used case-hardened and tempered steels, studies about the influence of the heat treatment conditions, especially case hardening depth CHD and surface hardness, exist. This affects both the grindability and the detectability of possible changes induced by the grinding process [15; 16]. Typically, the BN signal increases with beginning thermo-mechanical changes (tensile stresses without tempering effects and light tempering zones). For heavy tempering zones and rehardening, the signal amplitude often decreases again [17; 18]. Since a single parameter such as the maximum amplitude \(\mathrm{M_{MAX}}\) or the averaged BN does not guarantee unambiguous detection of all damage levels, multiparameter approaches for unambiguous distinction were developed [18; 19]. For in-process use of BN analysis, only single approaches exist, which have not been used industrially so far [20; 21; 22]. In addition to time savings by eliminating time spent on inspections after grinding, these approaches offer the potential for process control, possibly in combination with other in-process measured quantities [23]. A special characteristic of BN measurements during grinding is the occurrence of an effect known as passive BN. This describes a signal that occurs without external magnetic field and its occurrence is currently explained by mechanical excitation of the Bloch walls due to vibrations during the process [24; 25]. A similar effect was observed during rotational bending, called mechanical BN [26; 27]. It is explained by the invers-magnetostrictive (magneto-mechanical) effect, which describes the relationship among stresses, local changes in the magnetization, and hence, Bloch wall movements [26]. This relationship between magnetic and elastic states is measurable on almost all ferromagnetic materials [28].