Particularly for highly stressed components, it is important to have precise knowledge of the surface and subsurface properties and, thus, of the functional properties after final grinding at the end of a complex process chain in order to avoid rejected parts. Therefore, non-destructive testing methods have been the subject of research for several years. The Barkhausen noise analysis, as a micromagnetic measuring method, has the potential to characterize the subsurface area up to an analyzing depth δ non-destructively with micromagnetic parameters. In addition to micromagnetic multiparameter approaches, which allow post-process mode clear statements about the subsurface area state, the present research work deals with the concept of a connection of a single Barkhausen noise parameter with grinding process parameters. In combination with the analytical approach of Malkin for the thermal surface and subsurface area influence, which is based on the process parameters of grinding processes, a distinction between good and rejected ground parts can be achieved. The results show that, by post-process measurements of the Barkhausen noise on case-hardened workpieces made of steel 18CrNiMo7-6 (No. 1.6587, AISI 4820) and machined by a cylindrical grinding process, incipient changes in the residual stress state up to industrial-relevant limits, which distinguish between good and rejected parts, is possible. In the future, a combination of the Malkin grinding burning limit and sufficient condition monitoring based on in-process measurements of Barkhausen noise will be investigated. The application limits of the analytical approach of Malkin as well as the measurement of the Barkhausen noise in-process have to be determined.
The manufacturing process of grinding generally leads to a thermo-mechanical influence on the surface integrity. In addition to the intended development of residual compressive stresses due to the finishing process, disturbances in the grinding process can lead to negative effects such as tensile residual stresses, tempering and even rehardening zones and significantly reduce the component lifetime. In industrial applications, the analysis of Barkhausen noise is becoming increasingly important for the detection of this unwanted thermo-mechanically influenced surface integrity. The non-destructive method reacts sensitively to changes in, for example, the residual stress state as well as the hardness. In addition, other material-, process- and metrology-related influences are described in literature. The investigations presented in this paper deal with the influence of different material states (case-hardening depth, surface carbon content and alloy composition) on the signals of the Barkhausen noise as a function of the surface integrity. It is shown that the signal level is significantly influenced by the material condition and thus individual limit values must be used for evaluation of the surface integrity. ◼
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