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.
Grinding processes are often the last step in the value-added chain of high-performance hardened steel components. However, thermo-mechanical loads which can take place during the process can have a detrimental effect on the surface integrity of ground parts, which are generally tested by post-process measurements. In the present study, two different approaches for an in-process inspection of the workpiece surface integrity were assessed using magnetic Barkhausen noise analysis during cylindrical grinding of hardened workpieces. The results showed that both measuring systems are able to detect changes in the surface state of workpieces in-process or directly after grinding in the grinding machine. After preparations to protect the sensors from influences during the grinding process, changes in the residual stress state and a decrease of hardness could be reliably detected. Due to constant contact conditions between sensor and workpiece a high reproducibility of the measurements was achieved.
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 nital 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.
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