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Detection and quantification of grinding damage by using EC and 3MA techniques

: Wolter, B.; Theiner, W.; Kern, R.; Becker, R.; Rodner, C.; Kreier, P.; Ackeret, P.

Donzella, G. ; Associazione Italiana Prove non Distruttive -AIPnD-:
4th International Conference on Barkhausen Noise and Micromagnetic Testing 2003. Conference proceedings
Vaajakoski: Stresstech Oy, 2003
ISBN: 951-98400-4-4
International Conference on Barkhausen Noise and Micromagnetic Testing <4, 2003, Brescia>
Fraunhofer IZFP ()
3MA; 3MA technique

Grinding is used as manufacturing process for finishing of hardened surfaces. In dependence of the grinding parameters, the material and the component form the near-surface material warms up locally different during grinding. Local overheating can result in undesired structural changes, called grinding burns. In this case, surface tempering, cracking, rehardening and temperature induced tensile residual stresses can impair the desired surface treatment effect of grinding, which is the generation of compressive residual stresses in the substrate surface due to plastic deformation.
A safe proof of such grinding errors is necessary. Present quality management systems are based on tests performed according to relevant industrial standards, prescribing the complex and time-expensive natal etching. However, these tests are not suited for process-integrated non-destructive testing (PINT) respectively for fast post-process testing (FPPT). A very fast and reliable method to detect zones with grinding burns is MFEC (Multi-Frequency Eddy Current). It is able to be used as a fully automated inspection method within the production line, e.g. for cam slices.
In order to characterize grinding burns in a quantitative way, the non-destructive testing method must be sensitive to microstructure as well as to residual stress states. Furthermore it should be able to obtain depth dependent measuring results. If using only one micromagnetic measuring quantity, it is impossible to detect changes in hardness as well as in residual stresses at the same time. Due to the complex microstructure and stress gradients in near sub-surface zones, it is difficult to obtain trustworthy quantitative results with only one micromagnetic testing method. On the other hand, the 3MA (Multi-Parameter Micro-Magnetic Microstructure Stress Analyzer) technique combines the information content of four different methods, which are eddy current, Barkhausen noise, time signal of tangential magnetic field strength and incremental permeability. This multi-method, multi-parameter approach allows determining hardness and residual stresses in different material depths in a quantitative manner.
In this contribution, the methodical background as well as application examples for both methods will be discussed.