Monitoring Overheating when Grinding in Test Environment

January 9, 2018 by
Marco Marino

Scene Reference Block

The reference block according to DIN 14 104 is a cube made of hardened steel on the surface of which new hardening and tempering zones are introduced by means of laser radiation. In terms of hardness and residual stress, these zones represent real grinding burns which can occur during the machining of components due to unfavourable process parameters. These zones are dangerous for the components during their loading phase. New-hardening zones consist of untempered martensite, which shows a so-called glass embrittlement and may also have cracks. The area is generally harder than the surrounding structure. Tempered zones are softer than the surrounding structure and pose a danger to functional surfaces. The markings on the sample represent zones of different tempered areas and are etched with Nital. The reference block is originally used to control etching baths for the grinding burn test.

Scene Coordinate Table 

The specimen is clamped on a coordinate table and measured with the QASS μmagnetic. At the lower edge of the specimen, the new hardening zone is the first to be seen. The table passes under the comparison body. Subsequently, the depth of the error condition with regard to the volume decreases further.

The QASS µmagnetic induces an alternating magnetic field on the reference body. The inserted defects are detected as changes compared to the surrounding matrix. This results in an inductive current, the magnitude and temporal characteristics of which are indirectly dependent on hardness and the volume of the defect (principle of Barkhausen’s noise). The new hardening zone represents the most massive, the last and 11th defect, the smallest defect size.

Scene Fast-Fourier-Transformation

The resulting inductive current is available as an analogue measurement signal and is shown via a discrete Fast Fourier transformation in a spectral data diagram. Innovative filter technology improves the signal-to-noise ratio. The evaluation takes place via an analysis of the energy content of the generated spectra per area (defect and surrounding structure).

Scene Plot

The values of the energy analysis are plotted in a 2D diagram whose axes represent time in seconds[x-axis] and hardness in Vickers[y-axis]. The hardness is determined from a linear correlation between the energy amplitude and temporal occurrence between the hardest and softest area. A branch of the plot represents a measurement in one direction. Different magnifications of one branch are shown.

The grinding burn represents the respective minimum in the sequence Maximum/Minimum. The maxima values represent the measured values from the undisturbed environment. The trend in signal reduction is due to the movement of the measuring stage and the simultaneous magnetization of error areas and the surrounding matrix as well as a resulting mixed signal. However, the fault zones can be precisely delimited.

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