The wear resistance of a component depends significantly on its mechanical hardness. Traditional hardness testing methods according to Rockwell, Vickers, or Brinell are destructive testing methods. After the application of these test methods, the component is unusable and must be separated, except for a few exceptions. For this reason, these methods are generally only used on a random basis. On the other hand, especially in connection with safety-relevant components, there is often the desire for a non-destructive (NDT) hardness testing procedure and a 100% complete inspection.
QASS’ µmagnetic measuring system uses Barkhausen noise. Barkhausen noise was discovered about 100 years ago by the German physicist Heinrich Georg Barkhausen and is a phenomenon that occurs in ferromagnetic materials. By applying an external magnetic field, the randomly aligned elementary magnets inside the ferromagnetic material reorient themselves parallel to the external magnetic field. The ferromagnetic material is completely magnetized. The speed at which this happens depends, among other things, on the material and its magnetic hardness. The magnetic hardness is a measure of how easily a ferromagnetic material can be magnetized. Soft magnetic materials are easy to magnetize, hard magnetic materials are rather difficult to magnetize.
As magnetic hardness and mechanical hardness correlate, soft magnetic materials are mechanically soft and hard magnetic materials are mechanically hard. After calibration of QASS’ µmagnetic measuring system with two reference samples of known hardness, the hardness of unknown samples of the same composition can be measured in a non-destructive manner. Thereby, complete NDT hardness testing of components becomes reality.
To determine the hardness, the material to be measured is exposed to an alternating magnetic field and the reorientation of the individual magnetic domains inside its microstructure is measured. The time-amplitude signal is captured by the micromagnetic sensor, digitized by the QASS µmagnetic measuring system and processed in real-time by a Fast-Fourier Transform (FFT). The FFT is applied to obtain the hidden frequency information (pitches) from the signal. The self-developed method used by QASS for spectral analysis is called High-Frequency-Impulse-Measurement (HFIM). Machine and interference noise can be removed from the signal data by means of a spectral filter (which we call frequency mask). Thereby the signal-to-noise ratio is improved.
Data is processed in real-time by QASS’ Software Analyzer4D. The user receives the hardness values in the desired unit. Additionally, Analyzer4D visualizes the material’s properties in real-time in a 3D-spectrogram. Hard magnetic materials are also mechanically hard and show low amplitudes. Soft magnetic materials are also mechanically soft and display high amplitudes.
Cycle times in the order of seconds can easily be realized with the system. A typical hardness measurement with QASS’ µmagnetic system takes less than a second and consists of several individual measurements. Each single measurement represents a change of the magnetic orientation of the domains inside the material.
In addition to static measurement, our µmagnetic system enables measurement in motion. This allows hardness profiles to be recorded, for example to inspect the quality of welding seams or to examine tooth flanks for grinding burn. Since the measurement is contactless, contamination (oil, scale, etc.) or oxidation of the surface is irrelevant. Measurement in motion can be automized with QASS’ QOBOT handling system.