Q NET Engineering GmbH

Case Depth Testing

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Situation

Case hardening improves the wear resistance and the fatigue limit of dynamically stressed parts. These mechanical properties are primarily determined by the surface hardness, the hardening depth and the residual stress depth profile. The case hardness depth (chd) - or the thickness of the hardened layer - is essential for the quality of the case hardening process. Up to now, random samples using destructive methods were the only way to control the quality of hardening processes. Needless to say, the method was very costly and time-consuming.

Solutions

Case depth determination with the ultrasonic Case Depth Tester P3121

In collaboration with IZFP, Q NET has developed an ultrasonic method for fast and non-destructive case depth testing. We reduce the effort and the costs of testing and provide fast production control to ensure consistently high quality levels.
Q NET provides testing systems for optimizing production parameters, reducing downtime after inductor changes, for fast production control and for quality management.

CHD Tester
Figure 1: Case Depth Tester P3121, notebook version

Method Description

Ultrasonic waves are scattered where the density and/or the elastic material properties in a limited area of the part change within the sound beam. In general, the sound waves are scattered in all directions, some of them are also scattered back to the ultrasonic testing probe. On the one hand, the effect, meaning the magnitude of the scattered and backscattered sound energy, depends on the change of density and/or the elastic material properties within the volume range. On the other hand, it depends on the relation of the geometric size of this volume range (scatterer) and the wave length of the ultrasonic waves.

The magnitude of the ultrasonic backscattering can be influenced by adjusting the wave length of the ultrasonic waves, e.g. by adjusting the ultrasonic frequency. Is the average grit size small in relation to the wave length of the ultrasonic waves, the scattering is low; in case of larger grits the effect is stronger. Eventually, grits that are much larger than the wave length reflect the sound.

The determination of the case hardness depth (Chd) is based on ultrasonic waves that are scattered back by the grits of the core material. The ultrasonic frequency and thus the wave length is adjusted in a way which makes the microstructure of the hardened near-surface zone cause only negligible scattering. When the acoustic pulse reaches the core material with a rougher grit structure, the ultrasonic waves are significantly scattered. The time needed by the acoustic pulse to travel from the surface of the material to the point where it is scattered indicates the thickness of the hardened zone.

Needless to say, the case depth determined by ultrasonic backscattering does not necessarily equal the case depth measured with metallographic-, micro-hardness- or any other established method. However, it is also evident that there is often a close correlation between the ultrasonic chd and the reference chd. The reason is that both depths, the depth of the core material and the case depth determined by established methods are influenced by the process parameters of hardening.

Case Depth Determination With the Micromagnetic Microstructure Tester MikroMach ®

The MikroMach tester is a compact and inexpensive device for non-destructive microstructure characterisation for manual or routine testing of magnetizable parts. The device is designed for the characterisation of hardness, case depth, nitrided case depth and hardness depth curves.

mobile testing system
Figure 2: Mobile testing system (left) - measuring and sensor unit (right)

Method Description

The device uses the effect of the magnetic hysteresis that changes with the composition of the microstructure and the residual stress of the material. Alternating magnetic fields of amplitudes of up to 8 different frequencies are simultaneously launched into the test material. This so-called multi parameter measurement is based on the collection of 40 input data sets and up to 4 parallel measuring processes at a time.

The parameter determination and the calibration of the testing system are done by Q NET. For this purpose, we collect up to 40 different input data per measuring process of each model under production conditions. After that, the model undergoes a metallographic examination. The results of the destructive testing will be compared to the collected data.

Further Information

Contact

Yvan Lejeune
Phone: +49 (0) 6 81/9 76 71 53
Fax: +49 (0) 6 81/9 76 71 58