Q NET Engineering GmbH

Ultrasonic Backscattering Method

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Ultrasonic waves are scattered everywhere where the density and/or the elastic material characteristics alter within the sound radiation in a restricted area of a specimen. Generally speaking, the sound waves are scattered in all directions, part even back to the ultrasonic probe.
The effect, i.e. the size of the sonic energy scattered and also that of the sonic energy backscattered, depends, on the one hand, on the alteration of the density and /or of the elastic material characteristics in the volume range and, on the other hand, on the ratio of the geometric size of this volume range (scatterer) to the length of the ultrasonic waves.

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By means of adaptation of the ultrasonic wave length i.e. through adaptation of the ultrasonic frequency, the strength of the ultrasonic backscattering is influenced. If the average grain sizes are small compared with the length of the ultrasonic waves, the scattering will be small; with larger grain sizes the effect is stronger until, finally, sound reflection occurs on grains that are much larger than the wave lengths. When determining the Surface Hardness Depth (SHD), one uses the backscattering of the ultrasonic waves on the transi-tion area hardened / base material resp. on the grains of the base material (see above figure).

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The ultrasonic frequency and, as a result, the wave length, is set in such a manner that the structure in the hardened zone close to the surface, causes only minor scattering. Nevertheless, during the passing of the sound impulse into the basic material with the pronounced grain structure, significant scattering is caused. The thickness of the hardened zone results from the time required by the sound impulse to run from the surface of the component as far as the position at which the scattering is caused.

As a result it is clear: SHD determined by means of ultrasonic scattering is not necessarily identical with the SHD value determined metallographically and/or by means of micro-hardness measuring and/or via any other established procedure.

Microstructure

Also clear is, however, that in many cases, there will be a good correlation between the ultrasonic SHD value and the comparative value, because both depths, the depth position of the transition area hardened / base material resp. of the base structure and the Rht determined with established procedures, are influenced by the process parameters of the hardening.

Best results are achieved under following conditions:

  1. Parts to be inspected are indcution hardened;
  2. parts are forged, not cast;
  3. minimum SHD is 1.5mm;
  4. clear and sharp transition between base material and martensitic material without inter-mediate structures as bainitic material;
  5. grain size of the base material is sufficient for good backscattering of 20 MHz shear waves.