In cast iron the beam is not stopped by an obstacle: it is scattered by the structure of the material itself. The signal that returns is weak, and must be made readable with suitable frequency and energy.
- ✓ Low frequency to cross coarse-grained structures
- ✓ High-penetration instruments for the most difficult cases
- ✓ A-Scan to tell the backwall echo from the noise
- ✓ Sound velocity obtained from a sample of the same casting
Cast irons and some foundry alloys are among the most demanding materials for ultrasonic thickness measurement. The reason lies neither in the hardness nor in the thickness, but in the internal structure: in a lamellar graphite cast iron the graphite inclusions and the crystalline grains are of a size comparable with the wavelength of the pulse. Every internal discontinuity reflects and deflects part of the energy in random directions, and the advancing wave is progressively depleted along its path.
The result is twofold and needs to be well understood. On one hand the backwall echo returns with greatly reduced amplitude, because much of the energy never reached the opposite wall. On the other, the spurious reflections generated along the way arrive at the receiver as diffuse noise, in which the useful echo can be lost. An instrument that merely shows a number can in these cases lock onto an echo that does not correspond to the backwall, returning a stable but wrong value.
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The instrumental response follows two converging directions. The first is to lower the frequency: a wavelength larger than the size of the grains reduces scattering and allows the beam to cross the material, at the cost of lower resolution. The second is to increase the energy of the transmitted pulse, so that even a small fraction of the initial energy is still enough to produce a recognisable echo. Instruments described as high-penetration arise from this compromise.
How an attenuating material is tackled
| Aspect | Measure to take |
|---|---|
| Probe frequency | Low values, to reduce scattering on the grains of the material |
| Transmission energy | High-penetration instruments, designed for attenuating materials |
| Signal checking | A-Scan to verify that the echo locked onto really is the backwall one |
| Sound velocity | Obtained on a sample of the same material, not from a generic table |
| Preliminary check | Trial on a real part before defining the inspection method |
Sound velocity is not a table value
A thickness gauge does not measure a thickness: it measures a time of flight and converts it using the propagation velocity that has been set. In cast irons that velocity is not a single value, because it depends on the form of the graphite, on the content of the elements and on the solidification conditions: two castings nominally of the same grade can behave differently.
For this reason the calibration must be carried out on a sample of the same material with a known thickness, verified mechanically. Dakota instruments allow one- or two-point calibration; the calibration blocks remain the reference for the periodic checking of the measurement chain.
Recommended products
| Product | Why it is relevant |
|---|---|
| Dakota PMX4-DL | High-penetration gauge, conceived for strongly attenuating materials |
| Dakota CMX3-DL | A-Scan to assess the shape of the echo when the signal is weak |
| Transducers for corrosion thickness gauges | A range that includes low frequencies suited to cast irons |
| Calibration blocks | Reliable references for setting up and for periodic verification |
Frequently asked questions
Because the structure of the material scatters the pulse along the whole path: the backwall echo returns too weak to be recognised. A lower frequency and a greater transmission energy are needed.
It is not advisable. In cast irons the propagation velocity varies with the form of the graphite and with the solidification conditions: it must be obtained on a sample of the same material with a known thickness.
To see the signal instead of the number alone. With A-Scan the backwall echo can be told from the spurious reflections and an unreliable reading can be recognised, which would otherwise look like any other value.
Back to Materials and difficult cases or write to us from the Contact page.
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