On an as-cast blank the question is whether there is enough machining allowance to reach the final dimension. Finding out there is not, once machining has started, means machine hours already spent on a part that has to be scrapped.
- ✓ Check of the machining allowance before the part goes on the machine
- ✓ High penetration for cast iron and strongly attenuating materials
- ✓ Velocity Mode as an indirect nodularity test
- ✓ High gain on rough foundry surfaces
- ✓ A-Scan to tell the backwall echo from a porosity
- ✓ No cutting: the part stays machinable
A casting comes with shapes that follow the pattern, not the finished drawing: the as-cast wall has to contain the design dimension plus the allowance to be removed. If the casting has suffered a core shift or an unforeseen shrinkage, that margin can be missing at one specific point, and the part turns out to be unusable only when the tool reaches that area.
Measuring the thickness of the blank before machining closes this risk. The problem is that cast iron and many foundry alloys are difficult materials for ultrasound: they attenuate more than rolled steel, they have a coarse grain that scatters the beam and rough surfaces that worsen the coupling. High transmission energy, adjustable gain and an A-Scan are needed to understand what one is looking at.
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There is then a particular use of the method that concerns ductile iron: the speed of sound in cast iron changes with the degree of nodularity of the graphite. By measuring the velocity on a part of known thickness one obtains an indirect indicator of nodularity, used in the foundry as a quick process check.
Why cast iron is a difficult material
| Factor | Effect on the measurement |
|---|---|
| Coarse grain | Scatters the beam and generates background noise |
| High attenuation | The backwall echo comes back weak: more energy is needed |
| Variable velocity | It changes with the degree of nodularity: table values unreliable |
| Rough surface | Difficult coupling: more couplant and gain needed |
| Porosity and cavities | They can generate intermediate echoes that can be mistaken for the backwall |
Velocity Mode as a nodularity test
In cast iron the graphite can appear in lamellar or spheroidal form, and the form changes the elastic properties of the material.
- The speed of sound rises with the degree of nodularity
- Velocity Mode derives it by measuring a part of known thickness
- The value obtained is compared with that of reference samples
- It is a quick and non-destructive check, useful at the line
- It remains an indirect indicator: it does not replace metallographic examination
It is to be used as a process indicator, not as certification of the structure of the material.
Recommended instruments
| Instrument | Why in this application |
|---|---|
| Dakota PMX4-DL | High penetration: cast iron, castings and strongly attenuating alloys |
| Dakota ZX6-DL | Velocity Mode for the nodularity test, with data logger |
| Dakota CMX2-DL | 110 dB gain range and A-Scan: rough foundry surfaces |
| Dakota FX81-DL | Search for internal cavities and porosity, in the laboratory |
Frequently asked questions
Yes, with due care. Cast iron attenuates ultrasound more than steel and has a coarse grain that scatters the beam, so a high-penetration instrument with high gain is needed. The speed of sound also varies with the degree of nodularity and has to be derived on the material, not taken from a table.
No, it is an indirect indicator. The speed of sound in cast iron rises with the degree of nodularity, so comparing the measured value with that of reference samples gives a quick, non-destructive process check. The formal characterisation of the structure remains the task of metallographic examination.
A thickness gauge can give anomalous readings where there are cavities, because porosity generates intermediate echoes before the backwall: with the A-Scan the operator notices them. But this is not a flaw-detection check: for the systematic search for cavities a flaw detector with a dedicated procedure is needed.
Back to Mechanical Engineering and Steelwork. For a check on castings or cast iron, write to us giving the material, typical thicknesses and the condition of the as-cast surface.
Further information and contacts
Systems division and sensor integration partner : www.roder.it
More information about RODER SRL : about us
Contact for general information : info@roder.it
Contact for information about these products : info@roder.it
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