Heat is nice. A hot mug of coffee on a cold morning, a warm shower. It’s not as enjoyable when you have to perform inspections in sweltering heat, though. So what are the impacts of temperature on an electromagnetic testing (ET), beyond the obvious? A not-so-simple question.
Conductivity and Temperature
The relation between electrical conductivity and temperature is relatively well documented. The conclusion: conductivity drops proportionally as heat rises because electrical resistivity is a function of temperature and that conductivity is the inverse of resistivity. If you recall my article on skin depth, you remember that it’s essential to electromagnetic testing and that it varies in relation to conductivity according to this:
δ ≈ 1/√πfμσ
- f = coil frequency
- μ = magnetic permeability (in H/mm) of the material
- σ = electrical conductivity (in %IACS) of the material
So, at the same frequency, as conductivity decreases, skin depth increases.
Impacts on Ferrous Materials
On ferrous alloys like steel, this has very little impact on ET because the decrease in conductivity is marginal compared to the very high permeability. Heat, in this case, only affects the probe itself and the reliability of its data, if you recall this article — a probe’s sensitivity is affected by thermal drift. So, the material under test must not be so hot as to create a thermal drift effect in the probe, as this skews inspection results and may damage the probe.
In the specific case of remote-field testing (RFT), the change in conductivity causes the operational point of the probe to shift, therefore it’s sometimes necessary to perform a frequency compensation to maintain good sizing capabilities.
Impacts on Non-Ferrous Materials
Non-ferrous materials are a different matter, as they are less permeable than steel, especially when you’re looking for subsurface defects. The change in conductivity caused by heat requires that you modify the probe’s frequency slightly to compensate and maintain the ability to size defects.
The MONEL® 400 alloy is an interesting case study because it is ferromagnetic. It’s low Curie temperature (20–50 °C/68–140 °F), however, makes it become non-ferromagnetic, which also makes it much easier to inspect. So, in this particular instance, it’s better to use ET when MONEL is hot.
Heat is a formidable opponent when it comes to electromagnetic testing, but as you’ve seen, it’s possible to turn it into an ally or to dodge it using various methods. Mitigating the effects of heat on inspection, however, is very specific to your application. Ask your questions to one of our experts.
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