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Physical principles of magnetic-inductive measurement

With the magnetic-inductive method, the ferrite content can be determined quickly and non-destructively in accordance with the Basler standard and DIN EN ISO 17655. For example, weld seams on austenitic steel can be checked on site and repaired if necessary.

Here’s how the measurement works

The probe for ferrite content measurement consists of an iron core around which an exciter coil is wound. A low-frequency (168 Hz) alternating current flows through this coil. This creates an alternating magnetic field around the poles of the iron core.

When one of the probe’s poles approaches a steel part, the ferrite grains in the steel strengthen the alternating magnetic field. A measuring coil registers this increase as voltage. The magnitude of the difference in voltage depends on the magnetizable component in the crystal structure. For this reason, this method cannot differentiate between delta ferrite and deformation martensite.

Here’s what you need to pay attention to during the measurement

The magnetic field of the coil spreads about 2-3 mm all around the probe’s pole, both to the sides and in depth. This means that an approximately cone-shaped section of the sample is considered. The method doesn’t allow any conclusions to be drawn about the distribution or accumulation of the delta ferrite in the material. And it can also happen that the results of the magnetic-inductive method deviate greatly from a metallographic determination, because this method only records the surface distribution of the ferrite. 

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