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Testing of ferromagnetic tubes

Steel and ferrous material as well as nickel-containing alloys are ferro-magnetic within the technically relevant temperature range, i.e. below the Curie temperature.

This means that after switching off an exterior field (magnet), a solid object keeps exhibiting measurable magnetic characteristics, a residual magnetization or remanence. This continuation of an effect after discontinuation of the cause is called hysteresis. Hysteretic properties of ferromagnetic materials hamper eddy current testing severely. DELTA TEST has the required know how for this.

The magnetization of a body depends on the exterior field that magnetizes it. There is a difference in the behaviour of paramagnetic materials, like zinc and aluminium, and ferromagnetic materials, like iron, cobalt and nickel, with regard to their permeability and their response after magnetization is switched off. The permeability of ferromagnetic materials is a decisive interference factor in tube testing, which must initially be minimized as much as possible.

In paramagnetism, the molecular magnets are aligned by an exterior magnetic field so that their alignment matches the exterior stimulating magnetic field. If the paramagnetic material is located in a coil, the molecular magnets are rotated in direction of the exterior field. Once the exterior field is switched off, the molecular magnets return to their original alignment and the solid object does not exhibit any residual magnetic properties on the outside.

For ferromagnetic materials, an almost complete alignment of the molecular magnets also takes place, but this is partly irreversible. This residual magnetization can only be removed by applying an exterior opposing field with a material-dependent field strength, the coercive field strength. The cycle of magnetization, remanence, coercive field strength and back-magnetization is described in the hysteresis graph above.

DELTA TEST has developed a procedure with which eddy current testing can be successfully performed even for ferromagnetic materials like steel tubes. This is possible by applying premagnetization procedures. For this, specifically developed probes are used, which reduce the ferromagnetic properties – the permeability – of the material to a constant level which is a low as possible. The aim is to render the material-dependent relative permeability μr to a value near 1. This causes the eddy currents in the material to behave similar to the behaviour in paramagnetic materials. This enables an evaluation of the eddy current signals by particularly experienced and trained experts. Furthermore, based on technical auxiliary means like automatic locators, special eddy current visualisation software and intelligent automatic signal analysis algorithms precise test results can be achieved, which are almost as good as conventional eddy current test methods for paramagnetic tubes.