FI85772B - Method for detecting adequacy of magnetic flux produced by a magnetization yoke in magnetic flaw detection - Google Patents

Description

1 85772

Method for demonstrating the adequacy of the magnetic flux produced by a magnetizing yoke used in magnetic powder inspection

The invention relates to a method for demonstrating the adequacy of the magnetic flux produced by the magnetizing yoke used in the magnetic powder inspection 10 when magnetizing the ferromagnetic bodies to be examined.

Magnetic powder inspection belongs to the category of non-destructive material testing and is widely used in the testing of ferromagnetic materials for cracks, fissures and other discontinuities at or near the surface of ferromagnetic material. Magnetic powder inspection can be used on raw materials, semi-finished products (ingots, fusions, forgings), finished products and welds and is also useful in preventive maintenance. In magnetic powder inspection, fine magnetic powder is introduced onto the surface of the object under examination and a magnetic flux is applied to the surface by means of a magnetizing yoke. 25 A point of discontinuity on the surface, eg a crack or a crack, causes a leakage field and this can be seen in the change of magnetic powder. The method works if the leakage field is strong enough. The stronger the magnetic flux in the vicinity of the surface, the stronger the leakage field.

30

When testing a substance as described above, it must be ensured that the magnetic flux intensity is sufficient to obtain correct results. It has traditionally not been possible to measure the intensity of the flow from the surface to be inspected. Instead, 35 is the measured tangential field strength on the surface, for which a minimum value of 2400 A / m has been set in the British standard and which has set limits on the measurement. Another method has been to calculate the flux according to formula (1) 2 85772 B = μ0μΓβιΗ (1) where B = flux density, μ0 = constant, μΕβι = relative per-5 permeability and H = field strength. For flux adequacy, the above-mentioned standard BS 6072 sets the magnetic flux density at 0.72 T, provided that the substance has a relative permeability μΕβι> 240.

10 The US specifications for magnetic powder inspection generally do not set limits on field strength or magnetic flux, but require (ASTM SE-709 standard) that the magnetizing yoke have a certain lifting capacity. At the AC current and the maximum distance between the poles to be used, the lifting capacity is set to at least 44 N

(4.5 credits). According to the same standard, obtaining reliable indications requires that the field strength remain within acceptable limits, usually ± 25%.

The adequacy of the leakage fields, and thus the sufficient magnetic flux, has also been measured proportionally with eddy current devices, whereby the starting point for the calibration is the measurement of the leakage field of the discontinuity in a certain calibration body.

25

The latest method for measuring magnetic flux is induced flux measurement. In it, the probe is in electrical contact with the surface, whereby two contacts of the probe 1 cm apart measure the induced voltage, which is proportional to the change in the density of the magnetic flux, dB / dt.

On the other hand, V = d0 / dt. The induced voltage measured from the surface between the two measuring tips is a function of the electrical conductivity and permea-35 stability, and a connection is obtained.

3 85772 Ββ · δ · a VT (2) 5 where B = magnetic flux density at the surface 6 = electromagnetic depth effect a = distance between measuring tips.

The electromagnetic depth effect is in the order of millimeters for most steels used, so the voltage is completely independent of the thickness of the plate under investigation, with the exception of the thinnest steel plates.

The currently used measurement methods 15 listed above have some limitations.

The field strength measurement measures the scattered field even above the surface and the more erroneous the result, the more difficult the surface geometry or the worse the contact with the yoke on the surface.

Eddy current measurement only works if, in addition to the surface geometry, both the electrical conductivity and the permeability are known in both the calibration and the part under consideration.

25

The measurement of the lifting force from a known calibration piece alone does not take into account the properties of the test substance or their variations.

30 The induced flux measuring device is calibrated by means of a steel ring with a relative permeability = 1000 to show a certain flux value or, alternatively, to display a certain flux value when a "clear" indication is found. The "clear" indication is the minimum flow value at which the distortion in the calibration piece 35 is detected. The former method does not take into account the actual variations in permeability and electrical conductivity of the steel to be tested, and the latter method is based on a subjective assessment.

4,85772

The disadvantage of induced flux measurement is that the induced flux measurement used in the calibration gives an induced voltage that correlates only with the surface flux and does not give an image of the total flux. In practice, the voltage induced from the surface 5 by this method is only a few microvolts, and the reliability of the voltage measurement is affected by several factors. The absolutely good contact of the measuring tips with the surface to be measured is very important. This in turn requires careful grinding of the object and makes the method impractical under field conditions.

A patent device is known from patent application FI-884784 for demonstrating the adequacy of the magnetic flux produced by a magnetizing yoke used in magnetic powder inspection. The calibration of the device 15 utilizes induced flux measurement and lifting force measurement, and in field operation the device directly indicates whether the flux is suitable, too small or large. The device is placed on the other pole of the excitation yoke.

20 This known device may give a false, acceptable display even when one leg of the yoke has poor contact with the material to be inspected, as a result of which the magnetic flux in the inspected body may not be sufficient.

25

The present invention provides a measurement method that reliably provides information that a sufficient magnetic flux has been provided to an object to be inspected.

The measuring method according to the invention is characterized by what is stated in the characterizing part of claim 1.

According to the invention, the measurement method is double flux measurement, in which, when inspecting a substance, the circumference of both poles of both yokes is measured with an induced flux meter. Since most of the excitation yokes used in the world today operate on alternating current, a ring coil and a multimeter can simply be used as the flow meter, in which case the flux measurement takes place indirectly as a voltage measurement. Using a coil with e.g. 40 turns, the voltage induced by a conventional yoke is 2-6 V.

5 It has been shown that the measurement thus indicates how much magnetic flux passes through the foot. The ferromagnetic material under study under the foot acts as an antenna, effectively picking up magnetic flux from space. On the other hand, if the material to be inspected is missing underfoot or the contact to the material is poor, the magnetic flux from the yoke primary circuit will "correct" a different path back to the magnetizing transformer.

In order to know that sufficient magnetic flux has been produced in the object to be inspected, a limit must be set for the fluxes induced in the object from both legs of the yoke. In other words, it must be known that the specified voltage range obtained from the annular coil placed on each leg corresponds to an acceptable magnet-20 flux applied to the material to be inspected. Therefore, the ies must be calibrated to obtain a sufficient magnetizing force. This calibration can be performed by reference experiments known per se, e.g. using induced flux measurement from a known calibration piece of a certain size, lifting force measurements or using test specimens with known discontinuities.

Figure 1 below shows a measurement method according to the invention. Reference numeral 1 denotes a magnetization and reference numeral 4 denotes a ferromagnetic body to be inspected. The ring spools 2 and 3 are placed at each pole of the yoke just at the ends of the legs, so that they are close to the surface to be inspected. Voltage meters (not shown) are connected to the ring coils. Each ring coil can have its own voltage T: meter, or they can use a common meter with a toggle switch ’* · 35. The meters can be located in connection with the ring spools or further away from them, for example on a yoke. This is not essential to the invention.

6 85772

In field use, the magnetization element 1 with its measuring coils 2,3 is placed on the surface of the ferromagnetic body to be examined and a magnetizing current is applied to the yoke. The voltmeters connected to the measuring windings now show a certain voltage that is 5 proportional to the magnetic flux induced at the poles. Since the meters have been calibrated in advance, it is immediately seen whether the flux induced around both poles is sufficient. If either foot has poor contact with the surface of the body 4, it is immediately visible and the yoke can be moved to a better position.

The double flux measurement according to the invention easily ensures that a sufficient magnetic field is obtained in the object to be inspected.

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Claims (3)

7 85772 A method for demonstrating the adequacy of the magnetic flux produced by a magnetizing yoke used for magnetic powder inspection on a ferromagnetic body to be inspected in real time under field conditions during inspection, characterized in that the induced magnetic flux is measured at both poles at poles greater than the 10 magnetic flux predetermined under laboratory conditions, it can be assured that sufficient magnetic flux is obtained in the body (4) to be inspected. Method according to Claim 1, characterized in that the flow meter (2, 3) is a ring winding in which the induced 15 smv is detected by a voltage meter. Method according to Claim 1, characterized in that the excitation force (1) is calibrated before the body (4) is inspected. 8 85772