Eddy current flaw detection magnetic saturation

Eddy current testing is suitable for conductive material inspection, common metal materials can be divided into two broad categories: non ferromagnetic material and ferromagnetic material. The latter for copper, aluminum, titanium and its alloys and austenitic stainless steel; The former for steel, iron and its alloy. Their material is the essential difference between permeability mu is about 1 or greater than 1. In power plant, in addition to the complex water device such as a small amount of austenitic stainless steel pipe using copper, titanium, non ferromagnetic materials, a large number of pipes in ferromagnetic materials such as steel tube, a typical application of economizer, water wall, etc.

δ＝503.3／√fμrσ

Known in this case, the eddy current can only focused on the surface, cannot penetrate to the interior of the material. In addition, the magnetic domain structure of ferromagnetic materials, will have a great on eddy current testing signal interference, enough to put the defect signals completely submerged, and unable to get useful information。

Overcome the ferromagnetic metal on the impact of permeability on flaw detection method has two kinds: first, the far field eddy current testing method; Second, the saturation magnetization of steel pipe before inspection. Before the need to update the instrument of a kind of method, the latter only increase magnetic saturation in conventional instrument on the basis of the original device can be inspection for steel pipe and so on, has the advantages of less investment. After dealing with the magnetic saturation of the ferromagnetic materials can be treated by the non ferromagnetic material.

Usually steel tube eddy current flaw detection by magnetic saturation. It is composed of dc on coil steady magnetic field to produce, and with the aid of the guide sleeve to channel magnetic field into high permeability parts inspection position of the tested steel tube, to magnetic saturation state. In order to make full use of the coil magnetic field, device generally has made of ferromagnetic material such as pure iron shell. Because of the pure iron mu value is very big, magnetic resistance is very small, iron shell leakage magnetic field lines in the space will be collected, was also to the steel pipe detection area。

Due to a strong current through the magnetic saturation magnetization coil and can make the heating coil, therefore should have good thermal conductivity, in case the coil burned.。

Magnetic saturation eddy current flaw detection method should make testing coil magnetic flux density to make steel tube near saturation magnetization of the magnetic flux density of more than 80%. Therefore, before the inspection should be according to the material of steel pipe and specification choose magnetization current. The choice of the magnetizing current is usually in by comparing the sample). In theory, choose what should first calculate before detection of steel required for saturation magnetization of magnetic flux density, and then adjust the magnetizing current according to the above requirement, this method to be tedious calculation。In practice, can use simple adjustment method, namely in by comparing the sample back and forth, along with the increase of magnetizing current at the same time, observation instrument shows the change of the noise signals and artificial defects. When the minimum noise signal, signal, the largest artificial defects magnetizing current is the basic right. According to the general rule, the bigger the diameter, wall thickness is thicker, the magnetic properties of the material is soft, the magnetizing current is required, the greater the vice versa。

EM series magnetic saturation system,Designed specifically for steel tube eddy current flaw detection of current situation,It consists of magnetization of the magnetic saturation and constant current power supply,Conventional magnetic saturation of the device is composed of magnetization coil and iron components, big volume and heavy weight, suitable for use in the production of steel pipe used in fixed places, in this case, the magnetic saturation device without moving, the volume and weight are not considered, so can use ordinary materials, to reduce costs。For power plants, petrochemical plants and other users of steel pipe, steel tube eddy current flaw detection is usually at the scene of the flow, rather than in the workshop, for ease of use and mobile, device must be lightweight and efficient. To the EM series is specially designed magnetic saturation device, used the rational compact design, high permeability material and elaborate processing, greatly improve the efficiency of the device of magnetization, the weight is only 40% of the general plant, less volume in half. In addition, magnetic power supply voltage constant current power supply, it can well prevent voltage change or fever caused by magnetization coil resistance is bigger and change the disadvantage of magnetizing current。