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What is the preparation for eddy current testing

2021-01-14 16:05:30


It is necessary to adjust the experimental instrument before the on-line eddy current inspection of the automatic inner and outer ring assembly machine. Start the flaw detector (including part controlled by computer) in advance, select the appropriate camera diameter and guide posts of relative specifications for the cable diameter to be tested. Compared with offline flaw detection, online flaw detection has higher technical standards and higher difficulty coefficient. It should not only ensure the normal operation of the flaw detector itself, but also match with the static and dynamic data of the production line. For example, the straightener fixed in front of the camera is used to straighten the stainless steel wire and the precise positioning effect. If the straightening is too tight, although the actual effect is good, it improves the friction resistance and causes certain harm to the normal operation of the stainless steel wire in the heat treatment process line. Straightening too loose will enhance the vibration of the stainless steel wire, endangering its coaxiality.

When the equipment of the automatic eddy current flaw detector is online, part of the I/O electrical control system of the flaw detector is also closely connected with the production line. If the electric heater is not turned on in the heat treatment process line, the flaw detector can not work normally. If the common fault of the electrical equipment of the eddy current flaw detector causes the parking of the production line, it may cause the brittle fracture of the stainless steel wire stranded in the heat treatment furnace, and then destroy the continuity of the whole stainless steel wire. Eddy current flaw detection (ET) A flaw detection method used to examine surface and near-surface defects of conductive prefabricated components using the basic principle of magnetic effect of electric current. The basic principle is to use excitation current coil to create eddy current in the conductive prefabricated component, and then measure the variation of eddy current by the detection coil, so as to obtain the relevant information content of defects in the prefabricated component. According to the different appearance of the detection coil, it can be divided into three types: cross type (for the inspection of cables, rods and pipe fittings), camera type (for the inspection of parts of the surface of prefabricated components) and plug type (for the internal inspection of pipe orifice). The eddy current inspection method shall be used to verify that the magnetic flux density around the coil is above 80% of the magnetic flux density required to magnetize the seamless steel tube in the saturated state. Therefore, the magnetization current should be selected according to the material and specifications of seamless steel tube before flaw detection.

The selection of magnetization current is generally carried out according to the comparison of specimens. Theoretically speaking, the magnetic flux density of the tested seamless steel tube to be magnetized in the saturation state should be measured before selection, and then the magnetization current should be adjusted according to the provisions. This kind of method should be carried out complex calculation. In the process of operation, a simple adjustment mode can be selected, that is, in the back and forth according to the specimen, along with the gradual expansion of the magnetization current, in addition, the change of the noise data signal of the display information of the instrument and equipment and the data signal of human defects can be observed. When the noise data signal is minimal and the manpower defect data signal is large, the magnetization current is the basic fit. According to the general rules, the larger the specification, the thicker the wall thickness, the softer the magnetic characteristics of the raw material, the greater the required magnetization current, and the smaller the contrary. Eddy current flaw detection is only applicable to conductive raw materials, only to examine the surface or near the surface layer of defects, trouble to apply to the appearance of complex prefabricated components. In the thermal power station, it is mainly used in the inspection of condenser tube, turbine generator leaf, steam turbine rotor management center hole and welding. Rationale When AC current enters the coil, if the common operating voltage and frequency do not change, the current through the coil will not change either. If a metal hose is inserted into the coil, the surface of the pipe induces an axial current, an eddy current. The direction of the eddy current magnetic field is opposite to the magnetization direction of the additional current, so that the offset part of the additional current will be added, and then the characteristic impedance of the coil and the phase difference will change according to the size of the current. The characteristic impedance of the coil can be compromised by the diameter of the tube, the thickness of the tube, the changes in the conductivity and permeability of the tube, and the existence of defects. If other elements are maintained unchanged, only the data signal caused by the characteristic impedance caused by the defect is removed, and the instrument and equipment are enlarged and inspected, the inspection target can be achieved. The eddy current data signal can not only obtain the size of the defect, but also because the eddy current under the surface can be detected according to a certain phase difference between the eddy current under the surface and the surface eddy current, the phase difference analysis can be used to identify the bit T (deep) of the defect. The dissipation resistor of the eddy current safety channel and the anti-magnetic flux caused by the eddy current reflect the surface to the camera coil, changing the current size and phase difference of the coil, that is, changing the characteristic impedance of the coil. Therefore, video camera in the metal material surface move, encounter defects or the change such as material, specifications, prompt reaction of eddy current electromagnetic field on the coil, coil characteristic impedance transformation, based on the eddy current testing instrument precision measure this kind of variation can discern whether metal material surface defects, or other chemical shift.



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