Eddy Current Testing and Magnetic Flux Leakage Testing of Seamless Steel Tube

Apr 16 20:44 2014 Allice S Lee Print This Article

With the development of the national economy, all kinds of seamless steel tubes are widely used in petrochemical and boiler manufacturing and other industries, particularly high temperature, high pressure and harsh working conditions, on the quality of seamless steel tubes have higher requirements. 

Seamless steel pipe usually after smelting,Guest Posting casting, billet, rolling and drawing processes is made, and its defects on the slab in addition to a variety of metallurgical defects caused during the forming process, an extension along the circumferential direction of the pipe axis laminar imperfections, but also because of improper processing operation process, drawing die roll or other reasons caused by poorly designed cracks, folding, warping, scratches or other surface and internal strain on the pipe at each stage of the production process defects. To ensure the quality seamless steel pipe, according to the relevant technical standards, the seamless steel pipe production line is required for surface and internal NDT. Eddy current testing is to establish a non-destructive testing methods based on electromagnetic induction, which will probe sinusoidal current excitation coil, seamless steel pipe when the probe is close to the surface, alternating magnetic field is generated around the coil induced electromotive force induced in the steel surface current, that eddy currents. The eddy current and the induced magnetic field acting on the coil, thereby changing the electrical parameters of the coil. Eddy current loss resistance passage, and anti-eddy current magnetic flux generated by the coil to the probe and reflected, and phase change of the size of the coil current, i.e., the impedance of the coil changes. Moving the probe relative to the surface of the seamless pipe when the defect is encountered, so that the reaction of the eddy current type magnetic field to the coil, causing the coil impedance variation, which measured the amount of change identified this seamless steel surface defects. Eddy current testing is one of the hydrostatic test may be substituted detection methods. Eddy current flaw detection result can be directly output to an electrical signal, easy to automate testing. Non-contact eddy current testing methods, testing quickly. The basic principle of the magnetic flux leakage testing of seamless steel tubes based on the high-permeability properties of the ferromagnetic material, a ferromagnetic seamless steel is magnetized surface and near surface defects in seamless pipe surface leakage magnetic field , by detecting the change in magnetic field leakage signal can be detected to be defective. Defects in seamless pipe is much less than the permeability of the magnetic permeability of the steel pipe. When seamless steel pipe probe into pipelined line, fast through the detection zone will be subject to a DC coil magnetization transverse and longitudinal magnetic field. If no defects seamless steel pipe, seamless steel pipe by the magnetic field lines substantially, when the magnetic field lines uniformly distributed; if defective seamless steel pipe, the magnetic field lines are curved, and a portion of the leakage magnetic fluxes seamless steel surface. The use of horizontal and vertical probe coil (sensor) detects escaping seamless steel pipe leakage field surface, and then based on Faraday's law of electromagnetic induction, the leakage magnetic field into the defect signal (detection coil induced voltage generated in the probe), the defect signal for further processing and analysis, to determine whether there are defects and defect-related dimensions. Line eddy current testing, flaw detection through the use of the outer surface coil method, the inner surface flaw detection coil by interpolation method. The outer surface of the coil fixing common flaw was seized and sent seamless steel coil axially fixed seamless steel screw feed was seized in two ways. The mechanical transmission device is relatively simple testing equipment. Pipeline magnetic flux leakage testing methods are commonly used to detect the probe rotated straight forward and seamless steel pipe seamless steel screw fixing probe forward in two ways. Mechanical transmission detection equipment more complicated. Eddy current testing of seamless steel tubes suitable for surface inspection of a variety of conductive materials, regardless of seamless steel pipe is ferromagnetic or non-ferromagnetic, but also whether it is ferrous metals, non-ferrous metal or non-metal or as long as seamless steel is conductive , and the diameter and wall thickness to meet certain conditions can be detected using the eddy current method. It is more appropriate to detect common defects in the outer surface of the thin-walled seamless steel is widely used in carbon steel, alloy steel and stainless steel seamless pipe surface defect detection. Only magnetic flux leakage testing of seamless steel tubes suitable for the detection of ferromagnetic, it can detect the inner, the outer surface flaws of seamless steel pipe. This method is more suitable for large diameter thick-walled seamless steel ferromagnetic detection. It shows the basic characteristics of the eddy current, the eddy current density distribution in the vicinity of the main surface of the conductive material. Therefore, the more measured seamless steel surface defects exist, the use of eddy current effects more fully. Therefore, eddy current testing of seamless steel tubes for the detection of surface defects or near the conductive surface defects, in which case the detection sensitivity is higher than the magnetic flux leakage. For internal defects due to the presence of eddy current testing the "skin effect", the eddy current density in the conductors is an internal negative exponential decay law, and as the frequency increases, the conductivity and permeability of the penetration depth is reduced, the detection sensitivity. Eddy current testing is generally only one side of the surface defects detected seamless steel (inner surface or outer surface); detector can detect the magnetic flux leakage and external surface flaws of seamless steel pipe, the internal defects have a certain sensitivity. Relative to the eddy current, magnetic flux leakage testing to detect defects in low sensitivity. Eddy current and magnetic flux leakage testing are using the appropriate standard reference sample tube to verify comprehensive performance testing system setup and calibration detector operating parameters and operating conditions, and the comparative evaluation of the defect signal detection. Both methods are based on the detection output of an electric signal to a display defect, the signal processing more convenient. However, due to different physical principles, the signal processing and the sensor structures of the two methods are quite different. Eddy current testing due to the high-frequency excitation signal must exist to bring some of the signal processing difficulties, prone to signal interference with each other, signal processing and sensor structure is more complex. Detecting magnetic flux leakage can be a permanent magnet magnetization or AC DC magnetization, and DC magnetization of the permanent magnet, no high-frequency signal is present, interference, a great convenience to the signal processing, signal processing and sensor structure is simple.

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