With the development of the national economy, all kinds of seamless steel pipes are widely used in petrochemical, boiler manufacturing industry, especially the harsh working conditions such as high temperature, high pressure, seamless steel pipe quality higher requirements. Seamless steel pipe generally made after smelting, casting, cogging, rolling and drawing processes, defects in addition to the the various metallurgical defects on the slab during the forming process, along the pipe axially extending circumferential points laminar imperfections in the various stages of the production process, but also because of the improper process of machining operations, roll or drawing mold design improper reasons in the steel pipe caused the crack, folding, warping, scratches, or strained surface and internal defects. In order to ensure seamless steel tube quality, in accordance with the technical standards for products, the seamless steel pipe production line required nondestructive testing of surface and interior. Seamless steel pipe often used NDT eddy current testing and magnetic flux leakage testing two technologies, they have their own characteristics and scope of application.
Principles and characteristics of the eddy current testing
Eddy current testing is the establishment of a non-destructive detection method on the basis of the electromagnetic induction, it sine current excitation probe coil, the coil surrounding the alternating magnetic field generated in the pipe surface when the probe near the Seamless surface induced electromotive force generates an induction current, i.e. vortex. The eddy current and generates an induction magnetic field acting on the coil, thereby changing the electrical parameters of the coil. The loss resistance of the vortex channel and the eddy-current generation of anti-flux, but also reflected to the probe coil, changing the size and phase of the current in the coil, which changes the impedance of the coil. The probe in the seamless steel surface is moved relative defects encountered, so that the reaction of the eddy current magnetic field of the coil, causing the change of coil impedance, thereby measuring the amount of change identified seamless steel pipe surface defects. The eddy-current flaw detection method of one of the hydrostatic test may be substituted. All conductive seamless steel pipe, whether ferromagnetic or non-ferromagnetic, as long as the outer diameter greater than 2 mm, the wall thickness is not less than 011mm, the eddy current method may flaw. The eddy current test results can be directly output as an electrical signal, easy to automate testing. The eddy-current flaw detection using non-contact manner, the fast testing. Eddy current testing can detect defects limited depth, generally not more than 5 mm, and the test results are often affected by the function of testing instruments and testing conditions affect.
Principles and characteristics of the magnetic flux leakage testing
Seamless steel pipe of the basic principle of the magnetic flux leakage inspection is built on top of this feature of the high magnetic permeability of the ferromagnetic material, a seamless steel pipe of the ferromagnetism is magnetized, its surface and near surface defects in the surface of the seamless steel pipe to form a leakage magnetic field , by detecting the leakage magnetic field change signal can be detected in the presence of a defect. The seamless steel defects permeability much smaller than the magnetic permeability of the steel pipe. Seamless pipelined manner into the steel pipe detection wire, quickly passes through the detection zone will be subject to DC coils produce horizontal and vertical magnetic field magnetization. If seamless steel no defects, the most of the magnetic lines of force through the seamless steel pipe, the uniform distribution of magnetic field lines; if seamless steel defects, the magnetic lines is curved, and has a part of the leak magnetic field lines to a seamless steel pipe surface. Lateral and longitudinal coil probe (sensor) to detect the leakage magnetic field of the seamless steel pipe surface escape, then based on Faraday's law of electromagnetic induction, the leakage magnetic field is converted to the defect signal (the induced voltage generated by the detection coil in the probe), and further processing of the defect signal. and analysis, you can determine whether a defect exists and defect size parameters.
The pipeline detection method comparison
Pipeline eddy current flaw detection, through the coil to the outer surface of the flaw, the inner surface flaw detection using the interpolation formula coil method. The outer surface flaw Top coil fixed subject seamless steel axial feeding and the coil fixed to the subject seamless steel screw feed two methods. The mechanical transmission of the testing equipment is relatively simple. Pipeline magnetic flux leakage flaw detection method the probe rotation seamless steel pipe straight forward and probe fixed seamless steel pipe spiral forward in two ways. Flaw detection equipment, mechanical transmission is more complex.
Compare eddy current detection performance
Detecting the surface detection of the seamless steel pipe applicable to a variety of conductive materials, regardless Seamless is ferromagnetic or non-ferromagnetic, and regardless of whether it is black metal, or non-ferrous metal or a nonmetal, Seamless is conductive and the outside diameter and wall thickness to meet certain conditions can be detected using the eddy current method. It is more appropriate in the ordinary thin-walled seamless steel pipe outer surface defect detection, widely used in carbon steel, alloy steel and stainless seamless steel pipe surface defect detection.
Seamless steel tubes for the two surface-line testing methods have their own advantages and limitations, seamless steel pipe production line depending on the particular circumstances choose. The eddy-current flaw applies to various conductive seamless pipe testing, either stainless steel or carbon steel seamless steel tubes are suitable for detecting the influence of the surface of the oil and other impurities from seamless steel. Detection speed and high sensitivity. The magnetic flux leakage test can be detected not only the inner and outer surfaces, and subcutaneous defects, and without measurement from the established electrical signal amplitude and defect parameters relationship can informed of the defect depth and length characteristics such as size and whether the level of the set reject . Detection ability, speed detection. Seamless steel pipe, but it only applies to the ferromagnetic detection equipment investment, the lower the sensitivity of the detection of defects.