- Since the 1930s, with the rapid development of high-quality strip steel continuous rolling production and the progress of welding and inspection technology, the quality of the weld seam is constantly improving, the variety of specifications of welded steel pipe is increasing, and in more and more areas to replace the seamless steel pipe. Welded steel pipe is divided into straight seam welded pipe and spiral welded pipe according to the form of weld seam. Classification by production method: process classification - arc welded pipe, resistance welded pipe, (high-frequency, low-frequency) gas welded pipe, furnace welded pipe.
- Smaller diameter welded pipe using straight seam welding, large diameter welded pipe is more spiral welding; according to the shape of the end of the pipe is divided into round welded pipe and shaped (square, rectangular, etc.) welded pipe; according to the material and use of different types of welded steel pipe for fluid transport in mines, galvanised welded steel pipe for low-pressure fluid transport, welded steel pipe for belt conveyor rolls, and so on.
- Welded pipe commonly used materials are: Q235A, Q235C, Q235B, 16Mn, 20#, Q345, L245, L290, X42, X46, X60, X80, 0Cr13, 1Cr17, 00Cr19Ni11, 1Cr18Ni9, 0Cr18Ni11Nb and so on.
- Eddy current flaw detection, is one of the five conventional non-destructive testing methods. It can quickly and efficiently detect defects on and near the surface of the conductor, and is widely used in the detection of tubes and wires in aerospace, nuclear energy, petroleum and boiler fields.
- The basic principle of eddy current flaw detection:
Eddy current is a non-destructive testing method based on electromagnetic induction.
It is suitable for detecting defects on or near the surface of electrically conductive materials, and is used in the field of continuous oil pipe to mainly detect surface defects on steel plates, surface defects on continuous oil pipes, and welding defects on longitudinal welds of continuous oil pipes.
- How the eddy current flaw detector detects continuous oil pipe:
According to the classification of the method of use, the detection coil can be divided into: absolute coil, self-ratio coil, he ratio coil. Absolute coils and self-ratio coils are often used in the manufacturing process of continuous oil pipe.
The eddy current detector generates alternating current to supply the excitation coil, the excitation coil generates alternating magnetic field, the alternating magnetic field generates eddy currents on the surface of the continuous oil pipe, the eddy currents are affected by the defects and in turn affects the absolute coil or self-comparison coil to make it change, and then through the amplification and processing of signals, it gets the absolute phase signal, the absolute amplitude signal and the self-comparison phase signal and self-comparison amplitude signal.
- The characteristics of eddy current flaw detection methods are:
① No coupling agent is needed, and it can be carried out non-contact;
② fast flaw detection, can achieve online production (up to 3 m / s, 2m / s in normal production), in addition to adjusting and replacing the specification time is short (generally 10 ~ 20min);
③ It is possible to detect the weld seam of high temperature welded pipe;
④ high reliability of flaw detection results. Due to the fast speed of eddy current flaw detection, through the coil eddy current flaw detection method has been widely used for many years to detect the quality of welded pipe, especially its denseness, but it is difficult to detect the longitudinal cracks in the welded seam of ferromagnetic welded pipe.
- Therefore, in the high standard of eddy current detection, generally use the probe coil eddy current detection method. For the "internal burr adsorption" that often occurs in welded pipe seams, eddy current testing equipment often misreported it as a defect.
Frequent emergence of eddy current flaw detection process due to the electromagnetic force within the burr adsorption on the weld, leaving the eddy current magnetic field after the steel pipe contact automatic demagnetisation device, no magnetic attraction within the burr automatically fall, and then manual re-examination and no defects to show the embarrassing problem. Due to the skin effect, with the increase in the distance between the inspected welded pipe and the detection coil, its detection sensitivity will gradually decrease, therefore, for the same size defects, in the inner wall of the pipe reflected in the signal amplitude will be smaller than the defects on the outer wall of the pipe.
- Welded pipe full-body eddy current flaw detection equipment in the detection of defects on the outer and inner surfaces of the welded pipe, is determined by a variety of factors, but the most important still depends on the wall thickness of the inspected welded pipe and the eddy current excitation frequency and magnetic saturation intensity; at the same time, in the selection of instrument parameters, should also take into account the inspected welded pipe electrical conductivity and permeability and other physical parameters of the impact on the accuracy of the eddy current flaw detection.
