- Leakage Magnetic Flaw Detection of Welded Pipe Full Body
Leakage magnetic flaw detection refers to the non-destructive testing technology that the surface and near-surface defects of ferromagnetic materials form a leakage magnetic field on the surface of the material after being magnetised and the defects are found through the detection of the leakage magnetic field.
Leakage magnetic flaw detection on the surface state of the pipe does not require high, the detection depth is larger, in foreign welded pipe testing is used in a large number of domestic, especially for petroleum welded pipe testing has also been widely used.
In the production test, there has been a leakage of magnetic flaw detection of welded pipe through the wall of the phenomenon of large holes, in addition to the management and personnel factors, which has a relationship with the instrument, probe performance and defect size and shape.
The main factors affecting the accuracy of welded pipe leakage magnetic flaw detection of the whole pipe body are as follows.
(1) Magnetisation strength
When the magnetisation strength is low, the leakage field is small and increases slowly; when the magnetic induction strength reaches about 80% of the saturation value, the peak value of the defect leakage field increases rapidly with the increase in magnetisation strength, but when the ferromagnetic material enters the magnetic saturation state, the increase in the external magnetisation strength does not have much effect on the defect magnetic field strength.
Therefore, the design of the magnetic circuit should be as close as possible to the material under test to reach the state of near-saturation magnetisation.
(2) The direction, location and size of the defect.
The direction of the defect has a great influence on the accuracy of magnetic leakage detection. When the main plane of the defect is perpendicular to the direction of the magnetised magnetic field, the strongest magnetic leakage field is generated.
The same defects are located in the pipe surface when the leakage field is the largest, and with the increase in the depth of burial and gradually decrease, when the depth of burial is large enough, the leakage field will tend to zero.
Therefore, can be used to detect the welded pipe wall thickness is generally 6 ~ 15mm; reduce the sensitivity of the case, can be detected wall thickness of 20mm.
The size of the defect size of the leakage field also has a great impact, when the defect width and depth of the same time, the leakage field with the increase in the depth of the defect increases, within a certain range of the two approximately into a straight line relationship.
The effect of defect width on the leakage field is not monotonous, in the defect width is very small, with the increase of the width of the leakage field has a tendency to increase; but when the width is large, the width increases, the leakage field instead of a slow decline.
(3) Lift-off value
When the lift-off value is more than two times the crack width, the leakage field strength decreases rapidly with the increase of the lift-off height.
The design of the sensor bracket must make the probe in the detected steel pipe surface sweeping the lift-off value to maintain a constant, generally less than 2mm, often take 1mm.
(4) flaw detection speed
In the flaw detection process should try to maintain a uniform speed, different speeds will result in a different shape of the magnetic leakage signal, but generally does not cause misjudgment.
(5) Welded pipe surface quality
Welded pipe surface thickness of paint and other coatings on the sensitivity of the flaw detection is very large, with the increase in coating thickness, the sensitivity of the flaw detection decreases sharply.
From the current instrument performance, when the coating thickness ≥ 6mm, has been unable to obtain an effective defect recognition signal.
Welded pipe surface roughness of the different sensors and the detected surface of the lift-off value of the dynamic changes that will affect the consistency of the flaw detection sensitivity, in addition to causing system vibration and noise, so the surface of the welded pipe to be detected should be as smooth and flat as possible.
Welded pipe surface oxide and rust and other debris, may also produce pseudo-signal in the detection process, should be confirmed or re-examined in a timely manner.
- the welded pipe body eddy current flaw detection
The characteristics of eddy current flaw detection methods are:
① does not require coupling agent, can be non-contact;
② flaw detection speed, to achieve online production (up to 3 m / s, 2m / s in normal production), in addition to adjusting and replacing the specifications of a short time (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, the pass-through coil eddy current flaw detection method has been widely used for many years to test the quality of welded pipe, especially its denseness, but it is difficult to detect the longitudinal cracks in the weld 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 welds, eddy current flaw detection equipment often misreported as defects .
Frequent emergence of eddy current flaw detection process within the burr due to electromagnetic force adsorption on the weld, leaving the eddy current magnetic field after the steel pipe contact automatic demagnetisation device, no magnetic attraction after the inner burr automatically fall , and then manual re-exploration 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, the signal amplitude reflected in the inner wall of the pipe will be smaller than the defects on the outer wall of the pipe.
Welded pipe body eddy current 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 eddy current excitation frequency and magnetic saturation strength;
At the same time, in the selection of instrument parameters, should also take into account the physical parameters of the inspected welded pipe electrical conductivity and magnetic permeability and other physical parameters on the eddy current flaw detection accuracy.
- the whole pipe ultrasonic flaw detection
Welded pipe ultrasonic flaw detection is mainly divided into direct contact and liquid immersion method.
(1) Direct contact method
Ultrasonic probe and specimen detection surface is coated with a very thin layer of coupling agent, so it can be regarded as a direct contact between the two. Direct contact method is easy to operate, simple detection graphics, easy to judge, detect defects with high sensitivity, is the most common method used in the actual flaw detection. However, this method requires high accuracy of the detected surface of the tested specimen.
(2) Liquid immersion method
The ultrasonic probe and workpiece immersed in liquid as a liquid coupling agent for flaw detection method, known as liquid immersion method. Coupling agent can be oil, can also be water. Liquid immersion method is divided into full immersion and partial immersion.
This method is suitable for surface roughness of the specimen, the probe is not easy to wear, coupling stability, good reproducibility of the detection results, easy to achieve automated flaw detection.
However, the liquid immersion method in the propagation of sound waves in the pipe wall attenuation is very serious, a better solution is to replace the water film method instead of water immersion method for acoustic coupling.
- welded pipe seam ultrasonic flaw detection
(1) Welded seam offline flaw detection
Welded pipe is sent to the flaw detection area by the transmission roller conveyor, the rotating roller is raised, and the welded seam of the steel pipe is manually rotated on the roller to a certain point of the welded seam position.
Flaw detection trolley running, each group of probes in order to delay the drop in the welded pipe, the coupling agent for flaw detection for the welded pipe production line emulsion, accepting the sensing signals automatically sprayed on the weld seam and both sides.
Because the ultrasonic offline system is arranged in the water cooling and air cooling, the influence of weld deviation becomes very important, so in the weld offline flaw detection, welded pipe weld longitudinal line defect detection using two groups of six probes, each group of three probes, distributed on both sides of the weld seam, bi-directional detection, occupying a total of six channels, in the weld seam torsion (deviation) within the scope of the 15mm can be normal flaw detection.
A single longitudinal probe probe coverage for the vertical direction of the weld 12.7mm (6.4mm on each side of the weld), in the vertical direction of the weld 12.7mm wide area of the weld thickness (maximum 13.72mm) direction without blind coverage.
Weld transverse line defects and point defects detection should also take into account a small amount of torsion (deviation) of the steel pipe weld.
Therefore, the use of two groups of six probes, each group of three probes, before and after the distribution, even if the weld is twisted (deviation), the probe can always cover the detection area, due to the coverage of a wide area, the weld can be swept in and out of the left and right 5mm, to achieve the full coverage of the weld bi-directional detection, to better guarantee the quality control accuracy of the flaw detection, transverse probes take up a total of six channels. The total number of channels occupied by the equipment is 12.
