In the process of fabrication and use of the welded pipe, nondestructive testing is very important to ensure weld quality.

The commonly used NDT methods for welded pipe are as follows: suitable for off-line magnetic flux leakage (MFL) testing, eddy current testing, and ultrasonic testing of the whole pipe body above 5mm from the welded pipe surface;

On-line MFL and eddy current testing to verify the welding quality above 5mm on the surface of the tale-welded pipe; It is suitable for off-line ultrasonic flaw detection of the whole pipe body of thick wall welded pipe. Ultrasonic flaw detection to verify the welding quality of thick wall welded pipe.

In this paper, combined with production experience, the commonly used methods and techniques of flaw detection for welded pipes are briefly introduced, and their advantages and disadvantages are analyzed and compared.

I. Magnetic leakage testing of the whole welded pipe body

Magnetic flux leakage (MFL) testing refers to the non-destructive testing technology of detecting the magnetic field leakage on the surface and near-surface of the ferromagnetic material after it is magnetized.

Magnetic flux leakage testing has low requirements on the surface state of the pipe and a large detection depth. It is widely used in the inspection of welded pipe abroad, especially in the inspection of welded pipe for petroleum, which has been widely used in China.

In production testing, there has been a phenomenon that magnetic leakage testing can not detect large holes through the wall of welded pipes. In addition to management and personnel factors, this is related to the performance of instruments, probes, and the size and shape of defects.

2. Eddy's current inspection of the whole welded pipe body

The characteristics of the eddy current inspection method are as follows:

① It does not need a coupling agent, and can be carried out non-contact;

(2) Fast detection speed, can realize online production (up to 3 m/s, 2m/s in normal production), in addition, the adjustment and replacement of specifications time is short (generally 10 ~ 20min);

(3) can be high-temperature welded pipe weld detection;

(4) High reliability of inspection results. Due to the fast speed of eddy current inspection, the through-coil eddy current inspection method has been widely used for many years to detect the quality of the welded pipe, especially its compaction, but it is difficult to detect the longitudinal cracks in the weld of ferromagnetic welded pipe.

Therefore, in the high-standard eddy current inspection, the probe coil eddy current inspection is generally used. For the "internal burr adsorption" often produced in welded pipe welds, eddy current testing equipment often misreports as defects.

In the process of eddy current testing, the inner burr is often adsorbed on the weld by electromagnetic force, and the steel pipe contacts the automatic declassification device after leaving the eddy current magnetic field. The inner burr drops automatically when there is no magnetic force attraction, and then the manual retesting is carried out without a defect display.

Due to the skin effect, the detection sensitivity will gradually decrease with the increase in the distance between the inspected welded pipe and the detection coil. Therefore, for defects of the same size, the signal amplitude reflected in the inner wall of the tube will be smaller than that on the outer wall of the tube.

The ability of eddy current inspection equipment to detect defects on the outer and inner surfaces of the welded pipe is determined by many factors, but the most important is the thickness of the welded pipe wall, the frequency of eddy current excitation, and the intensity of magnetic saturation.

At the same time, when selecting the instrument parameters, the influence of physical parameters such as the conductivity and magnetic permeability of the welded pipe on the eddy current inspection accuracy should also be considered.

3. Ultrasonic flaw detection of the whole welded pipe body

Ultrasonic flaw detection of the whole welded pipe body is mainly divided into the direct contact method and the liquid immersion method.

(1) Direct contact method

The ultrasonic probe and the detection surface of the specimen are coated with a very thin coupling layer, so they can be regarded as direct contact. The direct contact method has the advantages of convenient operation, simple flaw detection pattern, easy judgment, and high sensitivity in detecting defects. It is the most commonly used method in actual flaw detection. However, this method requires high accuracy of the detection surface of the tested specimen.

(2) Liquid immersion method

The ultrasonic probe and the workpiece are immersed in liquid with liquid as a coupling agent for the flaw detection method, known as the liquid immersion method. The coupling agent can be either oil or water. The liquid immersion method is further divided into full immersion and local immersion.

The method is suitable for specimens with a rough surface, the probe is not easy to wear, the coupling is stable, the detection results are repeatable, and it is easy to realize automatic detection.

However, the acoustic wave propagation attenuation in the tube wall is very serious in the liquid immersion method. A better solution is to use the water film method instead of the water immersion method for acoustic coupling.

conclusion

(1) Magnetic leakage testing has high sensitivity, which can well distinguish the defects of the inner and outer walls of the welded pipe, but the long pipe body and the large wall thick pipe need to be degaussed after magnetic leakage testing.

(2) Eddy's current testing has a fast detection speed but is limited by skin effect, it is difficult to find the defects deep in the workpiece.

(3) Ultrasonic inspection has the characteristics of strong detection penetration, accurate defect location, low cost, and fast speed, but the inspection operation needs coupling (the water film method is recommended instead of the water immersion method for acoustic coupling). In the harsh winter environment in the north, the coupling welded pipe is easy to freeze, which brings inconvenience to inspection operations.

(4) In the construction of new projects and the production of welded pipes, the selection of detection methods, equipment selection, and process technology should be considered comprehensively according to the characteristics of welded pipes, quality requirements, implementation standards, and economic conditions of enterprises.

Cangzhou Shenlong Pipe Manufacturing Co., Ltd. is a high-tech enterprise specializing in the manufacture and sales of high-frequency straight-seam welded pipe. The main products of the company are oil and gas bushing, tubing, pipeline pipe, pile pipe, structural pipe, etc., with an annual design capacity of 100,000 tons. The company's products execute API SPEC 5CT, API SPEC 5L, FM 1630, UL 852, JIS G3444, JIS G3452, ASTM A53/A53M, AS/NZS 1163, AS/NZS 1074, AS/NZS 1396, EN10217, EN10219, EN10255, and GB/T9711, GB/T3091, and other domestic and foreign standards. Products are exported to overseas markets and widely used in petroleum, natural gas, solar energy, shipbuilding, automobile, chemical, environmental protection, boiler, fire protection, water conservancy, electrical industry, machinery and equipment, steel structure, construction, and other industries.