What are the common welding defects? What are the inspection methods?

Date：2022-05-26 17:01:52Views：938

There are many kinds of weld defects, which can be divided into internal defects and external defects according to their position in the weld. External defects are located on the outer surface of the weld, which can be seen with the naked eye or low magnification. For example, the weld size does not meet the requirements, undercut, weld bead, crater, air hole, crack, slag inclusion, incomplete penetration, incomplete fusion, etc. Internal defects are located inside the weld. Such defects are found by destructive inspection or flaw detection methods, such as incomplete penetration, incomplete fusion, pores, cracks, slag inclusion, etc.

Undercut: undercut refers to the occurrence of grooves in the transition zone between the weld edge and the pipe body along the weld centerline. Undercut is caused by improper matching of welding speed, current, voltage and other conditions.

Lap welding: the upper and lower edges of the steel plate are staggered and butted, resulting in the uneven phenomenon of the weld, which becomes the dislocation of the pipe seam or lap welding of the pipe seam.

Weld bead, a metal bead formed when molten metal flows to the base metal that is not melted outside the weld during welding.

Overburning: during welding, the molten metal flows out from the groove at too high temperature, forming weld defects.

Welding deviation: the weld bead deviates from the welding center line, resulting in the phenomenon of weld deviation.

Air hole is a hole formed in the weld when the bubbles in the molten pool fail to escape during solidification. Stomata can be divided into dense stomata, striped stomata and needle stomata.

Slag inclusion: during welding, the bubbles in the molten pool fail to escape during solidification and remain in the hole formed in the weld. Stomata can be divided into dense stomata, striped stomata and needle stomata.

Incomplete penetration, the phenomenon that the root of the joint is not fully penetrated during welding, also refers to the phenomenon that the weld depth does not meet the design requirements.

Thermal crack: in submerged arc welding, thermal crack can be generated in the weld, especially in the arc starting and arc extinguishing crater, which is prone to thermal crack due to large temperature difference. Hot cracks are most likely to occur when the weld stress is large or when the Si content in the weld metal is high.

Welding gray spot is a unique welding defect of high frequency resistance welding (HFW). It is characterized by a gray area without metallic luster on the macro port of the weld of the tensile sample or impact sample. It is generally believed that gray spots have no obvious influence on the strength level of the weld, but have a great influence on the toughness and plasticity of the weld.

Groove corrosion, groove corrosion is a special corrosion phenomenon in ERW steel pipe welds. The selective local corrosion of resistance welded pipes serving in seawater, industrial water and other media in the welding area is called groove corrosion. Most of them are continuous or discontinuous grooves from the surface, which can lead to the corrosion perforation of welded pipes in one to several years.

Indentation pit, roll pitting or hard object between roll surface and pipe blank, resulting in low indentation on the surface of pipe.

Common methods of welding defect inspection

1. Appearance inspection usually depends on visual observation. With the help of some tools, the accuracy of inspection can be greatly improved. The commonly used tools include: weld inspection gauge, tape, steel ruler, low power magnifying glass, etc. generally, it is to inspect the defects outside the weld.

2. Air tightness inspection generally refers to the air tightness inspection of melters, pipes, etc., which shall be carried out according to the different requirements of the tested object. ① For the submerged test, place the container filled with a certain pressure in the water tank and press it down to a certain depth, and then rotate it slowly to observe whether there are bubbles on the container to determine whether there is leakage. ② For the inspection of soapy water, apply it to the weld successively with a brush dipped in soapy liquid on the container filled with a pressure gas. If there are no bubbles, it is qualified.

3. Kerosene test, which uses the strong penetration ability of kerosene to test the tightness of the weld. Apply lime water on one side of the weld (outside of the container), and then apply kerosene on the other side of the weld (inside of the container) after the lime water is dry to test whether there are oil spots on the white lime.

4. Pressure test, also known as pressure test, includes hydraulic test and pneumatic test. The method of pressure test or pressure holding test is to check whether the vessel passes the pressure test or pressure holding test. The test pressure shall be higher than the working pressure, otherwise the safe operation of the container cannot be guaranteed. Pressure test is used to evaluate the overall strength performance, deformation and leakage of welded components such as boiler, pressure vessel and pressure pipeline.

On the one hand, the pressure test can test the compactness of the structure, on the other hand, it can also test the strength of the structure. Hydrostatic test: close the water valve when it is filled with water and completely exhaust the air, and then pressurize the container by stages with high-pressure water pump until it reaches the test pressure (generally 1.25 ~ 1.5 times of the working pressure); Check whether there are water drops (leakage) on the weld. If so, it indicates that there is leakage;

Check the pressure holding condition. After stopping pressurization, hold the pressure for 5 ~ 10min, and the pressure shall not drop significantly. For the air pressure test, the high-pressure air pump is used to step up the pressure of the container step by step, and the pressure is maintained for a certain time every step until it rises to the specified test pressure. Check whether there is leakage with soapy water and check the pressure holding condition.

5. Radiographic testing: in the process of penetrating the material, the intensity of the ray is weakened and attenuated due to absorption and scattering. The attenuation degree depends on the attenuation coefficient of the penetrating material and the thickness of the penetrating material. If there are defects inside the Transilluminated workpiece and the attenuation degree of the defect medium and the inspected workpiece to the ray is different, the intensity of the ray passing through the workpiece will be different and the sensitivity of the film will be different, After being processed in the darkroom, the defective part on the negative film has a large blackness, which can be used by the film evaluator to judge the defect. Radiographic testing shall be conducted by personnel with full-time qualification certificates.

6. Ultrasonic testing, which uses the acoustic characteristics of ultrasonic propagation in the medium to detect the internal or surface defects of metal materials and their workpieces. When the ultrasonic wave propagates in the metal, it will be reflected at the interface. During the detection, the ultrasonic wave has reflected pulses on both surfaces of the workpiece. If there is a defect inside the workpiece, the third pulse will appear between the pulses of the two boundaries. The defect position can be determined according to the position of this pulse. Ultrasonic flaw detection equipment is relatively light and flexible, with a wide detection range.

7. According to magnetic particle testing, the permeability of ferromagnetic metal material is much higher than that of air. When it is magnetized in the magnetic field, the magnetic lines of force will be concentrated in the material. If there are defects such as pores, cracks and slag inclusions on the surface or near the surface of the material, the magnetic lines of force will be difficult to pass through these defects, so a local magnetic leakage field will be formed at the defect. At this time, sprinkle magnetic particles on the material, and the magnetic particles will be attracted by the magnetic leakage field, Then it shows the macro trace of the defect. Demagnetization treatment shall be carried out for the workpiece after magnetic particle testing.

8. Other tests: ① magnetic yoke test; ② Penetrant testing; ③ Eddy current testing; ④ Bending test; ⑤ Impact test; ⑥ Metallographic examination.

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