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How to inspect welding products

2020-2-25

 
 
Welding is a manufacturing process and technology that joins metal or other thermoplastic materials such as plastic by heating, high temperature or high pressure. The welding process is widely used in machinery, construction, electronics and other fields. It mainly plays a key role in connection, sealing, and conduction. So it is very important for the quality control and inspection of welded parts!

 

 



According to commonly used welding standards, welding quality inspection includes the following three links:

A. Pre-weld inspection

1. Inspection of raw materials, including inspection of base materials, welding electrodes (welding wires), shielding gas, flux, electrodes, etc., whether they are in compliance with the certificate of conformity and national standards, whether the packaging is damaged, whether it has expired, etc.

2. Welding structure design and inspection of welding technical documents, whether the weldment structure is reasonable in design, convenient for welding, easy to guarantee the quality of welding, and whether the process requirements are fully expressed; whether new materials, new methods and new processes are subjected to welding process qualification tests .

3. Check the technical process of the welder, and clarify the welding process requirements, welding quality requirements and safety precautions.

4. Welding equipment quality inspection, including whether the welding equipment model, power source polarity meets process requirements, whether the welding torch, cable, gas pipe, welding auxiliary tools, safety protection, etc. are complete.

5. Check the assembly quality of the workpiece, including whether the assembly quality meets the requirements of the drawing, whether the bevel surface is clean, whether the fixture is installed, whether the assembly gap meets the requirements, and whether the welding shrinkage is considered.

6. Welder qualification inspection, including whether the welder qualification is within the validity period, and whether the test items are compatible with the actual welding, including welding methods, welding materials and workpiece specifications.

7. Inspection of the welding environment, including whether to consider wind, rain, snow (humidity) attacks in the welding environment and take protective measures.

When the welding environment temperature is lower than the allowable value of the specification, it is compatible with the welding material, weldment thickness and preheating measures.

B. Inspection during welding

1. Whether the welding process requirements have been performed during welding, including welding methods, welding materials, welding specifications (current, voltage, line energy), welding sequence, welding deformation and temperature control.
2. Whether there are surface defects such as cracks, pores, and slag inclusion between welding layers.


C. Post-weld inspection

Appearance inspection

(1) Use a low magnifying glass or the naked eye to observe whether there are surface defects such as undercuts, slag inclusions, pores, and cracks on the surface of the weld.
(2) Use the welding inspection ruler to measure the remaining height of the weld seam, welding knob, dent, staggered edge, etc.
(3) Measure the shrinkage deformation, bending deformation, wave deformation, angular deformation, etc. of the weldment with a template and a measuring tool.

2. Density test
(1) Leak test for liquid container: directly contain liquid without pressure equipment, and check the tightness of the weld.
(2) Air-tightness test: Compressed air is passed into the container or pipe, and soapy water is applied to the outside of the weld to check for leakage.
(3) Ammonia test: Ammonia gas is passed on one side of the weld, and the other side is pasted with phenolphthalein-alcohol solution test paper to check for leakage.
(4) Leak test of kerosene: chalk powder water is painted on one side of the weld, and kerosene is immersed on the other side, leaving oil stains on the chalk to leak.
(5) Helium test: Welds with strict requirements for tightness are measured with a helium leak detector.

3. Strength test
(1) The hydraulic strength test of the container is commonly performed with water, also known as the hydraulic pressure test. The pressure test pressure is generally 1.25 times the design pressure. When carrying out hydrostatic test on stainless steel, the chloride ion content of water should be controlled not to exceed 25ppm.
(2) A gas pressure medium is used for the pressure test. The test pressure is generally 1.15 times the design pressure. The pressure test is very dangerous, and measures should be taken to ensure safety.

4.Commonly used non-destructive testing methods for welds
Non-destructive testing of welding is essential in welding production.
Relevant non-destructive testing standards and non-destructive testing process specifications are specified in detail. Various methods of non-destructive testing of welds have specific applications, specifically:

Nondestructive testing methods for welds generally include radiographic inspection (X, γ), ultrasonic inspection, magnetic particle, penetration and eddy current inspection, etc. Among them, radiographic inspection and ultrasonic inspection are suitable for the detection of internal defects in welds, and magnetic particle, penetration and eddy current are applicable Inspection of weld surface quality. The non-destructive testing method should be selected according to the weld material and structural characteristics.

(1) Radiographic inspection (X, γ) method (RT): The penetrating radiation from X and γ-ray sources is used to penetrate the weld to make the film sensitive, and the defect image in the weld is displayed in the processed rays. Photographic films are currently widely used non-destructive inspection methods, which can find defects such as pores, slag inclusions, cracks and incomplete penetration in the weld. Radiographic inspection is basically not limited by the thickness of the weld, but it is impossible to measure the depth of the defect, the inspection cost is high, the time is long, and the radiation is harmful to the inspection operator.

(2) Ultrasonic flaw detection (UT): It uses a piezoelectric transducer to generate pulse vibration through instantaneous electrical excitation. It transmits ultrasonic waves into the metal with the aid of an acoustic coupling medium, and reflects and returns to the transducer when it encounters defects during propagation. Then, the acoustic pulse is converted into an electric pulse, and the position and severity of the defect in the workpiece can be evaluated by measuring the amplitude and propagation time of the signal. Ultrasonic ultrasonic detection has higher sensitivity, flexibility and convenience, shorter cycle, lower cost, higher efficiency, and harmless to human body, but the display defect is not intuitive, the defect judgment is not accurate, and it depends on the experience and technical proficiency of the inspector.

(3) Magnetic flaw detection (MT): It uses the surface and near surface defects of ferromagnetic materials to change the magnetic susceptibility. Leakage magnetic field is generated on the surface during magnetization, and then magnetic powder, magnetic tape or other magnetic field measurement methods are used to record and display defects. It is mainly used to detect weld surface or near surface defects.

(4) Penetrant flaw detection (PT): spray or apply the penetrant containing pigment or phosphor powder on the surface of the weld to be inspected, use the capillary action of the liquid to penetrate the defects in the surface opening, and then clean and remove Excessive penetrant is applied after drying, and the penetrant in the defect is adsorbed on the surface of the weld, and the traces of the defect are observed. This method is mainly used for welding surface inspection or root defect inspection after gouging.

(5) Eddy current flaw detection (ET): It uses the high-frequency current flowing in the probe coil to induce the effect of eddy current on the surface of the weld. Defects will change the eddy current magnetic field and cause the coil output (such as voltage or phase) to reflect the defect. . The control of its inspection parameters is relatively difficult, and it can inspect defects on the surface of conductive materials or on the surface or near surface of weld overlay and surfacing layer.

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