The hottest welding process Q & a NDT

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Welding process Q & A (nondestructive testing)

16. Try to describe the types and choices of common nondestructive testing methods

the method of detecting defects without damaging the performance and integrity of the inspected materials or finished products is called nondestructive (flaw detection) inspection. The commonly used nondestructive testing methods include ultrasonic, X-ray (x γ) Take photos, read the required load and the corresponding elongation value between markings, magnetic particle, penetration (fluorescence, coloring) and eddy current flaw detection, etc. Ultrasonic flaw detection and radiographic flaw detection are suitable for the detection of internal defects of welds; Magnetic particle inspection and penetrant inspection are used for weld surface quality inspection. Each nondestructive testing method has its advantages and limitations, and the detection probability of defects by various methods will not be 100%, nor will it be exactly the same. Therefore, it should be selected according to the weld material, structure, characteristics of flaw detection methods, acceptance standards, etc

see Table 8 for the selection of flaw detection methods for different weld materials

17. try to describe the principle of radiographic inspection and the image characteristics of welding defects

X and γ See Fig. 3 for the flaw detection principle of two kinds of rays. When the ray passes through the metal material, part of the energy is absorbed, making the ray attenuate. If the thickness of the penetrating metal material is different (cracks, pores, incomplete penetration and other defects, where cavities occur, making the material thinner) or the volume and mass are different (slag inclusion), the attenuation is also different. When passing through a thick or bulky object, the attenuation is large, so the intensity on the substrate is weak, the sensitivity of the negative is small, and the blackness obtained after development is light; On the contrary, the blackness is deep. According to the images with different blackness on the negative, the defects can be clearly displayed

γ The penetration ability of X-ray is stronger than that of X-ray, and it is suitable for weldments with perspective thickness greater than 50mm

see Table 9 for image characteristics of common welding defects in radiographic inspection

Table 9 image characteristics of radiographic inspection welding defects

most of them are round and oval black spots, with large blackness at the center, as well as needle and column pores. Its distribution is different, with dense, single and chain

1) the welding rod is damp

2) there is rust, oil stain, etc. at the welding place

3) the welding speed is too fast or the arc is too long

4) there is interlayer at the base metal groove

5) automatic welding produces open arc phenomenon

the shape is irregular, there are points, blocks, etc., and the blackness is uneven. Generally, the strip slag inclusion is parallel to the weld, or mixed with incomplete penetration and fusion

1) improper strip transportation, too small welding current, too small groove angle

2) rust is left on the weldment and the performance of the electrode coating is improper

3) during multi-layer welding, the slag removal between layers is not complete

regular, even linear black lines appear on the negative film, often accompanied by pores or slag inclusion. In the V-groove and V-groove welds, the root incomplete penetration appears in the middle of the weld, while the k-groove deviates from the weld center

1) the gap is too small

2) the welding current and voltage are improper

3) the welding speed is too fast

4) the groove is abnormal, etc.

the image of groove incomplete fusion is generally flat on one side, bent on the other side, light and uniform blackness, and often accompanied by slag inclusions. The image of interlayer lack of fusion is irregular and difficult to distinguish

1) the groove is not clean enough

2) the geometric size of the groove is improper

3) the welding current and voltage is small

4) the diameter or type of welding rod is wrong

it is generally in a straight line or slightly serrated fine grain, with clear outline, thin ends and slightly wide middle, Sometimes dendritic images appear

1) improper composition of base metal and welding material

2) improper welding heat treatment

3) too much stress or stress concentration

4) incorrect welding process

when tungsten gas shielded welding is used, tungsten electrode bursts or molten tungsten particles enter the weld metal

18. Try to describe the quality standard of radiographic inspection

according to GB "radiographic and quality classification of steel fusion welded butt joints", the quality standard of radiographic inspection is divided into two parts: photographic quality grade and weld quality grade

according to the type of source used and its energy level, the type of film, the sensitization method, the blackness of the negative, the distance between the source and the film and other parameters, the photographic quality level is divided into three levels: A, AB and B, and the quality level is increased in sequence. That is, when the latter distinguishes defects of the same size than the former, the transillumination thickness is larger. The seam photography quality of boiler and pressure vessel is grade ab

the weld quality grade is divided into four levels. Level I has the least defects in the weld and the highest quality; The defects in grade II and III welds increase in turn, and the quality decreases step by step. The number of defects exceeding grade III is grade IV, and grade IV is the worst. Regulations on the number of defects: cracks, incomplete fusion, incomplete penetration and strip slag inclusion are not allowed in grade I welds (a small amount of pores and dot slag inclusion are allowed); Cracks, incomplete fusion and incomplete penetration in double-sided welding and single-sided welding with backing plates are not allowed in grade II and III welds (a certain number of pores, strip slag inclusions and incomplete penetration in single-sided welding without backing plates are allowed)

19. Try to describe the principle and quality standard of ultrasonic flaw detection

the non-destructive testing method using ultrasonic to detect the internal defects of materials is called ultrasonic flaw detection. The principle of ultrasonic flaw detection is to use the defect in the weld and the normal tissue to have different acoustic impedance (the product of material volume mass and sound velocity) and the sound wave on the heterogeneous interface with different acoustic impedance to find the defect through the reflection phenomenon when passing the ultrasonic wave. During flaw detection, the piezoelectric transducer in the probe emits pulsed ultrasonic waves. It is transmitted to the weldment through the acoustic coupling medium (water, oil, glycerin or paste, etc.), and the reflected wave is generated after encountering the defect. Then another similar probe or the same probe is used to receive the reflected sound wave, which is converted into an electrical signal by the transducer, amplified and displayed on the fluorescent screen or printed on the paper tape. The defect position can be obtained according to the probe position and the propagation time of sound wave (echo position on the fluorescent screen); The amplitude of the reflected wave can approximately evaluate the size of the defect, as shown in Figure 4

quality standard: the more directions of ultrasonic detection of welds, the greater the probability that the beam is perpendicular to the defect plane, the higher the detection rate of defects, and the more accurate the evaluation results. According to the provisions of GB "manual ultrasonic flaw detection methods and classification of flaw detection results for steel welds", ultrasonic damage is divided into three inspection levels a, B and C according to the number of detection directions of welds. The degree of perfection of inspection increases step by step, of which level B is suitable for pressure vessels

see table 10 for the classification of weld quality grade. The figures in the table are the allowable maximum amplitude length

20. Try to compare the technical characteristics of radiographic inspection and ultrasonic inspection

see Table 11 for the comparison of technical characteristics between radiographic inspection and ultrasonic inspection

table 11 comparison of technical characteristics of radiographic inspection and ultrasonic inspection

there is a difference in penetration dose between intact parts and defective parts. The degree of difference is related to the material of these two parts, the direction of ray transmission and the size of defects

there is no reflected wave at the intact part, but there is a reflected wave at the defective part. The degree of reflection is related to the materials of intact parts and defective parts

21. Try to describe the principle and quality standard of magnetic particle inspection

the phenomenon that magnetic particles are attracted by the leakage magnetic field generated by the surface defects of ferromagnetic materials in a strong magnetic field is called magnetic particle inspection

principle of magnetic particle flaw detection: first, magnetize the inspected weld locally, and there will be magnetic lines of force passing through the weld. For welds with the same section size and uniform internal materials, the distribution of magnetic lines of force is uniform. When there are cracks, pores, slag inclusions and other defects on the surface or inside of the weld, the magnetic line of force will bypass the defects with large magnetic resistance and produce bending, as shown in Figure 5A. At this time, sprinkle magnetic particles on the weld surface, and the magnetic line of force will pass through the magnetic particles on the surface defects to form "magnetic leakage", and the magnetic particles will be adsorbed on the defects. See Figure 5B. The size and location of the defects can be judged according to the shape, number and thickness of the adsorbed magnetic particles. Because the internal defects are far away from the weld surface, the magnetic field line will not form magnetic leakage on it, and the magnetic particles cannot be absorbed and there is no accumulation, so the defects cannot be exposed

common magnetic particles are iron oxide (Fe3O4) and iron oxide (Fe2O3)

defect magnetic traces can be divided into three types according to shape:

(1) the display length of linear defect magnetic traces is more than 3 times the width

(2) circular defect magnetic marks except linear defect magnetic marks

(3) scattered defect magnetic traces there are several defect magnetic traces in a certain area at the same time

quality standard: according to the provisions of ZBJ standard, the grades of defective magnetic traces are divided into 7 levels, as shown in Table 12 and table 13 respectively

table 12 classification of linear and circular defect magnetic marks (mm)

22. Try to describe the principle and quality standard of penetrant testing

the non-destructive inspection method that uses the penetrant with fluorescent dye (fluorescent method) or red dye (coloring method) to show defect traces is called penetrant inspection

penetrant flaw detection principle: apply the penetrant containing dye on the surface of the weldment to be inspected, use the capillary action of the liquid to make it penetrate into the surface opening defects, then remove the excess penetrant on the surface, apply developer after drying, adsorb the penetrant in the defect on the surface of the weldment, and evaluate the quality of the opening defects on the surface of the welding structure by observing the defect display trace. The basic steps are shown in Figure 6

the display characteristics of various welding defect traces are shown in table 14

Table 14 display characteristics of various welding defects

defects are irregular, diverse in shape and different in depth

Quality Evaluation: the weld quality is evaluated according to the type of defect trace, the size of trace, the distribution and spacing of display trace, and the nature of defect. According to jbj59, t "weld penetration inspection method and defect trace classification", it is divided into four quality levels, of which level I is the highest and level IV is the lowest. See table 15

23. What does the mechanical property test of welded joints include

(1) tensile test of welded joints (including full weld tensile test) the purpose of the test is to determine the strength (tensile strength) of welded joints (welds σ b. Yield point σ s) And plasticity (elongation δ, Reduction of area φ), And some defects (such as white spots) on the fracture can be found

the test can be carried out according to GB tensile test method for welded joints

(2) bending test of welded joints the purpose of the test is to check the plasticity of welded joints, and at the same time, it can reflect the plastic difference of each area, expose welding defects and assess the quality of fusion line

the bending test can be divided into face bending, back bending and side bending. The test can be carried out according to GB test method for bending and flattening of welded joints

(3) the purpose of the impact test of welded joints is to determine the impact toughness and notch sensitivity of welded joints as an index to evaluate the fracture toughness and cold aging sensitivity of materials

the test can be carried out according to GB impact test method for welded joints

(4) hardness test of welded joints the purpose of the test is to measure the hardness of metal materials in the heat affected zone of the weld, and can indirectly judge the weldability of materials

the test can be carried out according to GB test method for hardness of welded joints and surfacing metals

(5) flattening test of welded joint (pipe butt joint). The purpose of the test is to determine the plasticity of pipe welded butt joint

the test can be carried out according to GB test method for bending and flattening of welded joints

(6) welded joint (weld metal

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