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Qualitative Dictionary of Ten Common Defects in Ultrasonic Testing
Release time：2023-09-26
The qualitative problem of defects in ultrasonic non-destructive testing refers to the quantification, localization, and determination of the types of defects. Although many predecessors in non-destructive testing have made continuous efforts, summarized many valuable experiences, and conducted a large number of anatomical experiments to verify, there are still considerable difficulties in the qualitative analysis of ultrasonic testing in practical testing. This is mainly because the reflection of ultrasonic waves by defects depends on their orientation, shape, length and thickness relative to the propagation direction of sound waves, surface roughness of defects, content of defects, and types and properties of defects. The acoustic signal obtained during ultrasonic testing is a comprehensive response. At present, the commonly used ultrasonic testing technology is still difficult to separate and identify the above factors from the reflected acoustic signal, which brings certain difficulties to the qualitative analysis.
In the actual testing process, due to the difficulty in determining the nature of defects or accurately locating defects, some workpiece defects may be missed or products that can be repaired or even eliminated during subsequent processing may be rejected, resulting in unnecessary waste. At the same time, some products containing dangerous defects (such as cracks) may also be overlooked, posing a potential threat to the safety of product use, And it may cause customers to claim compensation from the production unit, causing unnecessary trouble.
The qualitative evaluation of defects in ultrasonic testing technology is mainly based on the starting speed of the waveform signal, the steepness of the front edge of the echo, the speed of the descent of the trailing edge of the echo (descent slope), the shape of the wave tip, the width of the echo, and the changes in the defect echo when the probe is moved (wave amplitude, position, quantity, shape, etc.). It can also be based on the number of times the bottom wave is observed, the height loss of the bottom wave, and the position of the defect in the inspected piece Based on the distribution, equivalent size of defects (related to reflectivity), extension, and the characteristics of specific products, materials, and manufacturing processes, a comprehensive judgment is made to evaluate the types and properties of defects. Sometimes, the echo characteristics of defects can also be observed by changing the frequency of ultrasonic pulses emitted, changing the diameter of the sound beam (such as focusing or using probes with different diameters), in order to identify whether it is a defect in the material or a tissue reflection.
For example, judging white spots, inclusions, residual shrinkage cavities, coarse grains, central porosity, square segregation in steel forgings, as well as defects such as pores, slag inclusions, incomplete penetration, incomplete fusion, and cracks in welds, largely depends on the experience, technical level, and understanding of specific products, materials, and manufacturing processes of ultrasonic testing personnel, and its limitations are significant.
The following are the echo characteristics of ten common defects:
Coarse grains and porosity in steel forgings often occur in the form of clutter, cluster waves, or increased loss of bottom wave height, reduced number of bottom wave reflections, etc.
The central crack of the bar - When scanning along the circumferential surface with a 360 ° radial longitudinal wave, due to the radiation directionality of the crack, the reflected wave amplitude varies in height and has varying degrees of motion. When scanning along the axial direction, the amplitude and position of the reflected wave do not change much and show a certain extension length.
Cracks in forgings - due to the presence of gas in the contents of crack type defects, there is a significant difference in acoustic impedance compared to the base material. The ultrasonic reflectivity is high, and the defect has a certain extension length. The wave initiation speed is fast, and the front of the echo is steep, with sharp peaks. The slope behind the echo is large. When the probe moves over the direction of crack extension, the wave initiation and disappearance are rapid.
The white spots in steel forgings have sharp and clear peaks, often in the shape of multiple heads, with strong reflection and fast wave initiation speed. The front edge of the echo is steep, and the slope of the echo back edge is large. When the probe is moved, the echo position changes rapidly, one after another, mostly within the range of 1/2 radius from the center of the tested piece, such as steel bars, or the middle layer position of 1/4 to 3/4 of the maximum section thickness of the steel forging, which has the characteristic of appearing in batches (related to furnace batch numbers and hot processing batches). When there are many white spots, large areas, or densely distributed, it can also cause a significant decrease or even disappearance of the bottom wave height.
The non-metallic inclusions in forgings are mostly single reflection signals, with slow onset, less steep front edge of the echo, rounded and blunt peaks, less slope of the echo back edge, and a larger echo width.
The high-density inclusions (such as tungsten and molybdenum) in titanium alloy forgings are mostly single reflection signals, with a relatively small echo width, but larger than cracks. The front edge of the echo is steep, and the slope of the back edge is large. When the detection frequency and sound beam diameter are changed, the reflection equivalent size does not change significantly (such as large grains or other structures reflecting in this case, the echo height will have a significant change).
Pores in castings or welds - characterized by fast onset but low amplitude, and punctate defects.
Incomplete penetration in welds - mostly at the root (such as incomplete fusion of the blunt edge when welding on one side of a V-shaped groove) or in the middle (such as incomplete fusion of the blunt edge when welding on both sides of an X-shaped groove), usually with a straight extension, a single echo rule, strong reflection, and easy detection from both sides of the weld.
The inclusion of slag in castings or welds - the reflected waves are relatively disordered and irregular in position. When the probe is moved, the echo changes, but the waveform changes relatively slowly, the reflectivity is low, the starting speed is slow, and the slope of the trailing edge is not too large. The echo occupies a larger width.
Cracks in steel castings - The waveform has two main characteristics: it has an envelope line and the waveform is relatively independent; Moving the probe in both directions can reveal defect waves. The waveform of slag inclusions is not very independent, but defect waves can be checked from all four directions.
Generally, in order to further confirm the nature of defects, other non-destructive testing methods such as X-ray photography (inspection of internal defects), magnetic particle and penetration testing (inspection of surface defects) should also be used to assist in the judgment.
In the actual testing process, due to the difficulty in determining the nature of defects or accurately locating defects, some workpiece defects may be missed or products that can be repaired or even eliminated during subsequent processing may be rejected, resulting in unnecessary waste. At the same time, some products containing dangerous defects (such as cracks) may also be overlooked, posing a potential threat to the safety of product use, And it may cause customers to claim compensation from the production unit, causing unnecessary trouble.
The qualitative evaluation of defects in ultrasonic testing technology is mainly based on the starting speed of the waveform signal, the steepness of the front edge of the echo, the speed of the descent of the trailing edge of the echo (descent slope), the shape of the wave tip, the width of the echo, and the changes in the defect echo when the probe is moved (wave amplitude, position, quantity, shape, etc.). It can also be based on the number of times the bottom wave is observed, the height loss of the bottom wave, and the position of the defect in the inspected piece Based on the distribution, equivalent size of defects (related to reflectivity), extension, and the characteristics of specific products, materials, and manufacturing processes, a comprehensive judgment is made to evaluate the types and properties of defects. Sometimes, the echo characteristics of defects can also be observed by changing the frequency of ultrasonic pulses emitted, changing the diameter of the sound beam (such as focusing or using probes with different diameters), in order to identify whether it is a defect in the material or a tissue reflection.
For example, judging white spots, inclusions, residual shrinkage cavities, coarse grains, central porosity, square segregation in steel forgings, as well as defects such as pores, slag inclusions, incomplete penetration, incomplete fusion, and cracks in welds, largely depends on the experience, technical level, and understanding of specific products, materials, and manufacturing processes of ultrasonic testing personnel, and its limitations are significant.
The following are the echo characteristics of ten common defects:
Coarse grains and porosity in steel forgings often occur in the form of clutter, cluster waves, or increased loss of bottom wave height, reduced number of bottom wave reflections, etc.
The central crack of the bar - When scanning along the circumferential surface with a 360 ° radial longitudinal wave, due to the radiation directionality of the crack, the reflected wave amplitude varies in height and has varying degrees of motion. When scanning along the axial direction, the amplitude and position of the reflected wave do not change much and show a certain extension length.
Cracks in forgings - due to the presence of gas in the contents of crack type defects, there is a significant difference in acoustic impedance compared to the base material. The ultrasonic reflectivity is high, and the defect has a certain extension length. The wave initiation speed is fast, and the front of the echo is steep, with sharp peaks. The slope behind the echo is large. When the probe moves over the direction of crack extension, the wave initiation and disappearance are rapid.
The white spots in steel forgings have sharp and clear peaks, often in the shape of multiple heads, with strong reflection and fast wave initiation speed. The front edge of the echo is steep, and the slope of the echo back edge is large. When the probe is moved, the echo position changes rapidly, one after another, mostly within the range of 1/2 radius from the center of the tested piece, such as steel bars, or the middle layer position of 1/4 to 3/4 of the maximum section thickness of the steel forging, which has the characteristic of appearing in batches (related to furnace batch numbers and hot processing batches). When there are many white spots, large areas, or densely distributed, it can also cause a significant decrease or even disappearance of the bottom wave height.
The non-metallic inclusions in forgings are mostly single reflection signals, with slow onset, less steep front edge of the echo, rounded and blunt peaks, less slope of the echo back edge, and a larger echo width.
The high-density inclusions (such as tungsten and molybdenum) in titanium alloy forgings are mostly single reflection signals, with a relatively small echo width, but larger than cracks. The front edge of the echo is steep, and the slope of the back edge is large. When the detection frequency and sound beam diameter are changed, the reflection equivalent size does not change significantly (such as large grains or other structures reflecting in this case, the echo height will have a significant change).
Pores in castings or welds - characterized by fast onset but low amplitude, and punctate defects.
Incomplete penetration in welds - mostly at the root (such as incomplete fusion of the blunt edge when welding on one side of a V-shaped groove) or in the middle (such as incomplete fusion of the blunt edge when welding on both sides of an X-shaped groove), usually with a straight extension, a single echo rule, strong reflection, and easy detection from both sides of the weld.
The inclusion of slag in castings or welds - the reflected waves are relatively disordered and irregular in position. When the probe is moved, the echo changes, but the waveform changes relatively slowly, the reflectivity is low, the starting speed is slow, and the slope of the trailing edge is not too large. The echo occupies a larger width.
Cracks in steel castings - The waveform has two main characteristics: it has an envelope line and the waveform is relatively independent; Moving the probe in both directions can reveal defect waves. The waveform of slag inclusions is not very independent, but defect waves can be checked from all four directions.
Generally, in order to further confirm the nature of defects, other non-destructive testing methods such as X-ray photography (inspection of internal defects), magnetic particle and penetration testing (inspection of surface defects) should also be used to assist in the judgment.
