Ultrasonic testing can find defects such as shrinkage cavities, shrinkage porosity, porosity, inclusions, and cracks in castings with simple shapes and flat surfaces, and can determine the size and location of defects.
Ultrasonic testing refers to a method of injecting ultrasound (high frequency and short wavelength) into the casting, and then detecting the internal defects of the casting according to the characteristics of its refraction and waveform transformation at the interface. Ultrasound has the characteristics of beam directivity and propagation reflectivity.
There are three types of ultrasonic testing: pulse reflection method, penetration method and resonance method. The commonly used ultrasonic detection method is the pulse reflection method. It refers to a method of judging the size and position of the defect according to the echo of the defect and the echo of the bottom surface.
The basic principle of the pulse reflection method is that the piezoelectric element in the probe is excited by high-frequency pulses to generate ultrasonic pulses. When the sound wave propagates in the casting and encounters defects, a part of it is reflected back. The size of the reflected wave can reflect the size, location and depth of the internal defects of the casting. Ultrasonic waves that are not reflected continue to propagate forward until they are reflected back to the bottom of the casting. The sound energy reflected from the defect and the bottom of the casting is successively received by the piezoelectric transducer, and then displayed on the display of the ultrasonic flaw detector in the form of amplitude.
The sensitivity of an ultrasonic flaw detector refers to its ability to find the smallest defects. The sensitivity of ultrasonic testing is related to factors such as the frequency of the ultrasonic wave, the magnification of the flaw detector, the transmission power, the performance of the probe, and the stability of the power supply. In order to ensure the smooth transmission of ultrasonic waves into the acoustic medium, an appropriate coupling method must be adopted. This requires that the surface roughness of the casting must be Ra≤12.5 μm. At the same time, in order to enrich the air in the gap, the coupling fluid (water, lubricating oil, transformer oil, water glass, etc.) should be applied between the probe and the flaw detection surface of the casting.
Features of ultrasonic flaw detection:
1. High detection sensitivity. Ultrasonic flaw detection can detect defect signals with a pulse reflection wave sound pressure of only 0.1% of the incident sound pressure.
2. High defect location accuracy and high resolution
3. Strong applicability and wide range of use. Ultrasonic flaw detection can detect all kinds of castings except austenitic steel castings.
4. Low cost, high speed and large detection thickness.
The pulse characteristics and shape description of different internal defects of castings on the display screen:
1. Crack
Casting crack is a kind of metal fracture, which contains gas, has a certain direction, and is linearly distributed. When these defects are found by ultrasonic inspection, if they are perpendicular to the sound beam, the reflected pulses are obvious, sharp and strong. But when its distribution is parallel to the sound beam, it is not easy to be found. Therefore, when testing, it should be projected from multiple directions, so that the defects are perpendicular to the sound beam to the greatest extent, and it is possible to find cracks distributed in all directions.
2. Blowhole
Like cracks, blowholes in castings contain gas. The reflection interface of the air hole is regular and smooth, so when the sound beam is completely perpendicular to its reflection interface, the characteristics and shape of the reflected pulse are similar to the crack, and it is also obvious, sharp and strong. However, because most of the blowholes are circular or elliptical, when the probe moves slightly, the pulse disappears immediately. When the probe detects from all directions, blow holes can be found, and the characteristics of the reflected pulse are also small. This is not the case with cracks. Because the cracks are linearly distributed with strong directionality, their reflected pulses do not disappear immediately during the movement of the probe, and at the same time, not all of them can be found when inspected from all directions. Based on these features, we can distinguish between pores and cracks.
3. Shrinkage
The shrinkage cavity contains gas, and when its effective reflection surface is larger than the sound beam diffusion surface, the sound path is totally reflected, and the bottom surface pulse reflection is eliminated. The characteristics of the reflected pulse of the shrinkage cavity are also obvious, sharp and strong. However, in addition to the above judgment method, the judgment of shrinkage cavity defects should also use the multi-plane projection method.
4. Sand Inclusion and Slag Inclusion
Sand inclusion and slag inclusion refer to metal castings containing a small amount of gas and non-metallic inclusions. These impurities have the effect of absorbing sound energy, and because the reflecting surface is relatively single and smooth, the characteristics of its pulse reflection are between obvious, sharp, strong and dull, slow and short. The latter situation occurs when the interface between the inclusions and the metal is abnormally irregular and tightly adhered to the metal.
5. Shrinkage Porotity
The pulse reflection characteristic of shrinkage porosity is that there is neither a bottom surface reflection pulse nor a defect reflection pulse, but a creeping phenomenon on the sweep line of the display screen.
Post time: Sep-24-2022