Ultrasonic Flaw Testing of Metal Castings

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. 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.

Ultrasonic flaw detection equipment using the principle of reflection method generally includes four parts: high-frequency pulse generator, probe (transducer), receiving amplifier and display. Among them, the probes are divided into two types: straight probes and inclined probes. Straight probes are only used to detect defects perpendicular to the propagation direction of ultrasonic waves; while inclined probes are mainly used to detect defects parallel to the propagation direction of ultrasonic waves. The inclined probe mainly utilizes the existence of reflected and refracted transverse waves in addition to reflected and refracted longitudinal waves when entering from one medium to another during the propagation of ultrasonic waves.

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, fast speed and large detection thickness.

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