Metal ingots are generally detected by ultrasonic pulse longitudinal wave incident contact method. Because the grains inside the ingot are thick and not dense, the ultrasonic attenuation is serious. In addition, ultrasonic waves will generate diffuse reflections on the coarse-grained interface, causing cluttered forest waves to appear on the fluorescent screen. These forest-like reflection echoes will be mixed with defect echoes, making it difficult to detect large-diameter (Φ518-820mm alloy ingots, especially for extra-large diameter TC4 titanium alloy ingots such as Φ1000mm. The reflection characteristics on the curved surface of the concave column use the ultrasonic propagation characteristics of the ingot itself to adjust the flaw detection sensitivity for flaw detection of the ingot riser.
Due to the large internal structure of the ingot, the ultrasonic transmission is seriously attenuated and the penetration is weak. Usually, the flaw detectors commonly used in the market cannot obtain the bottom echo required for flaw detection due to their poor penetration ability, and cannot meet the flaw detection requirements. Considering the large size of the ingot, and because the ingot production process determines that the basic position of the riser is at the heart in the radial direction, the blind zone may not be considered. The key is the signal-to-noise ratio and penetration ability of the flaw detector.
The experiment selected an imported M340 ultrasonic flaw detector as the flaw detection instrument. The instrument has a high emission voltage, and the characteristics of damping and pulse width matching with the probe can be fully tested to meet the detection requirements. Longitudinal wave straight probe is used for flaw detection of ingot riser. Three probes with different frequencies were selected, and the bottom echo height value was measured on the same alloy ingot. From the experimental data, it can be seen that the low probe frequency penetration ability is conducive to the detection of coarse-grained materials. Through theoretical analysis and a large number of flaw detection practices, when using a probe with a frequency of 1.25MHz, the flaw detection effect is ideal. The diameter of the probe used in the actual flaw detection of the ingot was 20mm.
Considering that the surface of the curved surface is unlikely to have a high finish, the engine oil with higher viscosity was used as the coupling agent in the experiment. According to practical experience, the peripheral surface within 300mm of the head of the ingot is machined. The surface finish after processing basically reaches 6.3μm, which can meet the requirements of riser flaw detection. During flaw detection of the ingot, 3 to 5 spots on the machined peripheral surface of the ingot head along the circumferential direction are uniformly selected as the axial scanning surface. On the selected 3 to 5 scanning planes, move the probe from the head of the ingot in the direction of the axis in sequence, until the defect echo changes from high to low until it disappears.
Through the analysis of the characteristics of severe grain boundary reflection and large sound wave attenuation caused by the coarse ingot structure, combined with a large number of flaw detection tests, a practical flaw detection method was established. Using this method can accurately determine the position of the ingot riser.
Ultrasonic Flaw Detection Method For Large Diameter TC4 Titanium Alloy Iingot Riser
