When the actual flaw detection, flaw detector, frequency, probe and so on are fixed, in most cases directly use the corresponding standard, so clear these conditions, and strive to unify with the user's provisions.
Ultrasonic detection of large shaft forgings, the 2 ~ 3 small equivalent dense defects in essence, in addition to the part is white point, more are micro cracks, micro area loose, holes, pinholes and other defects.
Most of the defects affect the bottom wave, and the size of the influence varies with the nature, size, position and direction of the defects. The dense distribution of white points and inclusions all have an influence on the bottom wave, but to different degrees. The reflectivity of white point is high, and the transmittance of inclusion is high. Therefore, the attenuation of white point to bottom wave is greater than that of inclusion. Under the same sensitivity, multiple bottom waves of white point decline rapidly and show fewer times. The bottom wave of the inclusion decreases slowly for many times.Because the white point is a crack defect, it will reduce the sensitivity of detection. The defect wave drops slowly while the bottom wave drops quickly.
Crack more inclusions in presence of air, the air is the order of magnitude of the acoustic impedance of about 10 squared, and the acoustic impedance of metal materials orders of magnitude of about 10⁶, visible difference in the acoustic characteristics of the air and metal material. The boundary between crack and base material is soft boundary.
By analyzing the propagation characteristics of ultrasonic waves in the medium and the acoustic characteristics of the internal defects of large forgings, it can be seen that the boundary characteristics of various defects can be classified into two categories.
(1) Since the inclusions (slag) and other components are mainly oxides of Si, Al, Fe, etc., it can be said that the non-metallic inclusions in steel are a form of gas existing in steel.This kind of material in the acoustic characteristic impedance is less than the acoustic characteristic impedance matrix, and so this type of defect and the boundary of the substrate material can be regarded as "soft" boundary, namely, crack, flake, shrinkage cavity, air bubble, inclusion of gold and complete the boundary belongs to "soft" boundary, ultrasonic vertical person shot on this kind of interface echo phase change is 180 °.
(2) The boundary of the high-density metal inclusion belongs to the "hard" boundary, and the phase of the reflected ripple when the ultrasonic wave is vertically incident on this interface is the same as that of the human radiation wave.High density metal inclusions are tungsten, molybdenum and so on.The acoustic impedance of the tungsten is about (83.2 ~ 104.2) x 10 ⁵, molybdenum acoustic impedance is 63.8 x 10 ⁵, and general forging materials acoustic impedance (39.4 ~ 45.6) x 10 ⁵, so this type of high density metal materials acoustic impedance is greater than the forging the acoustic impedance of substrate material, this kind of defect and the boundary of the substrate material can be considered "hard" border. For cracks, white spots, shrinkage holes, bubbles and non-metallic inclusions, their reflectance and transmittance can all be calculated despite their different acoustic impedance, which reflect the amplitude attenuation level of ultrasonic echo signal. The difference of acoustic impedance between non-metallic inclusions and matrix tissues is the smallest, and the transmittance is high. Therefore, the echo attenuation of non-metallic inclusions is very serious, which shows that the bottom wave amplitude is small or even there is no bottom wave during detection. The white spot reflected wave is very strong, but because it appears in groups, the waveform is clear and clustered in the detection. The reflection wave of crack defect is strong and single, very clear. Due to the large number of inclusions around the shrinkage cavity defect and the roughness of the surface, the reflected wave will have a large attenuation.
Post time: Feb-02-2021