Basic Introduction of Ultrasonic Inspection
Ultrasonic nondestructive testing, also known as ultrasonic NDT or simply UT, is a method of characterizing the thickness or internal structure of a test piece through the use of high frequency sound waves. The frequencies, or pitch, used for ultrasonic testing are many times higher than the limit of human hearing, most commonly in the range from 500 KHz to 20 MHz. The time interval between the transmitted ray and reflected ray is recorded by a cathode ray oscilloscope. Any crack or void in the casting results in reflection or some of the sound from the crack which appears as a pip between the two pips representing the thickness of the casting. The depth of the crack from the surface of the casting can be easily calculated from the distance between these pips.
Ultrasonic inspection is generally carried out after heat treatment, because of the high requirements for surface roughness. After heat treatment, the surface of steel casting products have oxide skin, which has to be cleaned by surface treatment process. In other word, if ultrasonic testing is before heat treatment testing, there are two repeated and wasteful surface treatment processes during manufacturing. Moreover, heat treatment can also lead to the defects of casting parts, such as cracks.
What are Advantages & Disadvantages of Ultrasonic Inspection?
It is sensitive to both surface and subsurface discontinuities.
The depth of penetration for flaw detection or measurement is superior to other NDT methods.
Only single-sided access is needed when the pulse-echo technique is used.
It is highly accurate in determining reflector position and estimating size and shape.
Minimal part preparation is required.
Electronic equipment provides instantaneous results.
Detailed images can be produced with automated systems.
It can provide the results timely.
The operation process is safe for inspectors and the equipment is portable.
It has other uses, such as thickness measurement, in addition to flaw detection.
Surface must be accessible to transmit ultrasound.
Skill and training is more extensive than with some other methods.
It normally requires a coupling medium to promote the transfer of sound energy into the test specimen.
Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.
Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise.
Linear defects oriented parallel to the sound beam may go undetected.
Reference standards are required for both equipment calibration and the characterization of flaws.
Ultrasonic inspection is difficult to test the casting parts with rough surface, irregular structure, and thin thickness.
It is still difficult to make a very accurate qualitative and quantitative description of defects.
l It is not suitable for the casting parts with structure of the cavity. But this structure is normal for investment casting process manufacturing.