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.