Practical Welding Today - May/June 2009

TECHNOLOGY OVERVIEW

on which magnetic particle inspection would be useless.

The NDT technician may use VT, MT, or PT during the welding operation to assist the welder in revealing discontinuities (for example, performing MT on the weld after back-gouging to ensure sound weld metal has been obtained before welding the second side).

Subsurface discontinuities include under-bead cracks, wormhole porosity, lack of fusion, slag inclusions, voids, and laminations. Radiographic testing (RT) or ultrasonic testing (UT) help to locate these volumetric discontinuities which are not visible on the surface.

Ultrasonic Testing (UT). UT detects discontinuities within the internal structure of welds. The advantage of this testing method is its ability to help establish internal integrity without destroying the welded component.

UT is used on ferrous and nonferrous materials and is suited for testing thick sections accessible from one side only. It can also locate fine linear or planar defects. UT uses mechanical vibrations similar to sound waves but at a higher frequency. A beam of ultrasonic energy is directed into the object. The beam travels through the object with insignificant energy loss, until it is intercepted and reflected back to the UT instrument screen by a discontinuity.

UT generally uses the contact pulse reflection technique, in which a transducer converts electrical energy into mechanical energy. The transducer is excited by a high-frequency voltage that causes a piezoelectric crystal to vibrate mechanically. The crystal probe becomes the source of ultrasonic mechanical vibration. These vibrations are transmitted into the item through a couplant

fluid. When the ultrasonic wave pulse strikes a discontinuity in the test item, it is reflected back to its point of origin. Thus, the energy returns to the transducer, which also serves as a receiver for the reflected energy.

The detection, location, and evaluation of discontinuities become possible because the velocity of sound through a material is considered constant, making distance measurement possible, and the relative amplitude of a reflected pulse is proportional to the size of the reflector.

One of the most useful characteristics of UT is its ability to determine the exact position of a discontinuity in a weld. NDT technicians performing the tests are required to be qualified as a level II in accordance with ASNT standards. This method requires a high level of operator competence and depends on establishing and applying test procedures.