Ultrasonic testing is a form of quality assurance used for ensuring the strength and quality of rolled steel or steel created on the factory floor for a variety of purposes. This is important for buyers, who often want highly rated steel for specific projects. This is such an important test, many companies provide precise information on what ultrasonic devices are used and how they test the steel. Usually each individual sheet is tested, either on a conveyor belt (with a mounted ultrasonic device) or by hand (with a portable, wand-like device).
The goal is to weed out steel plates that have too many inherent flaws inside. Some flaws can be seen on the outside of the plate, which often means disqualification, but other flaws can hide inside the steel, where the metal particles may not have combined properly and formed problem areas. Like knots in a wooden board, these flaws can cause the plate to warp over time, or compromise its integrity so that it breaks too easily.
Ultrasonic testers are composed of two main parts: the transducer that sends the signal, and a receiver designed to pick it up again. A transducer is a device that converts one type of energy to another--in this case, electrical energy into acoustic sound waves. Essentially, the sound waves travel through the steel plate and return. Any imperfections in the plate with distort the waves, since it will take some parts of the waves a longer or shorter time to travel through flawed areas. These distortions are picked up by the receiver; if the waves are too distorted, the plate is disqualified, but if they fall within acceptable parameters, the plate passes. This does not mean that ultrasonic testing can rule out every flaw in the steel. Rather, it can show that the steel is good enough to be sold based on the manufacturer's standards, and any small flaws revealed by the test are dismissed.
There are two ways to perform the ultrasonic test. One of the first methods devised uses water and requires that the steel plate be submerged before the testing can begin. This is difficult to do on the factory floor, especially since freshly cooled steel may not be ready to be submerged in water again (unless the two processes are combined). A more simple method simply sends the sound waves through the tested object itself and no surrounding medium, but this requires different and more highly attuned sensors.
When tested, sound waves are sent in pulses. The goal is to have each pulse of sound make it through the object and be picked up again before the next wave is sent, with a carefully calibrated time interval between them. There is always the danger of picking up "false" pulses due to interference, but many sensors come equipped with the ability to sense false pulses.
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