Ultrasonic testing in stainless steel manufacturing
Ultrasonic evaluation is one form of non-destructive testing. It is based on how pulsed sound waves pass through different materials at different velocities. In other words, if a sound wave is passed through a piece of steel at a certain velocity and hits an air filled hole which has a much slower velocity, some of that sound wave will bounce back. That will show up as a spike on the scope.
Ultrasonic testing is a reliable method of examining the internal areas of steel forgings for defects without harming the steel or changing any of its properties. Internal defects such as holes, cracks, and non metallic inclusions are easily identified and located. Whether a piece passes or fails depends on the customer’s requirements and specifications.
A certified ultrasonic tech must perform these examinations. The tech must take theory training and accumulate a specified number of hours of experience to perform examinations. A pulse echo scope, transducers, test blocks, and couplant are among the tools the tech must learn to use. Calibration techniques and the ability to read and understand procedures including military standards are also required. Must be computer literate and have legible hand writing skills because everything from the setup and actual test has to be documented. Maps with indication locations must be recorded. As with any skilled trade, the experienced tech will be very valuable in maintaining a high level of quality control.
There are two main areas of ultrasonic testing; angle beam and straight beam. Angle beam is used in thin wall materials such as pipe and for checking welds. Straight beam is used for solid materials that have a parallel back-wall such as round bar, round or square billet, and plate. Angle and straight beam testing can be done by the contact method or non contact method. The contact method is hands on, the tech scans the test piece with transducer in hand and watches the scope for any indications and physically marks the test piece as required. The non contact method is done by a automatic machine usually with a water tank which is called immersion testing.
Many factors have to be considered before and during testing. Surface cleanliness and texture, ambient temperature, length and thickness, grade type, heat treat condition, and grain structure will have bearing on the test results. The most important factor in any test is the person doing the test, contact or non contact.
