Ultrasonic Testing of Metal Bars And Tubular Products

Whether manual, automated, or phased array, ultrasonic testing can help manufacturers of bars, tubes, and related metal products ensure product quality and customer satisfaction.

Ultrasonic non-destructive testing (NDT) is a well-established technique that uses high-frequency sound waves to locate cracks and other hidden defects in metals, composites, and plastics. In the metals industry, ultrasonic inspection is used not only to inspect finished parts, but also raw materials in the form of bars, blanks, and plates that are offered for further processing.

When used properly by qualified operators, ultrasonic non-destructive testing is fast and reliable, usually requires no preparation of the test piece other than wetting with an ultrasonic coupling agent or water, and has no specific safety hazards or regulatory licensing requirements associated with its use.

Phased Array Ultrasonic Automatic Testing Equipment For Bars

Ultrasonic testing uses sound waves at frequencies typically between 2MHz and 10MHz. The sound waves will travel through a medium like steel at a specific speed or rate and in a predictable direction, and when they encounter a boundary with a different medium they will be reflected or propagated according to simple rules. As a result, ultrasonic waves will be reflected from cracks, pores, inclusions or other discontinuities in the test piece. The frequency and aperture diameter of the ultrasonic transducer determine the beam diameter and beam spread characteristics, which also factor into the ability to detect and size defects. If the defect is larger than the beam diameter, and amplitude-based comparative measurement technique must be used.

Mechanical focusing using a fixed lens (conventional UT) or electronic focusing (phased array) can be used to establish a tighter beam diameter and focal zone, which in turn allows better definition of the edges of the defect and thus improved dimensional analysis. The choice of inspection technique is usually a compromise between inspection, size, yield, and documentation requirements.

Solid bars are often inspected for cracks, pipe voids, and inclusions. Tubular products containing resistance or arc welds are inspected for cracks and lack of penetration along the weld seam, as well as voids and inclusions. Standards, specifications, and customer requirements usually set the classification level for acceptance based on the location and size of critical defects in relation to the end-use of the product. Materials used for transport, power generation, and fluid/gas sealing or distribution often require more rigorous inspection than non-safety critical consumer products.

 Phased Array Ultrasonic Automatic Testing Equipment for the Seamless Pipe     

Traditional manual testing

Manual inspection using portable ultrasonic flaw detectors and hand-held transducers is usually the simplest and most cost-effective technique. The inspector monitors the waveform display while moving the transducer around the test piece. Bar testing can be carried out in up to three directions, using straight and angular beam transducers which, as the name suggests, aim the sound waves in a straight line or at a selected angle for optimum defect detection. The first test involves attaching a transducer to the end of the rebar to identify echoes from the distal end. The presence of an echo in front of the distal echo, or the attenuation of that echo, indicates a circumferential crack, a snowflake crack, or a porosity.

The transducer is then coupled to the side of the rebar and moved along its length while scanning at least 180° of the circumference. Internal echoes or distant wall echo attenuation indicate the presence of laminated cracks or pipes. Finally, an angular beam probe can be similarly scanned along the length of the rebar while scanning around the circumference to identify axial cracks near the surface. This third method is also used for pipes. These basic test protocols and echo interpretation also apply to the automated methods below, where automated alarms, part marking, and/or imaging software replace continuous manual monitoring.

source