Helical testing involves rotating the log while one or more probes move along its length. They inspect the log along a helical path. Your operator can adjust the pitch during testing by between 4 and 15 mm per revolution. Decisive here are parameters such as probe geometry, received signal-to-noise ratio, billet surface condition, internal grain structure, desired sensitivity class, etc. It inspects the entire log apart from an outer skin some 5 mm deep. To examine this skin layer you need further probes positioned at an angle to the billet surface (angle beam testing). Helical testing requires substantial capital investment and longer testing times per billet than the linear method. That’s why it’s only used for particularly meticulous quality testing standards.
Manufacturers tend to prefer helical testing for high-strength extrusion and forging alloys. Test stations are usually integrated in the casthouse or at a later production stage. Take a look at the photo to see a helical UT unit capable of examining 70,000 t of logs per year. It features nine probes, five volume test probes, and four angle beam probes for finding surface cracks at the log head and butt ends. First up, perpendicular probes scan the log volume. The system monitors the entry echo as a function check. Equally important are the back wall echo and any echo from a defect. They are continuously recorded as amplitude and delay time. Probes move along the rotating log in an axial direction. What determines the performance are the number and pitch of the probes. We engineer our plants so that the probes remain exactly perpendicular to the log surface – even if the logs are bent. The untested ‘skin’ below the surface occurs as a result of the back wall echo length (typically up to 5 mm). Yet that’s no problem, because our systems test this area using angle beam probes. Here is how: Two angle beam probes adjusted in clockwise and counter- clockwise direction scan the log surface for axially oriented cracks. Then a statistical analysis of the noise level detects differences in the grain structure. You can adjust the testing system to detect center cracks, inclusions, gas porosity, material grain structure variations, and surface cracks.
Also available to you for probe calibration are calibration billets with flat bottom holes according to ASTM E127. They are stored in a magazine integrated in the plant. You can easily move them to the probe using a crane or moveable table. To ensure periodic plant function checks, the software requests a dynamic reference log at predefined intervals. This involves loading a log with known defects. Then the UT system verifies that all defects are found within predefined limits. It also manages different diameter recipes for one dynamic log, so you need only 1 - 2 dynamic logs to cover the full range of variations. Calibrating probes is a time-consuming job. That’s why many production recipes are linked to a specific calibration recipe. The plant software dynamically utilizes the available signal-to-noise ratio to apply the greatest possible pitch. This directly affects production performance. A specially designed algorithm avoids any overvaluation. What you gain is improved productivity by 20 - 30%. Also accessible are graphs displaying the intensity as well as the location depth of faults within the billet. The charts, the relevant setup data, and the processed findings are stored in a database. You can install a viewer on any MS Windows PC in your network. It supports powerful search features so you can analyze the UT results.