Helical Testing is more often applied to high strength extrusion and forging alloys. Test stations are likely to be integrated in the production process in the casthouse, or at a later production step.
The photo to the right shows a Helical UT unit capable of examining 70.000 tons of logs per year. The UT-Station is equipped with nine probes, five volume test probes and four angle beam probes for finding surface cracks at the log head and butt end.
The log volume is scanned by perpendicular attached probes. The entry echo is monitored as a function check and the backwall echo as well as any echo from a defect continuously recorded as amplitude and delay time.
Probes are moved along the rotating log in an axial direction. Therefore the plant performance is determined by the number of probes and the pitch employed. The plant is designed so that even on bent logs the probes are exactly perpendicularly to the log surface.
The untested ‘skin’ below the surface occurs as a result of the backwall echo length (typically up to 5 mm). This area can be tested by angle beam probes. Two angle beam probes adjusted in clockwise and counterclockwise direction scan the log surface for axially oriented cracks.
Differences in the material’s grain structure within the log can be detected by statistically analysing the noise level.
The testing system can be adjusted to detect centre cracks, inclusions, gas porosity, material grain structure variations and surface cracks.
For calibration of the probes, calibration billets with flat bottom holes according to ASTM E127 can be supplied. They are stored on a magazine which is integrated in the plant and can be easily moved to the probe by an integrated crane / moveable table.
For periodic plant function checks, the software requests a dynamic reference log on predefined intervals. A log with known defects is loaded and the UT system verifies that all defects are found within predefined limits. Different diameter recipes for one particular dynamic log can be managed, so that only typically 1-2 dynamic logs are required for all production diameter recipes.
Calibration of probes is a time consuming job. Therefore many production recipes are associated to a particular calibration recipe.
The plant software manages dynamic utilization of the available signal to noise ratio to use the largest available pitch which directly affects the plant production performance. A special designed algorithm avoids any overvaluation. The result is a production performance gain of 20 - 30%.
Faults are displayed and stored as graphs showing the intensity as well as the location depth within the billet. All relevant setup data as well as all processed results are stored in a database.
A viewer, which can be installed on any PC in the network running MS Windows, supports powerful search features to analyse the UT results.