voestalpine Tubulars GmbH & Co KG (VAT) is a manufacturer of seamless steel tubes and operates a tube rolling mill with further processing facilities at its site in Kindberg. The seamless tubes are produced by means of the CPE process (Cross-Rolling, Piercing and Elongation), a combination of a cross-roll piercing mill and push bench. Due to the roll arrangement during cross-rolling, the rolled stock is transported helicaly along the rolled stock axis. Potential defects can thus form not only longitudinally and radially stretched, but also obliquely. The API Specification 5CT, 11th edition, prescribes obligatory oblique defect testing of certain grades. The aim is to determine and certify the angle of defects caused by the manufacturing process. Usually ultrasonic testing is used to analyse defects in tubes of various dimensions. In order to keep the inspection economically feasible, it is necessary to have exact knowledge regarding the orientation of the defect. Several test series should support and validate the mathematical model, however, the test runs showed a too differentiated picture of the defect orientation. A simple mathematical model showed strong mismatch with real data. The final angles showed clear tendencies to remain below 5°, even so a wide spread of the angles has been detected after the piercing process. Standard non-destructive testing in the seamless tube mill is capable of clearly detecting these defect angles so that tubes can be rejected automatically.