Determination of the Probability of Detecting Reflectors as a Function of Grid Size in Ultrasonic Testing of Large Forgings

In recent years, quality requirements for large forgings used in power generation equipment—particularly for turbine and generator rotors—have steadily increased. For this reason, forgings are now primarily inspected automatically using ultrasonic testing during the manufacturing process to detect even the smallest defects.
Due to the manufacturing process and heat treatment, these forgings have a fine-grained microstructure, which in turn results in low acoustic attenuation for ultrasonic testing. Consequently, high-amplitude phantom signals, known as “late returns,” are often observed. To avoid this, a low pulse repetition frequency must be selected for the inspection, which results in a long inspection time. Optimizing the inspection pattern therefore means optimizing both the inspection time and the inspection costs.
Upon closer examination, current regulations contain various instructions for defining the inspection pattern that are designed for manual testing and are therefore not suitable for automated testing or, in some cases, are ambiguous. For this reason, the DGZfP Subcommittee on “Automated Ultrasonic Testing Systems” the DGZfP Guideline US 07:2014 was developed, which describes the procedure for defining such an optimal inspection grid for the complete volumetric inspection of large forgings, taking into account the acoustic field geometry of the transducers used.
In Marcel Preißel’s thesis, the procedure developed in Guideline US 07:2014 for determining the inspection grid is presented and verified using simulation calculations and experiments, and the effect of the inspection grid on the probability of detecting reflectors is investigated. To this end, POD calculations were performed to determine the detectable reflector size as a function of the test grid size.