Guideline for Defining the Scanning Grid for Automated Ultrasonic Testing of Large Forgings (2025)

The contribution made by non-destructive testing methods to quality assurance exists in a state of tension between additional manufacturing and operating costs and the achievable reliability of quality assessments. In light of increasing cost and time pressures in recent years, testing methods must therefore become not only faster and more cost-effective to remain competitive, but also more reliable at the same time. Against this backdrop, existing NDT standards and codes must also be adapted and provide a more precise description of testing techniques as a result of this trend. Quality requirements have also steadily increased in recent years for large forgings, such as those used for turbine and generator rotors in power engineering. Consequently, a continuously growing proportion of large-volume forgings must undergo automated ultrasonic testing.

In the automated ultrasonic testing of such large components, the scanning grid—composed of shot spacing and track offset—takes on dual significance. A scanning grid defined too coarsely reduces the probability of detection, whereas test points spaced too closely increase the already very long inspection time for large-volume components. The often very low sound attenuation causes phantom echoes with significantly high amplitudes. Consequently, very low pulse repetition frequencies are required, which further extend the inspection time. Optimizing the scanning grid minimizes both the inspection time and, as a result, inspection costs.

However, currently valid codes and standards contain various instructions for defining a scanning grid that are partially based on overly simplified assumptions, are ambiguous, or are even completely unsuitable for automated testing. This can result in inspection gaps—that is, areas that are not inspected with the required sensitivity. A more exact approach must take into account, for example, the geometric characteristics of sound fields—particularly the changing sound beam diameter along the direction of sound propagation. Furthermore, sound beams do not represent physically sharply defined regions, but instead exhibit a continuous drop in sound pressure in the lateral direction.

This guideline describes a mathematically grounded procedure for determining an optimal scanning grid for the complete (100%) volumetric inspection of large forgings, taking into account the sound beam geometry of the respective probes used. In addition to beam entry through flat surfaces (e.g., end faces), the guideline also accounts for normal-beam and angle-beam scanning on convex and concave curved surfaces (e.g., outer cylindrical surface and bore). This provides users with a practical guide that supports them in optimizing inspection time, ensuring full inspection coverage, and thereby improving inspection reliability.