Guideline for the Determination of the Scanning Grid for the Automated Ultrasonic Testing of Large Forgings
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Beschreibung
The quality requirements for large forgings, especially those used in the manufacture of power generation machinery, i.e., for turbine and generator rotors, have increased continuously over the past years. A con‐sequence is the growing number of forgings, which have to be subjected to automated non‐destructive ultrasonic testing. When going from manual to automated ultrasonic testing it requires the specification of many parameters, including the sampling distance, i.e. the distance between adjacent scanning points in the scan direction, and the index direction, i.e., the distance between adjacent scan‐lines in index di‐rection. Scan step and index step define the scanning grid.
The considered forgings generally exhibit very low sound attenuation. This frequently results in "late re‐turns" with high amplitudes, which appear as "phantom indications". To avoid such indications very low pulse repetition frequencies must be used, resulting in long test duration. Optimization of the scanning grid therefore results in optimization of test duration and thus also of test costs. It is reasonable for pur‐chaser and manufacturer to agree on the scanning grid.
However, many currently valid standards and specifications contain various instructions for establishing a scanning grid, some of which are partially unclear and ambiguous or are even unsuitable for automated testing.
This guideline describes a procedure for establishing an optimum scanning grid for complete (100 %) vol‐umetric testing of large forgings under consideration of the sound field geometry of the probes used.
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Determination Large Forgings Scanning Grid