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Time of Flight Diffraction and Phased Array Techniques for the Detection of Small Transversal Cracks in Hydrocracking Unit Welds in CrMoV Steel

The paper deals with the detection and sizing of small transversal cracks in hydrocracking units. The minimum sensitivity code requirements for the tests were based on 4  4  0.3 mm planar discontinuities. This level of sensitivity was significantly higher than the normal requirements for sidedrilled holes from ASME Code Case 2235-9: Use of Ultrasonic Examination in Lieu of Radiography, Section V, Article 4; the difference was at least 12 to 18 dB (ASME, 2005). Special test blocks were prepared with electric discharge machining slits embedded in the weld thickness at different depths. Time of flight diffraction and phased array testing procedures were validated on these blocks. Height sizing was important in the testing as required by ASME Code Case 2235-9 (ASME, 2005). Both techniques optimized on the test blocks were able to detect and size small transversal cracks. Diffracted echoes were essential for sizing the height of cracks using the phased array technique.

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