Abstract：In order to analyze the unqualified causes of ultrasonic flaw detection for medium and heavy plates, the metallographic samples and Z-direction fracture samples are usually taken at the unqualified locations. Then the microscopic analysis of defects causing the unqualified ultrasonic flaw detection are conducted using optical microscope (OM), scanning electron microscope (SEM) and energy disperse spectroscopy (EDS) to confirm the type and source of defects. However, for the bubble defects, it is difficult to obtain the Z-direction fracture specimen at the defect as the defects are usually very close to the surface, and the microscopic characteristics obtained by the metallographic specimen are similar to those of the micro-crack defects, which affects the judgment of defect type. In order to better distinguish these two kinds of defects, the sample was lengthened by welding to obtain the Z-direction fracture specimen with bubble defects. Moreover, the micro-characteristics of bubble defect and micro-crack defect on the metallographic specimen and Z-direction fracture specimen were analyzed in detail. The results showed that both two defects presented as linear micro-crack morphology in metallographic examination, while there were significant differences on the Z-direction fractures. For the bubble defects, the smooth bubble wall morphology was observed on the fractures, and there were fine Nb and Ti composite secondary phase precipitation particles on the bubble wall. For the micro-crack defects, the white spot morphology was presented on the fractures. Moreover, there were MnS inclusions and large particles of Nb-Ti composite precipitate.
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YANG Xiao-jun, YANG Yong, ZHOU Kai, OU Yong. Microscopic characteristics of bubble and micro-crack defects in ultrasonic flaw detection of medium and heavy plates. , 2019, 39(11): 84-88.
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