中厚板超声波探伤气泡和微裂纹缺陷的微观特征

为分析中厚板超声波探伤不合格原因,通常采取在探伤不合格位置取金相试样或Z向断口试样,利用光学显微镜(OM)、扫描电镜(SEM)和能谱仪(EDS)等对引起探伤不合格的缺陷进行微观分析,以确认缺陷的类型及来源。但是对其中的气泡类缺陷,由于其通常距表面很近,不易获得缺陷处的Z向断口试样,而金相试样获得的微观特征与微裂纹缺陷具有相似性,影响了缺陷类型的判断。为了更好地区分这两种缺陷,通过焊接加长试样获得气泡缺陷的Z向断口试样,并详细分析了气泡缺陷和微裂纹缺陷在金相试样和Z向断口试样上的微观特征。结果显示两种缺陷在金相试样上均显示为线状微裂纹形貌;而在Z向断口上则存在显著区别,气泡缺陷在断口上显示出光滑的气泡壁形貌,气泡壁存在细小的Nb、Ti复合第二相析出颗粒;而微裂纹缺陷在断口上表现为白点形貌,且存在MnS夹杂及大颗粒的Nb、Ti的复合析出物。

Microscopic characteristics of bubble and micro-crack defects in ultrasonic flaw detection of medium and heavy plates

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.