在对锅炉的C类焊缝进行磁粉检测时，发现了与焊缝轴线夹角<30°的线状缺陷，而在理论上来说此类缺陷很难检测出来。进行了磁轭检测试验，通过磁粉指示磁力线以及磁轭悬空时的磁力线检测试验，说明在进行焊缝磁轭检测时，在空间位置有限的情况下，若磁场强度足够，仍然可以检测到磁轭触头周围的缺陷，但要注意区分相关显示和非相关显示。

A crack with angle less than 30° to the weld axis tested by yoke magnetic powder testing was found on a C-Class weld of a boiler, which was difficult to be detected in theory. Yoke magnetic powder testing was conducted, such as magnetic field indicating lines testing and field indicating lines by yoke suspending testing. Testing results showed that when testing by yoke at a limited space, if the magnetic field was sufficiently strong, the testing could still be proceeded for the defect at the magnetic yoke surrounding, but more attentions were needed to be paid to distinguish the relevant and non-relevant indications.