针对某厂生产的20SiMn钢水轮机主轴大型锻件在超声波探伤过程中发现的报废性缺陷，对探伤不合格部位进行了定位解剖、酸浸低倍检验、扫描电镜断口分析、能谱分析以及金相分析，查找出了导致该主轴超声波探伤不合格的原因。结果表明：主轴锻造用钢锭中高级别的一般疏松缺陷造成探伤时超声波声能大幅度衰减，导致无法探明主轴心部的质量；此外，钢锭心部硫化物夹杂聚集导致锻造变形时形成裂纹源，进而造成心部裂纹缺陷。

Aiming at the problem of discarding defects found in a heavy turbine shaft forging of 20SiMn steel produced by a factory during the ultrasonic flaw detection, the unqualified part was located and dissected, and means such as acid leaching macro-structure test, scanning electron microscope fracture analysis, energy spectrum analysis and metallographic analysis were used to find out the cause of the ultrasonic flaw detection disqualification of the shaft. The results show that the high-level general loose defects in the shaft forging caused the ultrasonic sound energy to be greatly attenuated during the inspection, which made it impossible to find out the quality of the shaft center. In addition, the sulfide inclusion center accumulation in the shaft caused the formation of crack sources during forging deformation and resulted in the center crack defects.