发电机护环内壁缺陷TOFD检测优化

超声波衍射时差法（TOFD）检测设备缺陷多采用45°、60°和70° 3种常规探头.基于发电机护环的壁厚和材质特性,即便采用45°探头也无法对其内壁缺陷进行准确评定.为此对TOFD检测工艺进行了优化设计,在分析衍射声波传播特性的基础上,提出了20°探头的检测模式,并研制出20°探头.通过对发电机护环内壁自然裂纹进行检测,确定出最佳检测频率为2.25 MHz.在该频率下对模拟缺陷试块进行检测,所得结果与实际情况吻合良好.研究得到的适合发电机护环内壁缺陷的最佳TOFD检测工艺为：20°探头,2.25MHz频率,晶片直径12 mm,声束聚焦深度为护环内壁.按此工艺对壁厚50 mm以内的发电机护环进行检测,能够检出内壁缺陷的最小临界尺寸为2 mm.

Optimization of TOFD Detection for Generator Retaining Ring Inner Wall Defects

The 45°,60° and 70° regular probes are widely used in equipment defect detecting with ultrasonic time-of-flight diffraction （TOFD） examination.However,even the 45° probe cannot accurately evaluate the defects in the inner wall of the generator retaining ring due to the wall thickness and the material characteristics.Therefore,the optimization is conducted to the TOFD detecting process.Based on the analysis on the propagation characteristics of diffraction acoustic wave,a detecting mode with 20° probe is proposed.Moreover,the 20° probe is developed.Through the natural crack detection in the retaining ring inner wall,the optimal detecting frequency is calibrated to 2.25 MHz.The detection results obtained under this frequency show the effectiveness of the new testing mode.The optimal TOFD process suitable to retaining ring inner wall defects detection is 20° probe,2.25 MHz and the 12 mm diameter transducer.The beam should be focused on the inner wall of generator retaining ring.With this optimized detection process,the minimum critical height of the inner wall defects that can be effectively distinguished is 2 mm in the 50-mm thick generator retaining ring.