Ultrasonic Qualitative And Quantitative Testing Of Defects For Narrow Gap Welding Of Nuclear Austenitic Stainless Steel

Austenitic stainless steel main pipe is the key structure of pressurized water reactor nuclear power plant,and its safety is a concern.The main pipeline is connected by narrow gap automatic welding technology.During the welding process,the included angle between the arc axis and the side wall is small,which is easy to form lack of sidewall fusion.Because the harm of lack of sidewall fusion is larger,nondestructive testing must be performed.Phased array ultrasonic testing(PAUT)technology has some advantages in the detection of narrow gap weld of austenitic stainless steel(NGWASS)in nuclear power main pipeline.When the NGWASS is detected by ultrasonic wave,the elastic anisotropic grains at different depths will cause different degrees of structural noise and scattering attenuation,reduce the detection signal-to-noise ratio(SNR),make it more difficult to defects of qualitative and quantitative.To solve the above problems,the electron back scattering diffraction(EBSD)observations was used to set up NGWASS model.To study the ultrasonic imaging characteristics of volumetric and area-based defects,and achieve the qualitative characterization of defects.Further study on the detection parameter optimization and signal post-processing method of phased array with lack of sidewall fusion at different depths,and achieve the precise quantitative of lack of sidewall fusion.The main research contents are as follows:(1)Metallographic and EBSD observation of NGWASS samples was carried out,the results show that the austenitic stainless steel base material was composed of fine equiaxed crystals and the weld was composed of thick columnar crystals.The NGWASS model with a thickness of 69.5mm was established.,and the model was simulated by finite difference time domain method.The comparison between simulation and experiment was made,it could be seen that the difference of velocity,attenuation coefficient,SNR,main frequency were 24.4m/s,0.01 dB/mm,0.6 dB and 0.1 MHz.The results of simulation and experiment were in good agreement,and the validity of the model is verified.(2)Eight defects with a height of 4.0 mm and a width range of 0.3 mm~4.0 mm were set at a depth of 26.5 mm in the model,and phased array ultrasonic simulation testing was carried out.The results showed that with the increase of aspect ratio,the amplitude ratio of the upper and lower tips in the fan scanning imaging decreases.The amplitude ratio between the upper and lower tips of the defect can be used as the basis for the qualitative differentiation of the defect.When the defect aspect ratio was less than 2,that was the corresponding volume defect,the amplitude ratio of the upper and lower tips was greater than 1.68.When the defect aspect ratio was greater than 3,that was the corresponding area-type defect,the amplitude ratio of the upper and lower tips is less than 1.25.In NGWASS block set ?3 while drilling and lack ofsidewall fusion with the aspect ratio of 3 mm for phased array ultrasonic testing.The experimental results showed that the amplitude ratio of the upper and lower tips in the fan scanning imaging of the while drilling was 5.38,and the amplitude ratio of the upper and lower tips in the fan scanning imaging of the lack of sidewall fusion was 1.22.The simulation was verified by the experimental results.(3)Due to the influence of coarse crystal structure,there are obvious structural noises in the imaging results,which affect the detection and quantification of lack of sidewall fusion.And the degree of difficulty in detecting the lack of sidewall fusion at different depths is different.For the lack of sidewall fusion with a height of 3.0mm and a center depth of10.0mm,26.5mm and 50.0mm in NGWASS,PAUT parameters were optimized respectively.The effectiveness of the detection parameters was verified by experiments.The amplitude of the defect signal was superposition by Total Focusing Method(TFM).The difference between the upper and lower tips of the lack of sidewall fusion was more obvious,and the maximum SNR was increased by 3.8 dB.Combined with Phase Coherence Imaging(PCI)method to further suppress structural noise,the image SNR after TFM and PCI treatment was increased by 5.9 dB to the maximum,and the quantitative and positioning errors were reduced by 6.7%and 3.0% respectively,thus improving the detection SNR and quantitative accuracy of the lack of sidewall fusion.