Modeling Of Ultrasonic Testing And Noise Suppression Using Phase Coherence Imaging Algorithm In Heavy-walled Cast Austenitic Stainless Steel

Because of its high fracture toughness, resistance to corrosion and weldability,casting austenitic stainless steel (CASS) has been widely used in primary circuit pipes with 66～96 mm thickness in pressurized water reactors. The mandatory requirements of non-destructive test (NDT) have been raised in inspection code of nuclear power plant, which are about the quality and service condition of CASS pipes. The existing inspection code requires radiographic testing (RT) and liquid penetrant testing (PT) are the NDT methods for CASS pipes. However, the disadvantage of RT is not sensitive to the harmful planar defects, incapable to determine the flaw position and radioactive. PT is incapable to inspect the subsurface and internal flaws. Compare with RT and PT, ultrasonic testing (UT) has prominent advantage such as sensitive to the harmful planar defects and capable to determine the flaw position. However, the CASS is composed of anisotropic coarse-grained structure which exhibits very large equiaxed or columnar or equiaxed-columnar mixed grains structure.Scattering effect of coarse-grain to ultrasonic wave will lead to serious scattering attenuation and structural noise during the UT for CASS pipes. When a flaw depth is greater than 50 mm, the sound energy of flaw echo is very weak. The signal to noise ratio (SNR) is even less than 10 dB and the probability of the flaw miss or false calling is significant increase. Hence, the UT of Heavy-walled CASS pipes is a challenging topic in NDT field.To overcome the above difficulties, including development of phased array ultrasonic testing (PAUT) equipment and probe, UT simulation modelling, exploring signal or image post-processing techniques has been carried out. UT simulation modelling is a very important methods for studying of ultrasonic wave propagation in coarse-grained structure.In addition, UT simulation has been widely applied to optimize PAUT parameters and develop advanced post-processing techniques. However, discrepancy occurs between the ultrasonic inspection simulation and experimental results for anisotropic coarse-grained materials due to the difficulty of the accurate description of the grain structure, orientation and elastic characteristic. Developing signal or image post-processing algorithms which improve the SNR is an effective way to solve the of the UT problem of CASS pipelines. The existing algorithms mainly includes filtering and delay-and-sum (DAS). The DAS algorithm is capable of improving the SNR of imaging zoom by delay-and-sum process. However, the signals for DAS imaging are acquired by B-scan using single probe. The method causes that sound energy is very weak in deep of heavy-walled CASS pipes and the SNR improvement is limited for deep flaw.Focusing on the UT simulation modelling and post-processing algorithm for SNR improvement for the heavy-walled CASS, this paper had done the related research and got the following conclusions:The electron backscattered diffraction (EBSD) technique was used to analyze the grain structure and crystal orientation of CASS. Then an EBSD map of CASS with 96 mm×12 mm radial-axial cross section was acquired. A model for UT simulation of CASS based on EBSD map was established using selection of misorientation angle and orientation unification.Based on the EBSD analysis, the difficulties to describe the grain structure and orientation of thick-walled CASS in the UT simulation modeling was overcome in this paper. The Bond transformation was used to determine the stiffness matrix of the grains with different orientations in the model, thus realizing quantitative description of the grain elastic characteristics.The CASS ultrasonic inspection model was calculated by finite difference time domain method. The results showed that the simulate A-scan signals reproduced scattering attenuation, decrease of central frequency and structural noise and it presented a good consistence with that in experiments. The noise amplitude and SNR of simulate PAUT S-scan images were quite well with experiments. The effect of grain shape (length-diameter ratio l/d and grain growth direction 0) to ultrasonic scattering degree was analyzed using numerical calculation. The results showed the following:The grains were regarded as equiaxed grain structure, when l/d=1, scattering degree does not change with θ(0°≤θ≤90°). The grains were regarded as columnar grain structure, when l/d>1, scattering degree will increase with θ. The amplitude of variation of scattering degree with θwill increase with l/d. In equiaxed-columnar mixed grain structure, the existence of the effect l/d and θ to ultrasonic scattering degree. The above work provides effective solution to reveal the mechanism of ultrasonic scattering in the anisotropic coarse-grained structure.To further improve the SNR of deep flaw in heavy-walled CASS, the transmit-receive probe was used to increase the sound energy of deep flaw using pseudo focus field.The comparison results of sound field energy showed that the energy of transmit-receive probe with 100 mm probe center spacing was 40～75% and single probe with 1.5 mm width was 25～40% on depth of 50～70 mm. Based on the above work, phase coherence imaging (PCI) technique is used to establish circular coherence factor (CCF) and sign coherence factor (SCF) which represent the phase dispersion for each pixel in the original DAS image. By dynamic weighting using CCF and SCF, the amplitudes of structural noise signals with high phase diversity are effectively suppressed. When the method combining beam propagation path optimization design and image post-processing algorithm was applied to CASS weld with 78 mm thickness, the SNR of Φ3 side drilled hole with 50mm depth had increased form less than 20 dB to more than 30 dB. According to their characteristics, SCF could acquire higher SNR, but it may cause the discontinuity along the depth of the defect in the image.CCF could acquire the continuity image, but it could only acquire higher SNR in the pseudo focus field. Therefore, SCF is suitable for preliminary test, while SCF is suitable for defect sizing and location.