The Study Of Phased Array Ultrasonic Testing Focusing Characteristics On Thick-wall CCASS Pipe

Ultrasonic propagation in CCASS (Centrifugally Cast Austenitic Stainless Steel) is resisted because of coarse-grain and anisotropism, result in bad penetrability, high background noise and some other problems. Based on traditional TRL probe, Low-frequency TRL ultrasonic phased array inspecting technology has new advantages:strong ability of electronic deflection and focusing, high resolution of space and contrast. It’s applied in the detection of CCASS pipeline used in nuclear power station. The focusing characteristic can affect the detecting precision. In this paper key factors affecting the detecting precision are analyzed. The effect law is explored by comparing the analogue simulation and experimental results to provide appropriate selection principles. The main content are as follow:(1) Focusing principles and characters are analyzed for traditional ultrasonic focusing, TR probe and phased array. Based on this the principles and advantages of TRL are explored, and the theory basis is established for the next study(2) The macroscopic and microscopic structure of CCASS are analyzed and the ultrasonic parameters of98mm-thick sample including ultrasonic testing signal, amplitude spectrum, sound velocity and attenuation coefficient are measured. The research shows the obvious correlation between coarse-grained columnar and equiaxed grains and ultrasonic parameters.(3) The effects of heterogeneity of CCASS base metal and weld on the TRL phased arrays focusing sound field of low frequency are researched. The results indicates the heterogeneity of ultrasonic parameters which caused by the heterogeneity and elastic anisotropy of CCASS. This will lead to the distortion of sound field and bad imaging results. The focusing sound field of low frequency TRL phased arrays caused by different zone in CCASS weld.(4) The TRL phased array ultrasonic testing focusing sound field of different frequency probe(0.5MHz,1.0MHz,1.5MHz) are analyzed. The results indicates that the distortion of sound field under different frequency is different.0.5MHz affected by the material is smaller, penetrating ability is strong, but the beam width is bigger, detection resolution is low, suitable for detecting deeper flaws, to improve the defect detection rate; And1.0MHz and1.5MHz focusing acoustic field affected by the material is larger, penetration is low, but the beam width is small, suitable for detecting shallow defects, to improve the defect detection resolution.(5) The TRL phased array ultrasonic testing focusing sound field of different focus depth on thick wall CCASS and homogeneous material are compared. The results indicates that with the increase of focus depth, focal spot and focal axis length increases, the energy reduction; The effect of different focus depth on CCASS is greater than the homogeneous material. With the increase of focus depth, beam scattering is greater, the distortion increase. focusing capability is reduce.