Design And Application Of Partial Immersion Focused Ultrasonic Transducers For Austenitic Weld Inspection

Austenitic stainless steels have many good mechanical properties,such as good corrosion resistance,creep resistance,and yield strength.They have been widely used in transportation,nuclear power,pipeline and other engineering structures in China.As many structures have complex shapes,austenitic stainless steels should be welded in the manufacturing process.If there are defects in welds,the safety of the structure will be seriously threatened.Therefore,it is essential to timely and effectively detect defects in austenitic stainless steel welds to avoid accidents.Among the current nondestructive testing methods,ultrasonic nondestructive testing is widely used in the detection and evaluation of weld defects because of its advantages of rapidity,safety and low cost.In the practice,the contact method is commonly used to detect defects in welds,but the signal-to-noise ratio(SNR)of the detection results is very low,and missed detection and false detection may occur.In this thesis,taking the detection in butt stainless steel welds as an example,aiming to solve the difficulties in current detection,a partial immersion focused(PIF)ultrasonic detection method is proposed.The design of PIF transducer and optimization method for detection results are mainly discussed,in order to improve the accuracy of weld ultrasonic detection.Firstly,the development of PIF ultrasonic transducer is carried out.Based on the basic ultrasonic propagation wave theory,a partial immersion ultrasonic detection technology using focusing method is proposed to improve sound energy density and signal-to-noise ratio of detection results.Considering the application characteristics,application difficulties and process requirements of butt weld detection,the design scheme of PIF testing is formulated.The PIF ultrasonic transducer is designed and manufactured,and the mechanical scanning frame is introduced in the weld ultrasonic testing device.Relevant testing experiments were carried out,and the results show that compared with the contact ultrasonic detection method,the proposed partial immersion focused testing scheme can be applied to different surface states of the weld and has a higher SNR.Then,the PIF transducer is optimized with consideration of wave propagation theory in welds.The grain characteristics of the weld area and the propagation characteristics of ultrasonic wave in the stainless steel weld area are studied.The grain distribution model for the weld area is established,and further combined with Dijkstra algorithm,the ultrasonic wave propagation path in the weld area is calculated.Based on the characteristic parameters of the weld and the propagation simulation of ultrasonic wave in the weld,the relevant parameters of the PIF transducer are optimized,such as frequency,underwater wave distance and incident angle.The PIF transducer with optimal parameters is used to detect defect in butt weld,and the detection results are compared with those obtained using PIF transducers with different parameters.It is observed that accurate detection results can be obtained when the optimized parameters are used.Later,the scattering noise theory of stainless steel is studied,and the time-dependent threshold theory for noise elimination is proposed.In order to reduce the effects of noise on the detection results using the PIF transducer,the generation principle of noise,the Huang-Hilbert transform is introduced to analyze the characteristics of noise signal,and the noise statistical method is studied.The influence of noise statistics on threshold selection is studied,the influence of optimal time-dependent threshold and threshold size on ultrasonic signal is explored,and the selection method of optimal threshold is determined.Combined with ultrasonic imaging technology,the internal defects of welding blocks are intuitively displayed to improve detection accuracy,and relevant experiments are carried out to verify and optimize the theory.Finally,the weld detection experiment is performed by using the PIF method proposed in this thesis.The butt weld specimen with artificial defects is designed and the weld ultrasonic detection system is built.The specimen for relevant detection is designed and the weld detection system is built.The detection experiments are conducted using contact transducer,partial immersion planar(PIP)transducer and PIF transducer,and the Bimaging display and signal analysis are carried out.The results obtained using different transducers are compared,and it is shown that all the defects in the specimen can be detected using the designed PIF transducer.When the imaging results obtained using PIF transducer is optimized using the time-dependent threshold,the false detection can be avoided,which verifies that the proposed method is effective in practice.It is also proved that this method can be used in weld ultrasonic testing to improve the detection rate and accuracy of defects.