Study On The Analysis Of Welding Defects And The Ultrasonic Testing Technology Of Nickel-base Alloy

Liquefied natural gas has the advantages of saving storage space and transportation,and promote the application and development of natural gas in various industries by the characteristics of high reliability and low risk.LNG not only has the characteristics of inflammable and explosive as natural gas,but also it is easily lead to other security issues with low temperature and liquid features.So it is very important to ensure the stability of LNG tank.The LNG tanks are welded by low temperature nickel-base alloy steel as 9%Ni steel and 5%Ni steel,a variety of defects may be leaded in welding.Such materials possess coarse weld grains,serious ultrasonic scattering and attenuation,which could easily lead to the missing of test and misjudge of imperfections.This paper analyzes the importance of nickel-base alloy welding defect,and designs and processes narrow pulse longitudinal wave angle probe,analyses the effects of complex wavelet denoising the echo signal.The paper analyzes the microstructure of weld zone、heat affected zone and the base metal of X7Ni9 steel and X12Ni5 steel,finding nickel-based alloy for the weld coarse grain size,such as 9%Ni steel weld zone average grain size 359.2μm.Construct nickel-base alloy welding defects fish-bone evaluation and analytic hierarchy process of weld defects.Welding hot cracking、lack of low-temperature toughness、welding magnetic blow are more seriously on security of nickel-based alloy.Workers mistakes、 people due to lack of technology、 lack of preparation before welding materials、process selection improper 、welding environment unclean are more seriously in factors leading to defects.Design suitable for nickel-based alloy narrow pulse longitudinal wave angle probe,the parameters of transducer are as follows: frequency 2MHz,the piezoelectric wafer material selected lead zirconate titanate(PZT-5),the wafer thickness 1mm,the wafer size 20×20mm,damping block component quality rich powder with an epoxy resin ratio of 20: 1,the cam processing plexiglass with toothed sound trap,the angle of incidence 45°and60°.Study based on the reference signal detection,stepwise analysis wavelet function,decomposition level,threshold rules,Threshold retune method and threshold function of the noise reduction effect by Haar、Daubechies、Coiflets、BiorSplines and Symlets wavelet basis function.Draw appropriate Wavelet Noise Reduction Methods by SNR comparison.The SNR is most high with 30.7501 dB when select db4 wavelet function,decomposition level 4,the soft threshold function threshold s threshold rules with rigrsure,threshold resetting method with sln.The ideal wavelet function of complex wavelet denoise is obtained by the optimization of unit-wavelet.The most effect complex wavelet denoise is composited by db4 wavelet function and sym5 wavelet function with 33.7500 dB.By ideal wavelet noise parameters obtained by ultrasonic detection signals for different types of nickel based alloy weld denoise can reach acceptable level.