Study On The Ultrasonic Testing Technology Of LNG Nickel-based Alloy Welds

With the vigorous implementation of the new national energy strategy,promotion of structural adjustment of energy,liquefied natural gas(LNG)production and consumption of has undergone a rapid growth year after year,the security of LNG storage device is faced with a severe test,The main materials of the LNG storage device are nickel-based alloys,such as X7Ni9 steel,X12Ni5 steel,316 L stainless steel,Such materials possess coarse weld grains,serious ultrasonic scattering and attenuation,prone-to-distortion sound beam,echo signals associated with forest-like noise,which could easily lead to the missing of test and misjudge of imperfections,Therefore,application of the appropriate method for noise reduction is the key to ensure the reliability and accuracy of ultrasonic test results.This paper combines hardware noise reduction and software noise reduction method,design and process wideband narrow pulse longitudinal wave angle probe,applies wavelet threshold denoise method to research to echo signal denoise.The paper has completed transducer of frequency 2.0MHz and 2.5MHz,detected X7Ni9 steel,X12Ni5 steel,316 L stainless steel weld.It is founded that lower frequencies are more conducive to improving the SNR of coarse grain weld.The parameters of transducer are as follows: the incident frequency 2.5MHz,the piezoelectric wafer material PZT-5,length and width of the wafer 13×13mm,thickness 1.2mm.The components of damping mass are tungsten powder,polysulfide rubber powder,and epoxy.The component of wedge block is plexiglass(processed into jagged sound trap).Through the ultrasonic detection test on block,the echo signals obtained are implemented single wavelet noise decrease.By wavelet analysis,decomposition level,threshold selection rules,re-adjust the threshold method,the impact on the SNR threshold function,the preferred single wavelet noise reduction parameters are determined as follows,wavelet function ’sym6’,’sym8’,’coif2’,’coif3’,’bior5.5’,’bior4.4’,’db7’,’db4’;decomposition level 6;threshold rule ’rigrsure’;threshold less than 76 is replaced by compromise threshold;threshold between 76 to 148 is replaced by soft threshold;threshold greater than 148 is replaced by threshold improved K = 1.Applying the above preferred parameters for complex wavelet noise echo signal analysis,complex noise reduction is better than each individual noise wavelet race results,among which bior5.5 and db9 complex,bior5.5 and db5 complex,bior5.5 and sym11 complex,sym7 and bior5.5 composite,coif1 and bior5.5 complex,coif3 and bior5.5 complex noise reduction effect are most significant.Complex wavelet noise reduction effect by the composite sequence,when Haar wavelet is situated at final composite can effectively improve the noise reduction.Based on the above analysis,the coarse-grained weld ultrasonic denoise software which is designed based on MATLAB graphical user interface GUIDE,enabling X7Ni9 steel,X12Ni5 steel,316 L austenitic stainless steel ultrasonic detection signal noise reduction,signal extraction,imperfections location through the software.