Research On The Ultrasonic Testing Method Of Austenitic Stainless Steel Welds

Austenitic stainless steel is widely applied to industry,but various kinds of defects will occur in welding process,which can easily lead to production accidents.Ultrasonic testing which can effectively find hidden defects in the weld is usually disturb by the noise result from the coarse grain of austenite.In order to solve this problem,the characteristics of austenitic material and its effect on ultrasonic are studied in this paper.Based on this,an improved de-noise method is proposed,which improves the reliability of ultrasonic detection results of austenitic welds.The main contents of this paper are as follows:1.In order to study the acoustic properties of welds,the formation principle of austenite grains and the growth of grains between layers of weld bead were analyzed,and based on this,the weld joint was divided into three regions.The reference blocks with different grain size corresponding to the three regions were fabricated,and the acoustic performance data of the reference blocks were actually tested.These data reveal the effect of material properties on ultrasound and the foundation for further research is established2.A multi-gaussian beam model was established,and the distribution of ultrasonic sound pressure in the welds was simulated.The austenite grain in the weld is simplified into a micro-reflector.Combined with the sound pressure distribution of the ultrasound and the acoustic properties of the material,the echo model of the ultrasonic wave is derived.Based on the echo model,the characteristics of austenite grain scattering waves are studied,which improves the mathematical model of structural noise.3.Structural noise can seriously affect the accuracy of ultrasonic testing,and traditional de-noising methods cannot effectively reduce this kind of noise.In order to solve this problem,an improved signal processing method is proposed in this paper: using CEEMDAN method to decompose the ultrasonic signal of welds made by austenitic material,and the IMF components associated to structural noise is extracted;then the local entropy parameter is introduced from the image processing field,and the local entropy threshold denoising method suitable for IMF components associated to structural noise is brought out.Finally,it is proved that the method brought in this paper is more advantageous than the traditional method through a series of comparative experiments.4.Aiming at the problem that ultrasonic waves cannot penetrate large-thickness austenitic welds,a set of double-crystal longitudinal wave focusing probes according to the design criteria of ultrasonic probes has customized in this paper.Tests show that this set of probes has a good detection effect on the weld test block.In conclusion,a local entropy threshold de-noising algorithm which is applied to the ultrasonic signal and achieves a higher signal-to-noise ratio than the existing methods is proposed based on the studying of the properties of austenite material and ultrasonic.After that,a set of special probes were designed for and customized on austenitic stainless steel welds contrast test blocks with large thickness,a good detection results were achieved.