| The pressure vessels are necessities in the petrochemical industry, and its security is directly related to people’s lives and property. There are many fillet joints on the pressure vessels, whose structure is complicated and it is always bearing poor stress, so it is difficult to detect and will miss some important defects easily. The national standard which is called “boiler and pressure vessel safety supervision regulations†had a regulation in the early years, which said that there should not have mainly compressive fillet weld on the equipment. With the development of science technology and the application of new materials, many fillet welds have been used in the pressure vessels in recent years, and it is under pressure all the time. So many new requirements are put forward to test the fillet weld. In 2013, the new national standard was published, which is the guiding principle of the framework, but for the various types of fillet weld in pressure vessels, it is lack of specific guidance. Due to the particularity of fillet weld position, general detect method is easy to cause mistakes, choosing a better detect method is the key to ensure the quality of fillet weld detection.Firstly, MATLAB was used to draw the figures about the sound pressure distributed situation of the ultrasonic sound field. By transforming different parameters, the specific change rule of the sound pressure distributed situation was summarized, which provided basis for the selection of the probe. In this text, the essential characteristics and the forming reasons of the crack were analyzed. By testing seven groups of simulated specimens with prefabricated cracks in the butt welds, the method of how to quantify and locate for the cracks were researched. Moreover, this text proposed that using the regression analysis method to reduce the uncertainty of measurement. These studies can be of great service to estimate the lifetime of the in-service devices.For the fillet weld detection, the T fillet weld specimen of single-sided groove type was studied in this paper, which is common to see in the engineering. But there are no corresponding types in the standard, so we need to reference related types of weld detection rules. Since the national standard is the terms of principle, there is no specific test conditions and process. In this article, the structural feature of the fillet joints was discussed. Through experiment contrast analysis, a set of testing technology of the fillet joints was proposed, which mainly studied the choice of the detect surface and probe, the mobile area of the detection and probe, the drawing method of the DAC, etc. Then, by testing three groups of specimens with prefabricated cracks in the fillet welds, the rationality of the testing technology was verified. The testing method was also used to test the using equipment of the factory. Due to the complicated construction of the fillet joint, the magnetic memory detector was used to check the fillet joint firstly, and then the ultrasonic was used, which improved the detection efficiency. Besides, combining with the figures, the specific location of the cracks will be more intuitive, and it will facilitate the positioning, qualitative and quantitative judgment of the defect.Since it is difficult to qualitative for the defects, the basic features of all kinds of defects were analyzed, and a qualitative method was summarized, which was successfully used to determine the nature of the defects. And by testing seven groups of specimens, the effect of the width and depth of the cracks on the wave shape were analyzed.At last, there are many interfering signals in the ultrasonic signals, which will have negative influence on judging the quality of the workpieces. In this text, the wavelet analysis theory was used to remove the noise. By comparing the different parameters, the best wavelet function, threshold value and the threshold processing method were found out. And a new threshold processing method was proposed. |
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