Formation Of Eigen-line In Cold Welding Ultrasonic Image Of Welded Ioints Of Polyethylene Pipes

As an effective and clean energy, the proportion of natural gas in national primary energy consumption will be enhanced. The widespread application of natural gas inevitably puts forward higher requirements to natural gas pipelines, especially the pipe network around the urban residential area.Polyethylene (PE) pipe, which has a long service life, good resistance to stress cracking resistance, low cost and multiple new trenchless construction methods etc, is the best option for constructing urban gas network. With the extensive use of PE pipe, more and more problems arise. The failure of pipeline welded joints accounts for about two-thirds of the total pipeline failures. Cold welding is one of the most dangerous defects in the welded joint of PE pipes. Ultrasonic phased array is an effective nondestructive testing technique to inspect the cold welding. It relies on the ultrasonic reflection from the boundary of molten zone during welding, which is called Eigen-line in the ultrasonic image. The Eigen-line has been used to inspect and evaluate the degree of cold welding. However, the formation mechanism of Eigen-line is still unclear. On the support of the National Natural Science Foundation of China (Project No.51305394), this paper focuses on the theoretical analysis, microstructure observation and macroscopic properties study on the Eigen-line and the material in nearby area, the main work is as follows:(1) Based on the reflection theory of ultrasonic enters through the interface perpendicularly and the movement pattern of the Eigen-line, heterogeneous thin layer interface reflection on the Eigen-line was confirmed. Considering from the thermal history, the formation of microcrystalline on the interface, and grains orientated along the interface may be responsible for the formation of the Eigen-line.(2) Differential Scanning Calorimetry (DSC) was applied to simulate the welding process to make the samples that have the same thermal history of different area in the welded joint. Field Emission Scanning Electron Microscope (FESEM) and Transmission Electron Microscope (TEM) were used to observe the microstructure of the simulated samples. The FESEM results showed that there is no obvious change in the grain size of Eigen-line, but the microstructure of the grain is different. And the TEM results showed that there is grain orientation exist on the Eigen-line.(3) The macroscopic properties which related to the Eigen-line formation, namely the elastic modulus was measured. The elastic modulus was measured by Depth Sensing Indentation (DSI). The results showed that the elastic modulus increased along the radial direction and didn’t change along the circular direction on the Eigen-line of butt-welded joint.(4) Grains orientated along the interface lead to the change of elastic modulus. The elastic modulus on the Eigen-line varied from other areas, thus the ultrasonic signal would reflect on the heterogeneous thin layer interface.