Abstract
The fracture toughness of the weld and heat-affected zone (HAZ) of the L360QS/N08825 composite pipe welded joint was evaluated by a crack tip opening displacement (CTOD) test. The fracture morphology, microstructure, and inclusion near fracture zones were observed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). The grain size and grain orientation of the crack propagation zone in the weld were investigated by electron back-scattered diffraction (EBSD). The results revealed that the average CTOD values of the weld and HAZ samples were relatively high, and a greater dispersion of CTOD values of the HAZ samples is due to the pop-in phenomenon in the P–V curve. The fracture surfaces of the weld and HAZ samples showed the characteristics of ductile fracture to a certain extent, whereas the fracture of the CTOD sample with the pop-in phenomenon exhibited a quasicleavage feature. High-density dislocation and a large number of inclusions were observed in the near fracture zone of the weld and HAZ samples. The stress concentration, caused by hindering the dislocation slip, was the main reason for microcrack formation. The existence of large-size grains and large-scale small-angle grain boundary in the weld implies that the cracks propagate toward the weld.
Highlights
Bimetallic composite pipes have been increasingly applied in the chemical industry, for environmental protection, for oil and gas transportation, and in the nuclear industry [1,2,3] because of their several benefits such as a reduction in material costs and a combination of good strength and excellent corrosion resistance of the two dissimilar materials [4, 5]
Crack tip opening displacement (CTOD) is thought to be the essential fracture parameter to assess the fracture behavior of steel and welded joints based on nonlinear elastic fracture mechanics [15]
To ensure the safety of bimetallic composite pipe welded joints during operation, in the present study, an experimental investigation was carried out using crack tip opening displacement (CTOD) tests to evaluate the fracture toughness of the weld and the heataffected zone (HAZ) in an L360QS/N08825 bimetallic composite pipe welded joint according to BS7448 [26,27,28], ISO12135 (2002) [29], and ISO15653 (2010) [30] fracture toughness test standards at room temperature. e fracture morphology, microstructure, and inclusion near fracture zones were observed by means of scanning electron microscopy (SEM), TEM, and EDS, and the differences in grain size and grain orientation of the crack propagation area were studied by the EBSD technology. e primary aim was to study the fracture toughness characteristics of each part of the welded joint to provide a theoretical reference for engineering applications
Summary
Bimetallic composite pipes have been increasingly applied in the chemical industry, for environmental protection, for oil and gas transportation, and in the nuclear industry [1,2,3] because of their several benefits such as a reduction in material costs and a combination of good strength and excellent corrosion resistance of the two dissimilar materials [4, 5]. Wang et al [18] studied the CTOD fracture toughness of the weld and heataffected zone (HAZ) of X80 steel at 0°C according to the BS7448 standard and found that the CTOD test could be used to evaluate effectively the fracture toughness of welded joints. To ensure the safety of bimetallic composite pipe welded joints during operation, in the present study, an experimental investigation was carried out using CTOD tests to evaluate the fracture toughness of the weld and the HAZ in an L360QS/N08825 bimetallic composite pipe welded joint according to BS7448 [26,27,28], ISO12135 (2002) [29], and ISO15653 (2010) [30] fracture toughness test standards at room temperature. To understand further the direction of crack propagation from the perspective of crystallography, the crystal structure, grain size, and grain boundary misorientation of the weld zone were observed and analyzed using EBSD technology
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