The suspected circumferential weld defects found in the pipeline inspection process were tested and studied. It was found that there was slag inclusion in the girth weld at 9 o’clock, which had not been found in the early stage or in the laboratory nondestructive testing process. The defects are most likely due to incomplete slag removal between layers and low welding wire energy. There was a root crack at the 6 o’clock position of the weld, and the crack length was 10 mm, which was close to the original film and the detection result of excavation. The misjudgment of the field rebeat and the laboratory test results may be caused by the pits in the weld cover and the bottom weld. The crack is caused by the crystal crack formed along the middle of the columnar crystal during the solidification process of backing welding due to the increase of residual stress caused by the reduction of welding layers and the group stress caused by the steel tube.
Duplex stainless steel has been widely applied to industry. In this paper, nondestructive testing, chemical composition analysis, metallographic analysis, hardness test, and scanning electron microscope were used to study the causes of the crack in the butt joint girth weld of pipe and flange. Both steel pipe and flange are duplex stainless steel S31803. Through analysis, it was judged that the crack was a crystalline crack, generated in the welding process, which was related to the welding process and the control of the welding installation gap between the steel pipe and the flange with unequal wall thickness.
The bolts of wind farm are prone to fatigue failure during service. Generally, the fatigue stress does not require too much external force. Sometimes the stress generated by bolts is much lower than the yield strength of bolts. In view of the failure problem of connecting bolts used in wind farms, the failure causes of the double-head bolt were studied by macroscopic inspection, chemical composition analysis, metallographic analysis, and scanning electron microscopy. The results showed that the fracture was a fatigue fracture, and the fatigue source area was located at the bottom of the thread tooth. Under the action of long-term alternating stress, the single source crack evolved into multi-source fatigue cracks, where the fracture was related to the larger stress in the process of installation and service. This paper analyzed the causes of bolt failure and put forward effective suggestions for the service environment of wind turbine bolts.
Given the quality problem of a batch of oil and gas long radius elbow found on site, a quality sampling inspection was carried out. The quality was tested by a penetrant test, geometric dimensions measurement, chemical composition analysis, and a hardness test. Metallographic analysis was carried out about the defects found. It can be seen that no obvious abnormality was found in the profile size, remaining wall thickness, chemical composition analysis, and hardness test results of this batch of elbows. There is a longitudinal crack with a length of 10 mm on the outer surface of the pipe end of the DN50 elbow. According to anatomical analysis, this defect is a shallow crack on the outer surface, with a certain angle between the crack and the outer surface of the pipe. The maximum depth in the cross-section is 0.24 mm, and the vertical depth from the outer surface is 0.12 mm. It is speculated that these cracks occurred before solution treatment and are related to the original defects of the parent pipe or mechanical damage during cold work.
In this study, the performance test and defect anatomy analysis were carried out for a girth weld of the pipeline. The results of the girth weld tensile test, groove hammer break test and Vickers hardness test showed no obvious abnormality, and the impact toughness and bending test results were not ideal. In the conventional inspection process, it was found that there were tiny unfused girth welds at 6 o 'clock and 9 o 'clock positions, and the maximum direction size of defects on the wall thickness section was 1.57 mm. The defects at the position from 6 o 'clock to 7 o 'clock found by on-site X-ray detection were identified as the existing non-fusion defects by anatomical analysis, and should be considered as the root of incomplete welding defects that could not be completely eliminated during repair welding. The length and height of the defects are 9 mm and 1.38 mm.
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