Failure analysis of Buried Natural Gas Pipeline Tee

In order to analyze the cause of the fracture of a high-pressure gas pipeline tee, the material metallographic examination, mechanical property examination and micro fracture scanning electron microscope were used to analyze the crack. It was concluded that the tee was not normalized or recrystallized annealed, and the external load was the main cause of the fracture.

A high-pressure gas pipeline directional crossing of the highway was completed after 2 years of use, due to traffic needs, the overpass was erected above the pipeline. The edge of the overpass foundation is about 0.5m from the tee setting, and the pipeline is about 0.6m above the foundation at the bottom of the overpass, as shown in Figure 1(a). Pipe specifications ∅ 168.3mm × 6.4mm, material 20# steel, design pressure 4.0MPa, tee pipe wall thickness 8mm, material L290, heat treatment state for tempering. There are often large vehicles passing above the bridge, imposing a large concentrated load on the pipeline, and the tee fractured and failed after six months of use. Crack in the straight section for the ring crack, and along the branch pipe axial development, the crack shape as shown in Figure 1 (b). In order to find the cause of the tee fracture, so as to take effective countermeasures and avoid the occurrence of similar accidents in the future, the cause of the tee fracture was analyzed.

Test specimen preparation and test method

Firstly, the fracture of the tee was analyzed macroscopically, and then the tee was sampled by wire cutting. Samples were taken from the crack source area of the tee for microscopic fracture analysis; one sample was taken near the crack source area for metallographic analysis; three samples were taken from the crack source area for tensile property test.

Figure.1 Macroscopic photograph of fractured tee
XJZ-1A metallographic microscope was used to analyze the microstructure, JSM-5610LV scanning electron microscope was used to observe the microstructure of the fracture, Q8MAGELLAN direct reading spectrometer was used to analyze the material composition of the tee, and WDW3100 micro-controlled electronic universal testing machine was used to test the tensile properties.

Test results and analysis

Macroscopic fracture analysis

Cracked tee coloring flaw detection photos are shown in Figure 2 (a), it can be seen that the straight section and the branch section transition zone annular crack depth (large penetration), while the branch section axial crack depth shallow (small penetration), indicating that the crack may originate in the straight section and the branch section transition zone. It was also found that the primary cracks were not continuous, but showed a multi-step expansion and abundant secondary cracks. Since fatigue fracture generally has only one main crack, and secondary cracks are rarely seen, the possibility of fatigue fracture of this tee can be preliminarily excluded [1].
The original macroscopic fracture shape of the tee is shown in Figure 2(b), and it can be clearly seen that there is an obvious herringbone pattern on the fracture surface.

Figure.2 Macroscopic photograph of the fractured tee fracture

Microstructure analysis

Figure 3 shows that the metallographic organization near the crack of the fractured tee is ferrite and pearlite, and there is a lot of deformation in both ferrite and pearlite, and there are obvious processing flow lines, which indicates that the tee is not normalized or recrystallized and annealed after processing [1,2], and the brittleness and toughness of the tee cannot be recovered, and the residual processing stress inside the tee cannot be eliminated, which is a safety hazard.
The tee cracking direction is parallel to the processing flow line direction, and the crack is parallel to the distribution of the pearl in the processing flow line. Processing streamline is also called streamline, during processing, the brittle impurities of the metal are broken up and distributed in the direction of the main elongation of the metal in the form of grains or chains; the plastic impurities are distributed in the form of bands along the main elongation direction with the deformation of the metal, so that the metal organization after hot forging has a certain directionality. The processing streamline makes the metal properties anisotropic; the strength and plasticity are higher along the streamline direction (longitudinal), while the tensile strength and plasticity are lower perpendicular to the streamline direction (transverse).

Figure.3 Metallographic microstructure of cracked pipe tee
It can also be seen from Figure 3 that the secondary cracks are consistent with the processing flow line, which shows that the processing flow line inside the tee tube is the main organizational cause of the cracking of the tee tube, while the improper forming process of the tee causes severe strain ageing embrittlement of the material is an important cause of the cracking of the tee.

Mechanical properties testing

The specimens were intercepted at the crack of the tee (see Figure 2(a)) and tested separately in tensile test. Due to the size and shape of the tee tube, the mechanical properties of the specimen sampling site is located in the combination of the straight section of the tee and the branch section, the area is also part of the crack extension range, should have a good representative; sample orientation is perpendicular to the crack direction sampling, so as to more accurately reflect the crack area material strength and fracture properties. The measured results are shown in Table 1.

Note: The standard properties of L290 steel are quoted from GB/T9711.1-1997 “Technical Conditions for Delivery of Steel Pipe for Oil and Gas Industry” [3].
From Table 1, it can be seen that although the tensile strength of the tee pipe meets the standard requirements, but the plasticity index (elongation) is seriously substandard, and the plasticity is also very uneven, especially the 2 # specimen, elongation is only 2.6%. At the same time, the flexural strength ratio of the tee is as high as 87%, and the safety plasticity margin is also very limited, which may lead to cracking of the tee due to short overload impact.

Microscopic fracture scanning electron microscopy

Figure 4 shows the SEM microscopic fracture morphology of the cracked tee. It can be seen that the original fracture of the tee has a typical brittle quasi-dissolution fracture morphology, and the microscopic fracture surface is basically composed of river patterns, with only a small amount of ductile tearing traces, indicating that the tee has a high local brittleness. At the same time, the microscopic fracture morphology analysis did not find fatigue glow lines, indicating that the tee tube fracture is not fatigue fracture, but a one-time (or a limited number of times) brittle fracture, the fracture cycle is very short, caused by overload impact.

Figure.4 SEM photo of the original fracture of the tee

Conclusions and recommendations

• (1) The tee is a brittle fracture caused by overload impact, and the source of the crack is located at the upper surface of the pipe (stress concentration) in the transition area between the straight section of the tee and the branch section, and then expands axially along the branch pipe and simultaneously to the depth of the inner surface of the pipe until it penetrates the wall of the tee and causes gas leakage.
• (2) The processing flow line inside the tee was the main organizational reason for the cracking of the tee; the improper forming process of the tee led to severe strain-age embrittlement of the material, which was an important reason for the cracking of the tee.
• (3) Due to the high pressure gas pipeline above the overpass bottom foundation of about 0.6m, the minimum cover thickness is not enough, under the design load and vehicle load, the tee produces a large local stress at the upper surface, at the same time, due to the high brittleness of the tee pipe, it leads to brittle fracture, the fracture cycle is very short, caused by overload impact.
• (4) It is recommended that the tee should be strictly normalized during the extrusion process, then tempered, or normalized or recrystallized annealing heat treatment should be carried out after the process is completed to eliminate the processing streamline, restore plasticity and the appropriate flexural strength ratio to ensure the normal service life of the tee.
• (5) The high-pressure gas pipeline is about 0.6m above the foundation at the bottom of the overpass, which does not meet the minimum cover thickness requirement (not less than 0.9m) of GB50028-2006 “Design Code for Town Gas” [4]. It is suggested that this high-pressure gas pipeline be protected by additional casing or increase the burial depth of the pipeline to reduce the impact of vehicle load and ensure the safe operation of the pipeline.

Authors: Chang Le, Qian Yinghao, Zhou Pengfei, Ding Yi, Dong Jinsan

References:

• [1] Chu Wuyang, Qiao Lijie, Chen Qizhi, et al. Fracture and environmental fracture [M]. Beijing: Science Press, 2000: 25 – 26.
• [2] Wu Hong, Peng Jianhong, Xu Yunhua, et al. Effect of strong cold drawing plastic deformation and annealing treatment on the organizational properties of pearlite steel [J]. Materials and Thermal Processing, 2007, 36(16): 13 – 15.
• [3] GB/T 9711.1-1997, Technical conditions for the delivery of steel pipes for the oil and gas industry [S].
• [4] GB 50028-2006, Design Code for Urban Gas [S].