Explosion Cracking Analysis of Sulfur Resistance Induction Heat Bend Pipe

The explosion cracking reason on induction heat bend pipe was analyzed through macroscopic analysis, chemical composition analysis, mechanical properties testing, metallographic analysis, scanning electron microscopy and XRD. The results showed that the chemical composition, tensile test, Vickers hardness results of the straight section of the bent pipe met the requirements of the technical specifications, and the metallographic structure of the straight section of the bent pipe was pearlite and ferrite. The Charpy impact test in the neutral zone of the bend section had a low shear fracture ratio, and the Vickers hardness test results were on the high side. The metallographic structure was granular bainite. The fracture surface of the failed bent pipe exhibited brittle fracture characteristics on the macro level. The metallographic analysis and hardness test results of the crack source area showed that the fracture surface had typical intergranular propagation characteristics and high hardness values. Energy spectrum analysis of the crack source area revealed the presence of S element. The XRD analysis of the product on the inner wall of the pipe contained sulfides. The comprehensive results indicated that the failed bend pipe undergoes stress corrosion and initiates corrosion cracks on the surface under the combined action of operating pressure, additional bending stress, residual stress, etc. in a sulfide corrosive medium, the cracks continued to propagate along the grain boundaries and ultimately caused the bend pipe to explode.


Introduction
Bend pipe was an important part of oil and gas pipeline, which was responsible for changing the direction of the line or improving the stress state of local steel pipe in the line [1] .Compared with straight pipe, bend pipe was subjected to more complex stress and harsh working conditions, and was a relatively weak point in the pipeline [2][3][4][5][6][7][8][9] .
Pipe bending burst failure occurred in a ground gathering and transportation project.The gas collection pipeline has been put into operation for 1 year and 4 months.The transportation medium was unpurified wet natural gas containing sulfur and saturated water.The design pressure was 8.0MPa and the design transportation temperature was 3℃-20℃.The size of the failed bend pipe was Φ323.9×10.5mm45° 6D, the steel grade was L360QS, and the mother pipe was seamless steel pipe.The bending process was based on SY/T 5257-2012 "Steel Induction Heating Bending Pipe for Oil and Gas Transmission" [10] .On the day of the accident, the operating pressure of the transport was 5.5MPa-5.6MPa, the temperature was 5℃-7℃, and the content of hydrogen sulfide in the transport medium was 732ppm.The bend pipe burst from the outer arc side with a large crack (a crater with a diameter of 6m and a depth of about 5m was rushed out at the site).The implementation standard of the failed bend pipe was the technical specification of Induction Heating Bend for Lines (hereinafter referred to as the Technical specification).

Macro Analysis
The length of the failed bend pipe was about 1.2m, which was a part of the total length of 2.5m bend pipe, including a 0.5m long straight pipe section and a partial bend section.One end was cut off by the onsite construction.The cracking place was located on the outer arc side of the bend.The macro picture of the failed bend was shown in Figure 1.
It was found that the fracture surface was flat without macroscopic plastic deformation, covered with corrosion products, and the color was dark.The analysis showed that there were two fracture source origins in the burst fracture.Source origin 1 was located near the girth weld of the pipeline, and source origin 2 was located near the maximum position of the fracture.The locations of the two fracture source origins were shown by the arrows in Figure 2.

. Chemical composition analysis
According to GB/T 4336-2016, samples were taken near the cracking location of the bend pipe for chemical composition analysis, and the results were shown in Table 1.The results showed that the chemical composition analysis results of the failed pipe meet the requirements of the technical specification.

Tensile property test
Due to the cracking of the outer arc side of the bend pipe and the deformation of the bend section, the mechanical properties of the intact parts near the neutral zone of the bend section were sampled.
According to GB/T 228.1-2010, the bend pipe was tested by a material testing machine.The tensile sample was a full-wall thick plate sample, the sample size was 300mm×38.1mm,and the standard distance length is 50mm.The results were shown in Table 2.The results showed that the tensile test source origin 1 source origin 2 results of the straight section and the neutral section of the bending pipe met the requirements of the technical specification.
Table 2. Tensile test results

Sampling position and orientation
Yield strength

Charpy impact test
Charpy impact test was carried out on bend pipe by impact testing machine.The test temperature was -10℃.Transverse sample was taken, the V-notch direction was perpendicular to the surface of the steel pipe, and the sample size was 10mm×10mm×55mm.The results were shown in Table 3.It could be seen from the results that the impact absorption energy of the straight pipe section and the neutral pipe body met the requirements of the technical specifications, while the shear section ratio did not met the requirements of the technical specification.The Vickers hardness test was carried out on the bend pipe with Vickers hardness tester.The position diagram of Vickers hardness indentation was shown in Figure 3, and the results were shown in Table 4.
The results showed that the Vickers hardness test results of the straight section of the bending pipe meet the requirements of the technical specification.The hardness value of the neutral zone near the failure part was higher than the technical specification.

Electron microscope and energy spectrum analysis of fracture
The high-resolution macro image acquisition and analysis system and scanning electron microscopy were used to observe the macro and low-power morphology of the specimens in the fracture source area.It was found that there were radial stripes on the fracture, and the convergence point of the radial stripes was the cracking source, as shown in figure 8-9.

3.2.2.Fracture metallographic analysis and Vickers hardness test
Because of the corrosion products on the surface of the fracture, it was not possible to observe the high-power morphology by scanning electron microscopy, so the metallography and hardness analysis of the source fracture were carried out.The results showed that no crack defect was found in the source origin 1 fracture, the fracture profile had the characteristics of intergranular fracture, and the fracture structure in the source zone was bainite.There were cracks in the fracture profile of source origin 2, and the crack propagation was intergranular, and the fracture structure of source origin was bainite, as shown in Figure 12-15.The Vickers hardness test was carried out on the fracture with the Vickers hardness tester.The results were shown in Table 6.The test results showed that the fracture Vickers hardness was higher than the technical specification.

XRD analysis of inner wall products
Black products were taken from the inner wall near the fracture of origin 1 and origin 2 respectively for XRD analysis, as shown in figure 16 and figure 17.The results showed that the inner wall products were mainly FeS, FeS2, Fe2O3 and so on.

Analysis and Discussion
The results of chemical composition analysis, tensile test and Vickers hardness test conformed to the requirements of technical specifications, and the metallographic structure of the straight pipe section was pearlite and ferrite.In the neutral section of the bending section of the bend pipe, the shear section ratio was low, and the Vickers hardness test results were high.The metallographic structure was granular bainite.The fracture of the failed bend pipe was granular in macroscopically, and radial stripes could be observed, converging on the surface of the pipe body, showing a brittle fracture feature [11] .The results of metallographic analysis and hardness test showed that the cracking source region had typical intergranular expansion characteristics and high hardness value.The presence of S element was found in the cracking origin by energy spectrum analysis.The products on the inner wall of the pipe contained sulfide by XRD analysis.
The transportation medium of the pipeline was acidic natural gas containing H 2 S, which was consistent with the results of energy spectrum analysis and XRD analysis, so there was a sulfur-containing corrosive medium in the environment of the bend pipe.When the H 2 S content was higher than the critical value of 50μg/L prescribed by NACE, there were conditions for sulfide stress corrosion cracking [12] .According to the information provided by the site, the hydrogen sulfide content in the natural gas transport medium was 732ppm, which far exceeds the critical value of sulfide stress corrosion cracking.At the same time, natural gas is unpurified wet natural gas containing saturated water.The two forms a wet H 2 S environment in the pipe, and the water containing H 2 S becomes the corrosive medium of the pipe metal.At the same time, there was internal pressure in the pipe bend.Under the combined action of the three, stress corrosion cracking occurred in the bent pipe [13] .
In addition, the higher surface hardness of the bend pipe made it easier to stress corrosion cracking.Therefore, in the sulfide corrosive medium, under the combined action of operating pressure, additional bending stress and residual stress, stress corrosion occurred, and corrosion cracks were generated on the surface, and the cracks continued to expand along the grain boundary and eventually led to the bending pipe burst.

Conclusions and Suggestions
(1) The bend pipe burst was stress corrosion cracking and had the characteristics of multi-source cracking.Under the combined action of aqueous H 2 S corrosive medium, internal pressure, additional bending stress and residual stress, cracks spread along the grain boundary and eventually led to the failure of the bend pipe.
(2) It was recommended to locate and test the hardness of the bend pipes in the same batch to eliminate potential risks.

Figure 1 . 2 . 3 .
Figure 1.Macro picture of the failed bend pipe Figure 2. Locations of fracture source origin 3. Detection and Analysis of Failure Samples 3.1.Physical and Chemical Properties Test 3.1.1.Chemical composition analysisAccording to GB/T 4336-2016, samples were taken near the cracking location of the bend pipe for chemical composition analysis, and the results were shown in Table1.The results showed that the chemical composition analysis results of the failed pipe meet the requirements of the technical specification.Table1.Chemical composition analysis results (wt×10 -2 )

Figure 8 . 9 .
Figure 8. Morphology of fracture Figure 9. Morphology of fracture source origin 1 source origin 2 An energy spectrometer was used to analyze the surface products of the fracture source origin, as shown in figure 10 and figure 11.According to the results of energy spectrum analysis, the spectral peak of S element existed in both cracking source origins, and the content was high.

Figure 10 . 11 .
Figure 10.Energy spectrum analysis curve of Figure 11.Energy spectrum analysis curve of fracture source origin 1 fracture source origin 2

Figure 16 .
Figure 16.XRD analysis of inner wall Figure 17.XRD analysis of inner wall products in origin 1 products in origin 2

Table 4 .
Vickers hardness test results Laser confocal microscope was used to analyze the structure, nonmetallic inclusions and evaluate the grain size of the bending pipe.The results were shown in Table5, as shown in figure4-7.

Table 6 .
Vickers hardness test results