Gong Guixian, Han Rongdong, Sun Yiqiang
(Research Institute of Wuhan Iron and Steel (Group) Corporation, Wuhan, Hubei 430080)
Abstract: Metallographic microscopy and scanning electron microscopy were used to analyze the cause of a burst failure that occurred in a swivel joint on a fracturing truck during fracturing operation in an oilfield. The results show that the burst failure of the swivel joint during fracturing operation was stress corrosion cracking. Because the material contained severe manganese sulfide inclusions, corrosion cracks initiated at the manganese sulfide inclusions under the combined action of tensile stress and chloride corrosive medium, then propagated further and caused the swivel joint to rupture.
During a drilling operation in an oilfield, a 3 in (76.2 mm), 105 MPa, Style 50 swivel joint used on a fracturing truck suddenly burst during fracturing operation.
The swivel joint was made of 20CrNiMo and had been in service for only about one year. Reinspection showed that its chemical composition met the standard requirements. The production process was forging, normalizing, rough machining, carburizing and quenching, low-temperature tempering and final machining. Before delivery, a body-strength test at 158 MPa had been carried out in accordance with the Factory Test Procedure for High-Pressure Flow Control Products, and the test result met the requirements.
During fracturing operation, the medium in contact with the swivel joint was acidizing fluid (20% HCl + 1.5% HF + adhesive). For one well, the acid medium injected during acid fracturing was 400 m3 to 1,000 m3, and the fracturing pressure was between 50 MPa and 90 MPa. To determine the cause of the burst failure, samples were taken from the leaking pipe for analysis. This article introduces the analysis process and results.
1. Inspection Results
1.1 Macroscopic Examination
Figure 1 shows the reassembled macroscopic photograph of the burst elbow. The fracture surface was severely corroded, but brittle-fracture features were still very obvious. Figure 2 shows the transverse fracture surface of the elbow, which was the main fracture surface. It can be seen that the fracture occurred at the sealing shoulder, with three obvious fracture origins located at positions A, B and C as indicated by arrows in Figure 2.
Figure 1. Macroscopic photograph of the fractured elbow
Figure 2. Matching fracture-surface features of the fractured elbow
Area A: a longitudinal crack penetrated through the wall thickness. The longitudinal fracture surface was covered with a severe layer of corrosion products. After cleaning with hot hydrochloric-acid aqueous solution, the fracture morphology is shown in Figure 3. The longitudinal fracture surface could be divided into two obvious regions. The side near the inner wall was relatively flat, with radial patterns extending from the inner-wall surface toward the wall thickness, indicating that the crack originated from the inner wall. The side near the outer wall was rough and fibrous; this region was the final tearing zone. At the same time, many longitudinal microcracks were found on the inner-wall surface, showing that formation of the longitudinal crack was related to microcracks on the inner-wall surface.
Figure 3. Fracture morphology of the longitudinal crack at Area A
Area B: the characteristics were generally the same as those of Area A.
Area C: this area accounted for about one fifth of the entire fracture surface. There were several small layered platforms on the section, with obvious radial stripes on the platforms. The stripes converged at the bottom of the outer sealing shoulder, where the wall thickness was relatively thin.
Based on the above features, the fracture-development sequence can be seen as follows: microcracks on the inner-wall surfaces of Areas A and B propagated in both longitudinal and transverse directions. After propagation reached a certain extent, a transverse crack formed at the bottom of the sealing shoulder in Area C and finally caused transverse fracture.
1.2 Metallographic Examination
Macroscopic examination found many longitudinal microcracks on the inner-wall surfaces at Areas A and B shown in Figure 2. After grinding the inner-wall surface, corrosion pits were observed at and near the cracks. These corrosion pits often initiated at sulfide inclusions (see Figure 4). A transverse metallographic specimen taken through the bottom of a corrosion pit showed that the crack propagated in a zigzag form along the wall-thickness direction of the elbow. Corrosion products were present inside the crack, and sulfide inclusions could often be seen at the crack tail.
A longitudinal metallographic specimen showed many fine strip-shaped sulfide inclusions (see Figure 5), rated as Grade A3 according to GB/T 10561-2005.
Measurement showed that the carburized-layer depth on the inner and outer walls of the specimen was 1.2-1.4 mm, meeting the relevant technical requirements.
The surface microstructure of the inner and outer walls of the specimen was coarse tempered martensite plus retained austenite. The prior-austenite grain size was Grade 8. Cracks propagated along austenite grain boundaries (see Figure 6) and stopped inside the carburized layer. The core was a tempered structure of martensite and bainite.
Figure 4. Sulfide inclusions and corrosion pits on the inner-wall surface
Figure 5. Sulfide inclusions in the sectional specimen
Figure 6. Crack propagation along austenite grain boundaries
1.3 Scanning Electron Microscope Observation
The main fracture surface shown in Figure 2 and the longitudinal fracture surfaces at Areas A and B were analyzed with a scanning electron microscope. The fracture surface was covered with corrosion products, in which chlorine was enriched (w(Cl) = 17.11%), as shown in Figure 7.
After the corrosion products on the main fracture surface were removed with acetone solution, observation showed that the surface layers of the inner and outer walls of the elbow, within a depth of about 0.84 mm, were dominated by intergranular fracture. Local secondary intergranular cracks were present, showing an obvious rock-candy pattern, as shown in Figure 8. The remaining areas showed quasi-cleavage plus cleavage features and a small number of fine secondary cracks.
The metallographic specimens were analyzed with the energy-dispersive spectrometer of the scanning electron microscope. In the corrosion pits at the inner-wall surface, in addition to oxides of Fe and Si, chlorine and a small amount of sulfur were enriched (Figure 9). The crack tail was branched and also enriched with chlorine (Figure 10). The sulfide at the crack tail was manganese sulfide, and chlorine was also enriched in the inclusion (Figure 11).
Figure 7. EDS analysis of corrosion products on the fracture surface
Figure 8. Intergranular fracture and secondary-crack features in the outer-wall surface layer
Figure 9. Morphology and EDS analysis of corrosion pits at the inner-wall surface of the section
Figure 10. EDS analysis of corrosion products inside the crack
Figure 11. Sulfide inclusion at the crack tail and EDS analysis
2. Result Analysis
Based on the above inspection results, the burst failure of the swivel joint on the fracturing truck in this oilfield during fracturing operation can be identified as stress corrosion cracking.
Stress corrosion cracking is a failure mode of metal components caused by the combined action of tensile stress, either applied stress or residual stress, and a corrosive medium. The factors causing the burst failure are analyzed as follows.
1) SEM-EDS analysis shows that chlorine was enriched in the corrosion pits on the inner wall of the elbow, inside the cracks and at the manganese sulfide inclusions. Intergranular failure is one characteristic of stress corrosion failure. This indicates that the medium causing stress corrosion in the elbow was chloride, a strong stress-corrosion medium. The swivel joint had been used for only about one year and was exposed to acidizing fluid (20% HCl + 1.5% HF + adhesive), showing that the chloride originated from HCl in the acidizing fluid.
2) For one well, the acid medium injected during acid fracturing was 400 m3 to 1,000 m3, and the fracturing pressure was between 50 MPa and 90 MPa. Therefore, the swivel joint bore a large tensile stress during operation, providing the stress condition for stress corrosion.
3) The swivel joint was made of 20CrNiMo, and its chemical composition and mechanical properties met the relevant standard requirements. Metallographic examination showed many manganese sulfide inclusions in the material, rated as Grade A3 according to GB/T 10561-2005. This indicates poor steel cleanliness. A large quantity of inclusions significantly reduces the stress-corrosion resistance of steel. Metallographic examination found that corrosion pits formed at manganese sulfide inclusions and cracks initiated at the bottoms of corrosion pits. Therefore, the severe manganese sulfide inclusions in the swivel joint material were initiation points for stress corrosion cracks and were one of the factors promoting stress corrosion failure.
3. Conclusion
The burst failure of the swivel joint on the fracturing truck in this oilfield during fracturing operation was stress corrosion cracking. Because the material contained severe manganese sulfide inclusions, corrosion cracks initiated at the manganese sulfide inclusions under the combined action of tensile stress and chloride corrosive medium, then propagated further and caused the swivel joint to rupture.
