GONG Gui Xian, Hanrong Dong, Sun Yijiang
(Wuhan Iron and Steel (Group) Company Institute, Wuhan 430080)

Abstract: Using analytical tools microscope and scanning electron microscopy of an oilfield fracturing car burst activity causes of Swivel joint during fracturing operations analysis. The results show that: the activity Swivel joint fracture occurs when part of the job stress corrosion crack damage. Because of the material in the presence of severe manganese sulfide inclusions in the role of tensile stress and corrosive chlorides by manganese sulfide inclusions at further expansion induced corrosion cracking caused by broken Swivel joint.

An oilfield in a drilling operation for fracturing a car 3 "(76. 2 mm) 105 MPa-50 type Swivel joint occurs when a sudden burst fracturing operations.
The event Swivel joint made of 20CrNiMo, using only about one year. After reinspection, its chemical composition meet the standards. Swivel joint production process: forging a normalizing a rough one case hardening and tempering a one machining factory by "high-pressure fluid control products factory experimental procedure," the requirements of the body through the pressure is 158MPa strength test The experimental results meet the requirements.
Acidic medium volume fracturing Swivel joint when in contact with the medium is acidified solution (20% HC1} 1.5% of HF + binder), playing a well of acid fracturing to be pushed in a 400m3 to 1000m3 ;, fracturing The pressure between 50-90 MPa. To determine the cause of the burst, the leaking tube sampling and analysis, this paper analyzes the process and results.
1. The test results
1.1 Macroscopic examination
Figure 1 is a fight burst Swivel joint complex macro photos. Severe corrosion fracture, but is still very obvious characteristics of brittle fracture, Figure 2 shows the lateral Swivel joint fracture surface, is a major fracture surface. As can be seen from the figure, fracture occurs at the mouth of the sealing stopper, the fracture more obvious source has three, as shown in Figure 2 arrows A, B, C 3 个 sites.

Figure 1 broken elbow macro photo

Figure 2 broken elbow fracture characteristics match
Area A: There is a penetration thickness longitudinal cracks, longitudinal surface covered with a layer of corrosion products seriously, wash with hot aqueous hydrochloric acid fracture surface as shown in Figure 3. Longitudinal fracture surface is divided into two obvious areas: relatively flat against the inner wall of the side, radial patterns extend from the inner wall surface to the thickness direction, indicating that the crack originated in the inner wall; the outer wall by the side of more rough fibrous fracture in the region is finally torn area. Also found that a plurality of longitudinal micro-cracks in the inner wall surface, showing the formation of vertical cracks associated with the inner wall surface microcracks.

Figure 3 A vertical crack and break parts and morphology
Area B: A district with roughly the same characteristics.
Area C: This area accounts for the entire cross-section of 1/5, there is a small section of layered multiple platforms, with a clear radial stripes, stripes converge to seal the outside spigot at the bottom of the platform, where the wall thickness is thinner.
According to the above features can be seen as breaking the order of development: A, B inner wall surface microcracks to the vertical and horizontal expansion, when extended to a certain extent, in the bottom of the Part C seal spigot generate transverse cracks, eventually transverse fracture.
1.2 Metallographic examination
Macroscopic examination revealed a number of vertical microcracks A shown in Figure 2, B site inner wall surface, polished inner wall surface corrosion pits were observed to have a crack at some of its vicinity, these corrosion pits often sprouted in sulfide at ( See Figure 4). By taking a cross-section of corrosion Bottom metallurgical sample, jagged cracks along the thickness direction of Swivel joint extension, within which there is corrosion products, tail often visible sulfide inclusions.
Take longitudinal section metallurgical sample observation, a lot sulfide thin strips (see Figure 5), according to GB / T 10561- 2005 standard assessment for A3 grade.
After measurement, the specimen, case depth of the outer wall 1.2 a 1.4 mm, in line with the relevant technical requirements.
The specimen, the outer wall surface tissue are COARSE tempered martensite + residual austenite former austenite grain size 8, cracks along the austenite grain boundaries (see Figure 6), terminating in Internal carburizing layer; a core part of tempered martensite and bainite organization.

Figure 4 the inner wall of the table and sulfide corrosion pits

Figure 5 sectional sample of sulfide inclusions

Figure 6 crack along the austenite grain boundaries
1.3 Scanning Electron Microscope
A main fracture and by scanning electron microscopy as shown in Fig. 2, B longitudinal side portion analysis, fracture covered with corrosion products, corrosion products enriched in C1 elements (w (C1) = 17. 11%) shown in Figure 7.
After cleaning with acetone solution to remove the main fracture was observed corrosion inside the Swivel joint, the outer wall surface (depth of about 0. 84 mm range) to intergranular fracture, local secondary intergranular cracks, show a clear pattern of sugar-like , shown in Figure 8. The remaining parts prevail cleavage cleavage + features, and a small amount of fine secondary cracks.
Scanning electron microscopy, energy dispersive spectroscopy for metallurgical sample for analysis, in addition to the inner wall surface corrosion pit Fe, Si oxides, but also enriched with a small amount of chlorine and sulfur (Figure 9); crack bifurcation trailing its chlorine is also on enrichment (see Figure 10); crack trailing sulfide manganese sulfide inclusions are also enriched in chlorine (see Figure 11).

Figure 7 fracture corrosion product spectrum analysis chart

Figure 8 wall intergranular fracture surface characteristics and secondary cracks

Figure 9 section inner wall surface pitting morphology and EDS analysis chart

Figure 10 EDS analysis of corrosion products within the crack map

Figure 11 the sulfide inclusions and cracks spectrum analysis chart
2. The results of the analysis
According to the test results, it is considered an oilfield fracturing Swivel joint car accident occurred during fracturing operations burst belonging to stress corrosion cracking.
Stress corrosion cracking is a destructive form of a metal component in the joint action of the tensile stress (the stress or residual stress) and corrosive media, caused. Analysis of the factors causing burst as follows:
A) SEM EDS analysis results show that internal corrosion pit Swivel joint, cracks and manganese sulfide inclusions are enriched at the chlorine, intergranular stress corrosion damage is a characteristic damage, indicating that cause stress corrosion medium Swivel joint is chloride, it is a strong stress corrosion media. The activity Swivel joint using only about one year, of the contact liquid medium is acidified (20% HC1 + 1.5% of HF + binder), visible from the acidified solution of chloride HCI.
2) to play well in acidic medium volume of acid fracturing to be pushed in the 400 m3 to 1 000 m3, fracturing pressure is between 50 ~ 90MPa. Thus, in the course of their work Swivel joint to withstand great tensile stress, the stress areas to provide the conditions for stress corrosion.
3) The activities of Swivel joint made of 20C rN iMo, its chemical composition, mechanical properties are in compliance with the relevant standards. Metallurgical test results indicate that the material manganese sulfide inclusions lot, according to GB / T 10561-2005 standard assessment reached the A3 grade. This indicates that the purity of steel is poor, large number of inclusions will significantly reduce resistance to stress corrosion of steel. From metallographic examination found that corrosion pits produced in the manganese sulfide inclusions at corrosion pits and crack initiation at the bottom, showing activity in serious Swivel joint material manganese sulfide inclusions are induced stress corrosion cracking point, which is to promote stress corrosion, damage one of the factors.
3. Conclusion
An oilfield fracturing activity vehicle Swivel joint occurs when fracturing burst belong to stress corrosion cracking. Because of severe manganese sulfide inclusions in the material, in the role of tensile stress and corrosive chlorides by manganese sulfide inclusions induced corrosion cracking at the further expansion of the activities that cause Swivel joint fracture.