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Failure Causes of Titanium Alloy Bends

By means of metallographic microscope, scanning electron microscope and tensile test, the failure causes of TA10 bend used for wastewater treatment were analyzed. The results show that the inner wall of TA10 bend is seriously oxidized. The disappearance of dispersed phase at higher temperature leads to the decline of its mechanical properties, and erosion occurs under the action of high pressure fluid, leading to the fracture of the weak area of the bend.
Titanium alloy has good mechanical properties and corrosion resistance, and titanium alloy pipes are also widely used in nuclear power [1], offshore engineering [2], etc. Under complex operating conditions, titanium alloy may still experience common material failures such as wear and corrosion due to various factors in the design, use and maintenance processes. The sudden bursting of a pipeline line may cause significant safety accidents and economic losses [3]. Therefore, this work analyzes a failed pipe fitting to determine the cause of failure in order to provide a theoretical basis for various aspects of design, production and maintenance to ensure the safe operation of the system.

1. Test

An explosion failure occurred in a bend serving in a chlorine-containing wastewater treatment system of a chemical plant, whose material is TA10 titanium alloy. Bend in the material composition of complex, solid particles, the normal service temperature of 220 ℃, if there is a blockage, the local temperature will rise, the pressure is 7MPa, the internal air, the part had a blockage before the burst failure, the blockage will be unblocked soon after the burst failure. Cut titanium pipe bend and straight pipe failure site for macroscopic analysis, and then in the bend fracture failure, the bend is not fractured, the straight pipe intercepted specimens, using OLYMPUS GX51 microscope to observe its cross-sectional microstructure; JSM-6460 type scanning electron microscope to observe the failure of the inner wall and fracture and analysis of its internal surface products. Tensile specimens were intercepted at the straight and bent tubes respectively with the dimensions shown in Figure 1, and their mechanical properties were analyzed by tensile tests.

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Figure.1 Dimensions of tensile specimens

2. Results and Discussion

2.1 Macroscopic morphology and microstructure

Figure 2 (a) shows the failed bend, the macroscopic morphology can be seen in the outermost part of the bend, the failure of the thinning is obvious, after bending the outermost wall of the titanium tube is the thinnest, the maximum tensile stress, so here is the weak link of the entire bend. The inside of the tube wall as shown in Figure 2 (b), near the location of the fracture appears obvious oxidation discoloration, due to the presence of corrosive media and solid particles, the inside of the wall has obvious signs of erosion, intercept the fracture, 100mm from the fracture, the straight tube at the production of metallographic specimens and its cross-sectional morphology to observe.

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Fig.2 Macro morphology of failed bend: (a) outside; (b) inside
As shown in Figure 3, it can be seen that there are many tiny cracks at the bend fracture, and the closer the inner surface cracks, there are also micro cracks at 100mm from the fracture, but the number is significantly reduced, and the internal organization at the straight pipe is clear and clean.

2.2 Fracture morphology

The fracture after failure is observed as shown in Fig. 4, and it can be seen that the oxide layer near the fracture is not complete, and there is the phenomenon of spalling of the oxide layer. Under multiple cycles of stress, a large number of parallel stripes appear perpendicular to the crack expansion direction, and these stripes are caused by the deformation of the bend tube during each stress. From the above results, it can be concluded that the cumulative deformation of the titanium tube under multiple cycles of stress leads to the gradual expansion of cracks, and the internal oxidation layer of the titanium tube flakes off under multiple flushing, while the high temperature aerobic environment is conducive to the generation of the oxidation layer again, and the multiple oxidation and flaking process leads to the thinning of the tube wall [4].

2.3 SEM morphology and EDS analysis of the inner surface

As shown in Figure 5, it can be seen that the material inner surface corrosion is serious, especially at the grain boundaries gathered more corrosion products, while the internal corrosion of the grain is lighter, showing a more obvious along the grain corrosion morphology.

As shown in Figure 6 and Table 1, due to the high temperature environment and the presence of oxygen, the inner surface of the titanium tube fracture formed a relatively uniform and dense oxide layer, the mass fraction of oxygen in the oxide layer is about 40%. Compared with the fracture, the oxide layer on the inner surface of the straight tube was looser and unevenly oxidized. The EDS results also show that in the dense oxide layer region (region B), the mass fraction of oxygen is about 32%, while in the non-dense region (region C) the mass fraction of oxygen is about 9.6%. In summary, it can be concluded that the degree of oxidation at the fracture is much greater than that at the unbroken area, and the oxide layer at the fracture is more uniform and dense than that at the unbroken area.

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Fig.3 Cross-sectional microstructures of failed bend: (a) break; (b) 100mm from the break; (c) straight pipe

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Fig.4 Fracture morphology of failes bend: (a) internal surface; (b) crack tip

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Figure.5 Corrosion morphology of the inner surface of the failed bend

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Fig.6 EDS analysis locations on internal surface of failed bend: (a) break; (b) straight pipe

Table.1 EDS analysis results of the inner surface

Region Wo Wp WK WTi WSi
A 39.72 59. 93 0.35
B 32. 18 0.48 0. 33 67.01
C 9.59 90.41

2.4 SEM morphology and EDS analysis of cross-section

As shown in Fig. 7, in order to analyze the changes of the internal structure of the material before and after the fracture, the cross-section at the fracture and the straight tube were selected as specimens, polished and then analyzed by SEM. At the straight tube, the diffuse second phase is uniformly distributed on its surface. After local magnification, it can be seen that there are some small pits on the surface of the substrate in addition to the second phase, see Figure 7(b), and some diffuse phases that have not completely disappeared remain in these pits. At the fracture failure the dispersive phase basically disappears, and the surface shows a dense distribution of small pits. The analysis suggests that these pits should be the residual morphology after the disappearance of the second phase on the substrate.

The change of internal composition is an important factor affecting the material properties. To confirm the change of composition before and after the fracture failure, EDS analysis was performed on the organization of the cross-section. From Figure 8 and Table 2, it can be seen that the overall surface Ni mass fraction at the straight tube (region A) is 2%. On the dispersion phase (region C, region D) the Ni mass fraction is 20%,Ti mass fraction is 80%, while in the matrix (region B) the Ti mass fraction is 100%, indicating that almost all Ni at the straight tube is gathered on the Ti-Ni dispersion phase, while the composition at the fracture has changed significantly. As shown in Figure 8(c), the composition at the fracture (region F), which is mainly Ti, Ni and a small amount of Mo, the Ni content here is basically the same as the Ni content at the straight tube. The composition inside the corrosion pit is examined (area E, area G), and the Ni content inside the corrosion pit is not significantly different from the scan results at the straight tube. Due to the low migration ability of Ni elements in titanium alloys, Ni elements are more likely to migrate at higher temperatures [5]. Before the failure occurred, the blockage occurred at the bend, and the temperature at the bend increased after the blockage, and the high temperature provided the conditions for the migration of Ni elements. In summary, at the straight tube Ni elements are basically all concentrated in the second phase, while the Ni elements at the fracture migrate from the second phase to the matrix.

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Fig.7 Cross-sectional morphology of failed bend: (a) straight pipe; (b) enlarged straight pipe; (c) break

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Figure.8 Cross-sectional EDS analysis position of the failed bend

Table.2 Cross-sectional EDS analysis results

Region WTi UNi WMo
A 98 2
B 100
C 75.74 24.26
D 84.09 15.91
E 98.37 l.63
F 98.65 l.35
G 97.88 l.27 0.85

2.5 Mechanical performance analysis

The tensile strength of the straight tube and the bend part of the titanium tube were selected for tensile testing, as shown in Figure 9. The tensile strength at the bend was 580 MPa, the elongation after break was 15.33%, and the shrinkage at the section was 43.3%. Due to the effect of work hardening during bending, the strength at this place is obviously increased, but at the same time, the plasticity of the material is greatly reduced, so its post-failure elongation and cross-sectional shrinkage decrease, and its post-failure elongation is already lower than the requirements of GB/T 3624-2010 “Titanium and titanium alloy seamless tube”, and it is easy to fracture here after the force is applied due to the insufficient deformation capacity at the bend.

3. Conclusion

(1) Due to the bending process, the process hardening generated by deformation leads to a reduction in material plasticity and poor deformation capacity. In addition, the bending process leads to a thinning of the wall thickness and the presence of large tensile stresses at the outer wall of the bend, making the outer wall of the bend a weak link when the titanium tube is pressurized internally.

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Figure.9 Engineering stress-strain curve of the failed bend
(2) The tube is a high-temperature wastewater environment, fluid pressure is high and contains solid particles, this high-pressure liquid at the bend to form turbulence, the impact on the bend to produce a large shear force, and the fluid contains solid particles and air bubbles will strengthen the shear force, so that the inner wall of the tube oxide film destruction, the reciprocal process of oxidation erosion leads to the bend wall thickness is getting thinner.

(3) When the bend is blocked, the temperature and pressure here will rise, high temperature so that the material internal dispersion reinforced phase disappears, the material strength is reduced, the pressure-bearing capacity becomes poor. Eventually in the joint action of internal and external factors, the material occurs along the crystal corrosion, accompanied by a number of large tensile stress effect, the pipe gradually accumulated deformation, and finally rupture.

Source: China Titanium Bend Manufacturer – Yaang Pipe Industry Co., Limited (www.titaniuminfogroup.com)

References:

  • [1] Zhao B, Zhao YQ, Zhang PX,et al. A high-impact toughness low-activation titanium alloy for nuclear reactors: CN106521239A [P].2017-03-22.
  • [2] A. Orensheko,Zhang H. Extending the service life of titanium alloy marine naval structural parts[J]. China Titanium Industry, 2017(2):42-46.
  • [3] Lu M X, Bai Zhenquan, Zhao X W, et al. Current status and typical cases of corrosion in oil and gas gathering and storage [J]. Corrosion and Protection, 2002 ,23(3):105-113.
  • [4] Wu Chenghong, Gan Fuxing. Scouring corrosion of metals in two-phase flowing water[J]. Materials Protection, 2000,33(4):33-35,61.
  • [5] Zhang Lianji, Hu Qingmiao, Yang Rui. First-principles calculations of atomic migration properties in titanium alloys[J]. Rare Metal Materials and Engineering, 2017 ,46(S1):246-249.
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