Comparison of stress corrosion behavior of U-tubes in titanium (TA2), SAF2205, 304 and 316L heat exchangers

Shell and tube heat exchanger is the most commonly used heat exchanger in petrochemical industry because of its simple structure, solid structure, wide range of materials, free expansion of tube bundle, no temperature difference stress caused by the wall temperature difference between the heat exchanger tube and the shell, and the tube bundle can be pulled out, the weight of the extracted part is light, the design and manufacturing process is mature, the safety is relatively high and the applicability is relatively wide and is widely used.


See Figure 1 for the structure of U-tube heat exchanger. Although there is no thermal stress between the heat exchange tube and the shell of the U-tube heat exchanger, there are two necessary conditions for stress corrosion in the practical application of the U-tube heat exchanger:

  • (1) The member is in the state of tensile stress. In the U-tube structure, the temperature stress is caused by the different expansion of the two straight pipe sections, and there is a large residual tensile stress on the outer edge of the bend section of the U-tube after bending.
  • (2) Members are in stress corrosion environment. As the water evaporates, the concentration of CL will increase. According to the investigation, stress corrosion cracking of austenitic stainless steel occurs when the temperature is lower than 200 ℃ in the chloride solution containing 2ppm.

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Figure.1 Structure of U-tube heat exchanger

Therefore, it is necessary to study the stress corrosion behavior of U-tubes with different materials and different states.

Stress corrosion test

Select test criteria

According to the situation of U-shaped heat exchange tube, in order to simulate the use state of U-shaped tube, GB / t17898 stress corrosion test method of stainless steel in boiling magnesium chloride solution is selected as the test standard, and the U-shaped bending test conditions are adopted. As there is no corresponding stress corrosion test standard for titanium, stainless steel and other materials, stress corrosion performance evaluation test shall be conducted according to this standard.

Test materials

Titanium (TA2), SAF2205, 304 and 316L heat exchange tubes, with the specification of Φ 25mm × 2mm.

Sample type

The specimen is U-shaped, with two bending radii (R1 = 67.5mm, R2 = 52.5mm) and L = 120mm, as shown in Figure 2.

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Fig.2 U-tube sample

Sample status

According to different materials, stainless steel materials can be divided into two states: non solution treatment and solution treatment (1050 ℃× 3min); TA2 heat exchange tube can be divided into stress relief heat treatment (540 ℃× 15min) and non stress relief state; SAF2205 materials can be divided into two states: non solution treatment and solution treatment (1050 ℃× 3min).

Test conditions

According to GB / t17898, magnesium chloride aqueous solution with a concentration of about 42% is used, with a boiling point of (143 ± 1) ℃; the test process ensures that the solution is in a micro boiling state, with a temperature of about (143 ± 1) ℃; the constant displacement method is used in the test, and the open end of the U-shaped heat exchange tube is fixed through a fixture; the test vessel is a reflux cooler with cold capacity to prevent the concentration of the test solution; the electric furnace is used for heating.

Test process

Prepare the sample according to figure 2, compress the width between the two straight pipe sections with clamps to a certain displacement to apply pressure, pad insulating materials between the clamps and the sample, check whether the sample surface is defective, and then clean the sample surface. Heat the magnesium chloride solution to boiling in the flask, then inject it into the test vessel and heat it. After the solution completely boils, put the sample under pressure, and record the time as the start time of the test. During the test, take out the sample every 24 hours, rinse it with distilled water, and then observe the surface of the sample with a magnifying glass. If there is no crack after observation, put the sample into the boiling solution immediately, and continue Test; repeat the observation, record the time from the beginning of the test to the occurrence of the cracks as the macro crack occurrence time, record the time from the beginning of the test to the crack penetrating the pipe wall as the crack penetration time; change the solution at least once in 7 days during the test cycle.

Test results and analysis

Specimen failure

Most of the samples produce cracks near the top of the arc and lead to fracture (see Fig. 3). A small number of samples produce cracks at many places of the arc section (see Fig. 4 and Fig. 5). The cracks originate from the outer fixed points of the arc and propagate to the inside.
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Fig.3 Failure diagram of 304 (non solid solution) U-tube under 60% stress

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Fig.4 Crack initiation and propagation of 316L (unsoluble) U-tube under 100% stress

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Fig.5 Failure diagram of SAF2205 (not solid solution) U-tube under 40% stress

Performance analysis

See the attached table for fracture data of U-tube of various materials. The results show that the stress corrosion resistance of titanium material is very good, no matter whether the elbow is heat-treated or not, there is almost no stress corrosion tendency in the boiling 42% MgCl2 solution; except for TA2 material, other samples break in the boiling 42% MgCl2 solution, and the threshold value of stress corrosion cracking is below 40% SS; the solution treatment has better resistance to chloride ion stress corrosion of 304 and 316L materials The fracture time of the material after solution is increased by 2-3 times.

U-tube test data U-tube fracture data of various materials

Time/h

Material

Stress level

40%ss

60%ss

80%ss

100%ss

316L

Solid solution

384

168

384

384

Unsolid solution

168

168

168

168

304

Solid solution

216

216

336

216

Unsolid solution

72

72

72

72

2205

Solid solution

504

504

456

456

Unsolid solution

168

144

72

72

Ti

Heat treatment

Unbroken

Unbroken

Unbroken

Unbroken

Non heat treatment

Unbroken

Unbroken

Unbroken

Unbroken

Fracture analysis

See Fig. 6 for typical sample fracture photos. Figure 6 shows the typical stress corrosion fracture morphology of SAF2205 U-shaped tube. It can be seen from the figure that regardless of solution treatment, the fracture mode is classic brittle fracture, the fracture surface is covered with electrolyte deposition layer or crystal ball, and the fracture has the stress corrosion cracking feature of intergranular cracking. Micro cracks with uneven distribution appear in the fracture surface of the material, which is the obvious feature of transgranular cracking. After the solution treatment, there are obvious moire accumulation on the fracture surface, which effectively increases the path of fracture crack propagation and decreases the transgranular crack, which is the main reason for the improvement of stress corrosion resistance of U-shaped tube after the solution treatment. In addition, after the solution treatment, the grain distribution at the fracture surface of austenitic stainless steel is more uniform, and the defects in its structure are significantly reduced, which is at the fracture surface There are a few dimples to verify the ductile fracture, which is the secondary reason for the improvement of its properties. This phenomenon is related to the phase transformation and the change of phase distribution of austenitic stainless steel after solution treatment.
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Fig.6 Stress corrosion fracture morphology of SAF2205 U-tube

Concluding remarks

According to the test results and analysis, the following conclusions can be drawn:

  1. The top of R section of U pipe is the main area of stress corrosion crack.
  2. After the U-shaped heat exchange tube of titanium material is bent, the stress corrosion resistance is still superior without stress relief heat treatment. In the actual manufacturing process, the stress relief heat treatment can not be carried out after bending.
  3. For the cold-formed U-shaped pipes made of SAF2205 duplex stainless steel, 316L, 304 and so on, the resistance to chloride ion stress corrosion after solution treatment is obviously better than that of the U-shaped pipes without solution treatment. Therefore, after U-shaped pipe bending, solution treatment should be carried out to improve its stress corrosion resistance.
  4. After solution treatment, the stress corrosion resistance of SAF2205 duplex stainless steel U-tube is better than that of 316L and 304 austenitic stainless steel U-tube.

Source: China Tube Sheet Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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References:

  1. https://www.yaang.com

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comparison of stress corrosion behavior of u tubes in titanium ta2 saf2205 304 and 316l heat exchangers - Comparison of stress corrosion behavior of U-tubes in titanium (TA2), SAF2205, 304 and 316L heat exchangers
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Comparison of stress corrosion behavior of U-tubes in titanium (TA2), SAF2205, 304 and 316L heat exchangers
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Although there is no thermal stress between the heat exchange tube and the shell of the U-tube heat exchanger, there are two necessary conditions for stress corrosion in the practical application of the U-tube heat exchanger:
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