Stress corrosion behavior comparison of titanium and SAF2205, 304, 316l heat exchange tubes processed into U-shaped tubes

Shell and tube heat exchanger is the most commonly used heat exchanger structure in petrochemical industry because of its simple structure, strong structure, wide range of materials, free expansion of tube bundle, no temperature difference stress caused by wall temperature difference between heat exchange tube and shell, and tube bundle can be drawn out with light weight, mature design and manufacturing process, high safety and wide applicability And is widely used. The structure of U-tube heat exchanger is shown in Fig. 1. Although there is no thermal stress problem between the heat exchange tube and the shell of U-tube heat exchanger, there are two necessary conditions for stress corrosion in practical application of U-tube heat exchanger:

• (1) The component is in tensile stress state. In the U-tube structure, the temperature stress is caused by the different expansion of the two straight sections, and there is a large residual tensile stress at the outer edge of the bent section after bending.
• (2) The components are in the stress corrosion environment. Due to the continuous evaporation of water, the concentration of Cl ion will increase continuously. According to the investigation, the stress corrosion cracking of austenitic stainless steel occurs when the temperature is lower than 200 ℃ in the chloride solution containing 2ppm.

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

Stress corrosion test

Selection of test criteria

According to the condition of U-tube, in order to simulate the service state of U-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. Since there is no corresponding stress corrosion test standard for titanium, stainless steel and other materials, the stress corrosion performance evaluation test is conducted according to this standard.

Test materials

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

Sample type

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

Fig. 2 sample of U-tube

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 is divided into stress relief heat treatment (540 ℃× 15min) and non stress relief state; SAF2205 material is divided into two states of no solution treatment and solution treatment (1050 ℃× 3min).

Test conditions

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

Test process

Prepare the sample according to Fig. 2, compress the width between the two straight pipe sections by a certain displacement with a clamp to apply pressure, pad insulating materials between the fixture and the sample, check whether there are defects on the surface of the sample, and then clean the surface of the sample. The magnesium chloride solution is heated to boiling in the flask, and then injected into the test vessel and heated. After the solution is completely boiling, 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, wash it with distilled water, and then observe the surface condition 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 observation of the crack 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; during the test period, change the solution at least once every 7 days.

Test results and analysis

Specimen failure

Most of the samples have cracks near the top of the circular arc and lead to fracture (see Fig. 3). A small number of samples have multiple cracks in the arc section (see Fig. 4 and Fig. 5). The cracks initiate from the outer fixed point of the arc and expand to the interior.

Fig. 3 failure diagram of 304 bend under 60% stress

Fig. 4 crack initiation and propagation of 316L bend under 100% stress

Fig. 5 failure diagram of SAF2205 bend under 40% stress

Performance analysis

See attached table for fracture data of U-tube of various materials. The test results show that the stress corrosion resistance of titanium material is very good, no matter whether heat-treated or not, there is almost no stress corrosion tendency in boiling 42% MgCl2 solution; except for TA2 material, other samples have no stress corrosion tendency The stress corrosion cracking threshold of MgCl2 in boiling solution is less than 40% SS; the solution treatment has a great influence on the chloride stress corrosion resistance of 304 and 316L materials, and the fracture time of these materials increases 2-3 times after solution treatment.
Test data of U-tube fracture data of various materials

Fracture analysis

See Fig. 6 for fracture photos of typical samples. The stress corrosion fracture morphology of typical SAF2205 U-tube is obtained in Fig. 6. It can be seen from the figure that 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 characteristics of intergranular cracking. Microcracks distributed unevenly in the fracture surface of the material, which is an obvious characteristic of transgranular cracking. After solution treatment, there are obvious cloud pattern accumulation on the fracture surface, which effectively increases the propagation path of fracture crack, and the transgranular crack decreases obviously, which is the main reason for the improvement of stress corrosion resistance of U-tube after solution treatment; in addition, after solution treatment, the grain distribution at the fracture surface of austenitic stainless steel is more uniform, and the defects in the microstructure are significantly reduced A small number of dimples were found to verify the ductile fracture, which was the secondary reason for the improvement of properties. This phenomenon was related to the phase transformation and phase distribution change of austenitic stainless steel after solution treatment.

Fig. 6 fracture morphology of U-tube sample after stress corrosion

Conclusion

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

• (1) The top of R section of U-tube is the main area of stress corrosion cracking.
• (2) The stress corrosion resistance of U-shaped titanium heat exchange tube is still superior without stress relief heat treatment after bending. In the actual manufacturing process, stress relief heat treatment can not be carried out after bending.
• (3) The results show that the corrosion resistance of Sau type and 2205 type stainless steel is better than that of stainless steel after cold treatment. Therefore, solution treatment should be carried out to improve the stress corrosion resistance of U-tube after bending.
• (4) The stress corrosion resistance of SAF2205 duplex stainless steel U-tube is better than that of 316L and 304 austenitic stainless steel after solution treatment.

Author: Zhao Yanfeng

Source: China Titanium Pipe Manufacturer: www.titaniuminfogroup.com