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Study on the forming process of titanium 180 degree pipe bend for high temperature equipment

The 180 degree pipe bend of TA10 heat transfer oil with a design temperature of 280 ℃ burst, and all the 180 degree pipe bends were bulging. 180 degree pipe bend is formed by medium frequency heating and push bending. The U-bend is analyzed. Through analysis, it is found that the bulging and cracking of the 180 degree pipe bend are caused by the high temperature oxidation of the material. After the 180 degree pipe bend is formed by medium frequency heating, the bending point is bulged and the material surface is oxidized at high temperature. The oxidation of the titanium tube may indirectly lead to the bursting of the titanium tube. Titanium bends for high-temperature equipment shall not be formed by intermediate frequency heating push bending, but only by cold forming. If heating bend has little effect on normal temperature water pressure. If heating push bending can be used for titanium bend of normal temperature equipment.

On April 27, 2016, at the site of Gu’an Tianhao additives Co., Ltd., a group of 180 degree pipe bend bends exploded and bulged in the straight pipe section. The specification of 180 degree pipe bend is: Ф 45×3, material TA10, design temperature 280 ℃, design pressure 7.7mpa. Design verification, raw material inspection, material wall thickness sampling analysis, aging analysis, intermediate frequency heating push bending test, cold bending test, normal temperature water pressure blasting test and a series of experimental analysis were carried out for 180 degree pipe bends.

1. Design verification

The material is verified by design, and the material thickness can meet the design requirements of the drawings.
Table.1 Design conditions and minimum wall thickness

Design Pressure Mpa) Design Temperature (°C) Allowable Stress Mpa Minimum Thickness
7.7 240 91.6 1.91
7.7 260 88.4 1.97
7.7 280 85.2 2.04
7.7 290 83.6 2.08
7.7 300 82 2.12
10 280 85.2 2.62

2. Raw material inspection

Re inspection of raw materials: tensile, flattening and flaring tests. The performance of raw materials shall comply with the provisions of the standard GB/T 3625-2007 titanium and titanium alloy tubes for heat exchangers and condensers [1].

3. Sampling analysis of material wall thickness

The broken section is sampled and tested. The minimum wall thickness of the bend is 2.87mm. The wall thickness of the material after the final forming of the pipe meets the requirements of the standard minimum wall thickness (2.7).

4. Aging analysis

4.1 Macroscopic observation

The outer surface of the 180 degree pipe bend is golden yellow, and the nozzle is bell shaped. The length of the crack is about 45mm and the width is about 13mm. The pipe wall is obviously thinned, and the edge is very sharp. It is characterized by ductile fracture; The inner wall morphology of the 180 degree pipe bend near the crack was observed. There was obvious corrosion morphology on the surface and the oxidation was serious. Close observation of the fracture surface shows that the color near the fracture surface is purple blue. The surface of the bulged 180 degree pipe bend is bulged in the circumferential direction, and the inner wall is oxidized yellow.

4.2 SEM analysis

Immerse the fracture of the cracked 180 degree pipe bend in acetone and put it into the scanning electron microscope after ultrasonic cleaning. Because the fracture section is very narrow, it is impossible to observe the fracture characteristics of the fracture comprehensively, but it can be seen that there is obvious oxidation on the fracture surface and a small amount of dimples can be observed.

The surface morphology of the inner wall of the cracked 180 degree pipe bend shows that there are obvious pitting corrosion pits on the inner wall, and the adhesion of corrosion products is observed. Energy spectrum analysis of corrosion products showed that there were more N and O elements besides matrix elements.

20220814084424 10399 - Study on the forming process of titanium 180 degree pipe bend for high temperature equipment

Fracture morphology 

4.3 Metallographic analysis

The two 180 degree pipe bends were sampled for metallographic analysis. The metallographic structure of the cracked 180 degree pipe bend near the crack and the metallographic structure far away from the crack were α+ A small amount of granular β, No obvious difference; The metallographic structure of the bulged 180 degree pipe bend is also α+ A small amount of granular β.

4.4 Analysis and discussion

Plastic deformation at the crack of cracked 180 degree pipe bend: the pipe diameter is obviously enlarged, the pipe wall is thinned in the shape of a knife edge, and the crack opening is in the shape of a trumpet. The main characteristics of the fracture that can be observed are dimples. The inner wall is obviously oxidized, and the color near the crack is purple blue, which is an obvious characteristic of high temperature. This phenomenon is generally caused by excessive local heat load in the pipe.
Poor circulation of steam and water in the pipe or serious slagging in the pipe are unfavorable to heat transfer, which will cause obvious rise of wall temperature and cause overtemperature.
There is corrosion on the inner wall of the cracked 180 degree pipe bend. Energy spectrum analysis shows that there are more N and O elements in addition to the matrix elements, which indicates that the 180 degree pipe bend is corroded by NO3 – ions during the working process. NO3 acid ion will increase the corrosion ability at high temperature.
There is obvious oxidation on the inner wall of the bulged 180 degree pipe bend.

5. Intermediate frequency heating push bending test

The medium frequency bending test was carried out on the straight pipe. The surface of the bend is oxidized at high temperature, and there are bulges in some parts. The color of titanium oxide film is related to the production temperature. It is silver white below 200 ℃, light yellow at 300 ℃, golden yellow at 400 ℃, blue at 500 ℃, purple at 600 ℃, red gray at 700 ℃ ~ 800 ℃, and gray at 800 ℃ ~ 900 ℃ [2]. The oxidation color of the titanium tube surface is blue purple, indicating that the forming temperature of the titanium tube is 500 ℃ ~ 600 ℃.

20220814084515 25049 - Study on the forming process of titanium 180 degree pipe bend for high temperature equipment

Surface oxidation

6. Normal temperature hydrostatic test of bend

Jiangsu special equipment safety supervision and Inspection Institute (national pressure pipeline element quality supervision and inspection center) conducted normal temperature hydrostatic test on 2 cold bends and 2 Intermediate Frequency bends.

20220814084556 80195 - Study on the forming process of titanium 180 degree pipe bend for high temperature equipment

Blasting test
The bursting pressure of two intermediate frequency bends is 96.1mpa and 96.0mpa respectively, and the bursting pressure of two cold bends is 95.4mpa and 91.1mpa respectively. The burst pressure resistance of medium frequency bend and cold bend is similar.

7. Conclusion

  • 1. After the 180 degree pipe bend is formed by medium frequency heating, the bending point is bulged and the material surface is oxidized at high temperature. The oxidation of the titanium tube may indirectly lead to the bursting of the titanium tube.
  • 2. If heating bend has little effect on normal temperature water pressure, if heating push bending can be used for titanium bend of normal temperature equipment.
  • 3. The titanium bend used for high-temperature equipment shall not be formed by medium frequency heating push bending, but only by cold forming.

Author: Yan Jianwang

Source: Network Arrangement – China Titanium 180 Degree Pipe Bend Manufacturer – Yaang Pipe Industry Co., Limited (www.titaniuminfogroup.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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at [email protected]

Reference:

  • [1] Huang Yongguang, GB / t3625-2007, 2007, 4, 30, titanium and titanium alloy tubes for heat exchangers and condensers
  • [2] Shoubinan (shoubinan), JB / t4745-20022002,8,22, titanium welded vessel
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