Design, Manufacture and Application of Titanium Bellows

Titanium and titanium alloy bellows have been widely used because of their light weight, high strength, non-magnetic and high corrosion resistance. In this paper, the design basis of titanium corrugated pipe is introduced, the performance parameters of titanium corrugated pipe under different materials are described, the commonly used forming and manufacturing methods are discussed, and the application fields are summarized. The development situation and research status of titanium corrugated pipe are discussed, and the design, manufacturing and application prospects of titanium and titanium alloy corrugated pipe are also prospected.

As a key component for thermal compensation of heating pipes and equipment, bellows expansion joints play a vital role in the safe operation of the entire ship’s steam pipeline. With the rapid development of marine equipment in China in recent years, the service environment of equipment components has become increasingly harsh. In addition to the weight reduction of the bellows itself, the bellows itself is also required to be resistant to seawater and marine atmosphere corrosion [1,2]. Because titanium and its alloys not only have good comprehensive mechanical properties, but also have strong corrosion resistance, titanium and titanium alloy bellows are increasingly used in marine equipment in recent years, and the demand is increasing year by year. However, due to the high purchase cost of titanium and titanium alloys, and its low elastic modulus, large deformation rebound, high deformation strengthening rate and other cold forming characteristics, the use of titanium bellows is limited. At present, there is little research on titanium and its alloy bellows at home and abroad [3, 4, 5]. In terms of design and forming, it is also necessary to borrow the calculation data of austenitic stainless steel, which brings hidden dangers to the safety of equipment during service. This paper summarizes the common material selection, fatigue design, forming methods, application fields and prospects of titanium bellows, It is expected to provide certain design basis and theoretical basis for the further development and application of titanium bellows.

1. Classification and characteristics of titanium

The industrial applications of titanium are mainly divided into two categories: pure titanium and titanium alloys. Titanium and titanium alloys have good corrosion resistance. Compared with traditional austenitic stainless steel, titanium and its alloys have higher corrosion resistance in media containing oxidizing, chloride ions and alkali solution. For example, in the NaOH solution with 70% content at 130 ℃, the corrosion rate is only 0.18mm/year. Till now, titanium and its alloys have been successfully applied in metallurgy, paper making, shipbuilding, seawater desalination and other fields [6].
Industrial pure titanium has good plasticity and low temperature properties, but its strength is low. Generally, its tensile strength is 350MPa-700MPa in the annealed state. The linear expansion coefficient and thermal conductivity of titanium are small, and the welding performance is good. In the processing and manufacturing of conventional titanium corrugated pipes, the corrugated materials themselves are often required to have high elongation. In titanium and titanium alloys, only industrial pure titanium can reach more than 30% at room temperature, which has hydraulic formability. Therefore, pure titanium is often used as the main material of corrugated pipes. In pure titanium, TA2 has high strength and good elongation, so it is a common material for titanium bellows.
For titanium alloy materials, due to their high specific strength, superior comprehensive performance, and excellent corrosion resistance toughness and heat resistance, they were initially used in aviation. However, due to the high yield ratio, large cold working springback and high deformation strengthening rate of titanium alloy, bellows cannot be formed at room temperature. Instead, it can only be formed by heating it to a high temperature and taking advantage of its superplasticity. From the current research level, conventional titanium and titanium alloys can achieve different degrees of superplasticity α+β Titanium alloy has the best superplasticity. For example, TC4 can reach 100% elongation at high temperature. Therefore, TC4 can be considered as an ideal material for titanium alloy bellows with high specific strength.
Table 1 shows the chemical composition, mechanical properties and process properties of TA2 and TC4 currently used to make titanium bellows.
Table.1 Chemical composition, mechanical properties and process properties of TA2 and TC4

2. Fatigue life of titanium bellows

At present, a large number of data studies have been carried out on the fatigue life design of bellows at home and abroad, and the corresponding design fatigue formulas are also specified in EJMA, GB/T 12777 and other design standards. However, these fatigue formulas are based on the corresponding test data of austenitic stainless steel. There is no definite conclusion on whether the corresponding design fatigue formulas are applicable to the design and manufacture of titanium bellows.
Among many corrugated pipe design standards, only in Appendix X302.1.3 Fatigue Analysis of ASME B31.3, the corresponding provisions are given for the calculation of corrugated pipe design fatigue formula for new materials. The so-called new material is that the design material is not austenitic stainless steel or the corrugated pipe is heat treated after forming. Duan Mei and others of Yang Shipbuilding Materials Research Institute calculated the fatigue life formula of pure titanium reinforced U-shaped bellows according to the corresponding regulations as [7]:

In the formula:

• N_c – design fatigue life;
• S_t – allowable stress range.

Chen Yong et al. used the fatigue life formula of austenitic stainless steel bellows to calculate the fatigue life of pure titanium unreinforced U-shaped bellows [8]. Wang Gang [9] of Harbin University of Technology and others only measured the fatigue life of TC4 unstrengthened U-shaped bellows, and did not estimate its fatigue life before the bellows was formed. Table 2 shows the waveform parameters, design and actual fatigue life of titanium bellows from different manufacturers [7, 8, 9, 10].
Table.2 Design parameters and fatigue life of titanium bellows

3. Welding and processing of titanium bellows

There are mainly three key pressure bearing welds in the processing and manufacturing of titanium corrugated pipe, namely, the longitudinal seam of pipe blank and the connection weld between corrugated pipe and left and right end pipes after forming. For the longitudinal seam of small size corrugated pipe blank, seamless pipe can be selected for pressure forming and manufacturing during corrugated pipe forming to avoid the reduction of base metal and weld toughness caused by welding heat. Especially in the welding and manufacturing of large diameter corrugated pipes, the pipe blank before the forming of corrugated pipes needs to be welded after the sheet is rolled, so the longitudinal seam of the pipe blank needs to have good comprehensive mechanical properties. The connection between the corrugated pipe and the left and right end pipes is usually welded by argon tungsten arc welding, which is mainly due to its good engineering applicability, wide welding process window, small welding heat input, relatively stable welding quality, and low welding cost. It is the welding method selected by most corrugated pipe manufacturers. As for the longitudinal weld of pipe blank, since the pipe blank needs to be pressure formed after welding, and the deformation is large, the secondary weld needs to have good strength, good toughness and a certain amount of extension. Therefore, the longitudinal weld welding of pipe blank is the most critical process in the manufacturing process of corrugated pipe.

3.1 Argon tungsten arc welding

Argon tungsten arc welding is a common welding method for enterprises to weld pure titanium corrugated pipe blanks due to its relatively stable heat source, easy operation of the welding process, and low equipment procurement cost. Its welding quality is generally superior to manual arc welding and MIG welding. However, due to the wide welding arc and dispersed distribution of welding heat sources, tungsten argon arc welding often causes excessive growth of grains in the welding heat affected zone during the welding process, resulting in reduced plasticity and strength. The dispersion of welding heat sources also brings difficulties to the protection of the welding process. Therefore, tungsten argon arc welding for longitudinal seam welding of corrugated pipes with large deformation rate often cannot bring ideal results, At the same time, when argon tungsten arc welding titanium alloy, the superplasticity of titanium alloy will disappear, which will lead to the failure of forming and manufacturing high-strength titanium alloy bellows.

3.2 Plasma arc welding

Plasma arc welding and argon tungsten arc welding are both non consumable gas shielded arc welding, but they are different from argon tungsten arc welding in that they compress and restrict the arc by adding nozzles. Compared with argon tungsten arc welding, plasma arc welding has a more concentrated welding heat source through the arc compression effect, which is very suitable for titanium alloy welding. The titanium corrugated pipe blank is thin, and the penetration type welding method is often used in the welding process. Due to the concentration of arc energy in plasma arc welding, the weld and welding heat affected zone are narrow and the grain coarsening is not obvious during the welding process, which promotes the plasma arc welding to have good comprehensive mechanical properties, and the superplasticity of the raw material will still be retained after the titanium alloy is welded by plasma arc welding [11,12,13]. Compared with laser welding and electron beam welding, plasma welding has lower cost and is an ideal and economical welding method for welding corrugated pipes with large deformation curvature. Wang Gang, et al. [14] of Harbin University of Technology used plasma to weld TC4 pipe blank, and used superplastic forming process to shape it. The test results show that the plasma weld has good superplasticity at 927 ℃, and successfully manufactured U-shaped corrugated pipe of high-strength TC4 material.

3.3 Electron beam welding

Electron beam welding is an ideal method for titanium alloy welding. After welding, the welded joint still has superplastic properties. This is mainly due to the high concentration of electron beam heat sources, the narrow weld during welding, and the fast cooling rate, which make the weld and heat affected zone have smaller grain structure, so the welded joint has good comprehensive mechanical properties; As the welding process is carried out under the vacuum chamber, the weld and welding heat affected zone will not be polluted by air, and the post welding performance is much higher than other welding methods. However, due to the excessive concentration of the heat source of electron beam welding, the requirements for welding assembly clearance and misalignment are high, and the transition between the weld and the base metal is not smooth, which is easy to cause local stress concentration at the weld toe during the forming of the corrugated pipe, leading to forming failure; The size of the vacuum chamber will also limit the size of the bellows.

3.4 Laser welding

The energy density of laser welding is equivalent to that of electron beam welding. However, compared with electron beam welding, laser welding is characterized by being able to weld in the atmosphere and obtain structures similar to electron beam welding. It is the same as electron beam welding. Before welding, there is a high requirement for the assembly gap and misalignment of the workpiece. The welding process window is narrow, and penetration welding is often used to weld titanium tube blanks. Wang Gang, et al. [15] of Harbin Institute of Technology used laser welding to weld TC4 material, and investigated the superplastic properties of its welded joints. The test results show that the weld microstructure is fine grain acicular martensite α+ strip  α+β The maximum elongation of the weld is 154%, and the maximum expansion is about 1.54 times of that of the master mold.

4. Forming method of titanium bellows

There are many forming methods for corrugated pipes, and the forming methods applicable to austenitic stainless steel corrugated pipes are also applicable to the manufacturing of titanium corrugated pipes. At present, there are six main forming methods for titanium bellows.

4.1 Hydroforming

Hydroforming is a common method for most bellows forming, and it is also suitable for hydroforming of pure titanium bellows. The hydraulic method is to use oil pressure or other similar equipment to apply pressure on the medium in the pipe blank, and then bulge under the constraint of the external forming die to finally obtain the bellows of the specified size. Hydroforming has the characteristics of uniform pressure and moderate thinning. For corrugated pipes with special structure, the distributed multiple hydroforming method can be used.

4.2 Rolling forming

Rolling forming is to place the tube blank in the forming machine in advance, and then use the friction force between the working wheel and the tube blank to drive the whole tube blank to rotate, and then the working wheel and the shaping wheel coordinate to press the U-shaped ripple. Rolling forming efficiency is relatively low, and it is often used to process corrugated pipes with large diameters. In recent years, some enterprises have also used this principle to shape titanium bellows with a diameter of less than 100 mm.

4.3 Spinning forming

Spinning forming is mainly used in the production of titanium spiral bellows. Compared with other forming methods, the spinning die is relatively complex, including working diaphragm and spacer. In spinning forming, the diaphragm and the spacer rotate each other and extrude the tube blank to form a spiral corrugated pipe. Corrugated light with smaller diameter (within 50mm) is usually produced by this method.

4.4 Pressure forming

This method requires that a multi petal circular inner membrane is placed in the tube blank in advance, and the tube blank is expanded to form ripples by the side pressure generated by the cone placed in the center of the inner membrane after moving downward. The module is reset by the spring force after the cone moves upward. However, hydroforming, rolling forming, spinning forming and bulging forming are all cold working methods, which are only applicable to pure titanium bellows with good plasticity.

4.5 Welding forming

Welding forming is often used to manufacture corrugated pipes that cannot be hydroformed. This method is to alternately weld the inner and outer edges of several thin annular diaphragms into titanium corrugated pipes with transverse ripples through precision welding technology. The wall thickness of welded bellows is easy to control compared with other waveform dimensions, so it is often used in the manufacturing of precision bellows. The axial compensation of welded bellows is large, even reaching 80% of the total length of bellows. However, due to its small bearing margin and high manufacturing cost, it is commonly used for titanium bellows with non-standard size and large compensation amount under low pressure. The welding method is generally laser welding or micro plasma arc welding.

4.6 Superplastic forming

Superplastic forming is to use the superplasticity of titanium alloy at high temperature to form titanium alloy bellows with large wave depth parameters. The diameter of bellows manufactured by this method mainly depends on the capacity of superplastic forming machine, and the diameter range is generally 50-800 mm. The superplastic forming process is relatively complex, requiring the synergistic effect of superplastic air bulging and axial loading. The forming mainly includes the following three stages:

• 1) Bulging stage: firstly, argon is filled into the tube blank to make the tube blank produce a certain plastic deformation through air pressure, so as to achieve the fixation of the preset intermediate module on the cylinder.
• 2) Mold closing stage: subsequently, the press presses each module.
• 3) Filling stage: the press continues to apply pressure and maintain the pressure to make the corrugated part of the corrugated pipe all stick to the mold. In this method, the size of corrugated pipe is accurate, the residual stress is small, but the forming process consumes more energy, and the wall thickness of workpiece is easy to be uneven.

5. Application field of titanium bellows

Titanium bellows has been used in many industries due to its light weight, high flexibility and strong corrosion resistance.

5.1 Chemical industry

The titanium corrugated pipe has a considerable application prospect in chemical plants such as salt plants and chemical fertilizer plants. Due to the high corrosivity of the medium in the chemical industry, the traditional stainless steel can no longer meet the actual production needs. The use of titanium bellows is the main trend of development at present. In particular, titanium bellows can effectively prevent corrosion in equipment with more chloride ions.

5.2 Oil refining industry

Corrugated expansion joints are the most widely used bellows in the oil refining industry. Corrugated pipes used in the oil refining industry have high requirements for corrosivity, which are characterized by large diameter, easy corrosion of corrugated pipes, and more damage and failure. The main reason is corrosion damage. In recent years, with the increase of corrosive medium content in crude oil and the increase of operating temperature during use, the working conditions of corrugated pipes are increasingly harsh. Some bellows expansion joints have been corroded after several months or even hours of operation, which seriously affects the safe operation of the overall equipment and reduces the economic benefits of the enterprise. In this regard, some Japanese enterprises have applied titanium bellows to the oil refining industry, and all performance tests have shown satisfactory results.

5.3 Aerospace industry

Titanium bellows has good comprehensive performance and light weight. It was first used in aerospace. At present, the bellows used in aerospace are made of titanium and its alloys.

5.4 Medical devices

Because titanium has good affinity with human tissues, and titanium ions are not easy to lose, titanium corrugated tubes also have good application prospects in medical devices in recent years. In the United States, enterprises have used welding to manufacture titanium corrugated pipe containers with wavelet pitch as drug containers for peristaltic pump devices.

5.5 Ship industry

In recent years, with the rapid development of the ship industry in China, the service environment of the pipeline inside the ship hull has become increasingly harsh. In addition to certain strength, toughness and stiffness, the pipeline structure also requires that the pipeline have strong corrosion resistance. The titanium alloy bellows can well meet the above requirements. In addition, the titanium bellows also has the characteristics of light weight, which also brings benefits to the weight reduction of the ship hull. In terms of military ships, due to the invisibility of titanium alloy and the higher corrosion resistance of titanium compared with other metal materials in seawater, the use of titanium bellows has certain strategic significance to some extent. Some Japanese enterprises have already used titanium bellows in the water steam latent heat equipment of ship hulls, and the number of titanium bellows used on ships in China has also been increasing in recent years.

6. Conclusion

Although the cost of titanium and titanium alloy bellows is higher than that of stainless steel and nickel base alloy bellows, its service life can reach several times that of conventional material bellows. From the perspective of long-term and comprehensive economic benefits, titanium bellows has considerable application prospects.
At present, due to the cost problem, titanium corrugated pipes are used in high-tech industries such as aviation, aerospace and high-energy physics, while chemical and oil refining industries are less used; At the same time, due to the lack of manufacturing technology and less design basis, such as the engineering applicability of forming methods, the fatigue life of titanium bellows under corrosion and cyclic stress, and the design of bellows with complex dimensions, the promotion and application of titanium bellows are limited. Some enterprises choose conventional carbon steel titanium composite plate or stainless steel for cost saving pipeline, and titanium is selected as compensator material. However, there is a lack of reliable and practical engineering connection technology for dissimilar metal connection between titanium and conventional materials, especially for structures that cannot be connected with flanges due to structural size limitations, the welding of dissimilar materials is also an urgent problem to be solved. However, it can be predicted that with the development of manufacturing technology and the reduction of production cost, titanium bellows will be more and more widely used.
Authors: Qin Jian, Zhong Yuping, Zhang Aiqin, Zhang Xiaowen, She Dongsheng
Source: China Titanium Bellows Manufacturer: www.titaniuminfogroup.com

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