Titanium Welds In Heat Exchanger

Titanium welds

Welding plays a crucial role in the manufacturing of titanium heat exchangers. Titanium welding techniques are used to join the different components of a heat exchanger together, such as the titanium tubes, plates, and fittings. The most common welding techniques used for titanium heat exchangers are TIG (Tungsten Inert Gas) welding, plasma arc welding, and electron beam welding.

TIG welding is the most commonly used welding technique for titanium heat exchangers. This welding technique uses a non-consumable tungsten electrode to produce the arc, which melts the base metal and filler wire, creating a fusion between them. The filler wire used in TIG welding is typically the same grade of titanium as the base metal to ensure a strong and corrosion-resistant joint. TIG welding provides excellent control over the welding process and produces high-quality welds with minimal distortion.

Analysis of Titanium Welding Performance

1. The influence of gas and impurity pollution on welding performance

1.1. The influence of hydrogen is the most serious factor affecting the mechanical properties of titanium among gas impurities. The change in the hydrogen content of the weld has the most significant impact on the impact performance of the weld. The main reason is that the increase in the amount of hydrogen bomb in the weld increases the amount of flake or needle-like TiH2 precipitated in the weld. TiH2 has very low strength, which significantly reduces the impact performance of the weld. Pay attention to the use of low-hydrogen electrode or wire when welding.

1.2. The influence of oxygen. Oxygen has high solubility in both α and β phases of titanium, and can form interstitial solid solution phases, causing serious distortion of the lattice phase, resulting in increased hardness and strength of titanium and titanium alloys, but a significant decrease in plasticity. In order to ensure the performance of welding joints, in addition to preventing oxidation of the weld seam and welding heat-affected zone during the welding process, the oxygen content in the base metal and welding wire should also be limited.

1.3. The influence of nitrogen. At a high temperature above 700°C, nitrogen and titanium react violently to form brittle and hard titanium nitride (TiN), and the degree of lattice distortion caused by the formation of interstitial solid solution between nitrogen and titanium is more serious than that caused by oxygen absorption. Therefore, nitrogen is more significant than oxygen in improving the tensile strength and hardness of titanium and titanium alloy welds, and reducing the plastic properties of welds.

1.4. The influence of carbon. Carbon is also a common impurity in titanium and titanium alloys. Experiments show that when the carbon content is 0.13%, the carbon is deep in α titanium, the weld strength is improved, and the plasticity is reduced, but it is not as strong as oxygen and nitrogen. However, when the carbon content of the weld is further increased, the weld appears net-like TiC, the amount of which increases with the increase of the carbon content, so that the plasticity of the weld decreases sharply, and cracks are prone to appear under the action of welding stress. Therefore, the carbon content of the base metal of titanium and titanium alloy shall not exceed 0.1%, and the carbon content of the weld shall not exceed the carbon content of the base metal.

2. Welded joint crack problem

When welding titanium and titanium alloys, cold cracks may appear in the heat-affected zone, which is characterized by cracks occurring several hours or longer after welding, which is also called delayed cracking. Studies have shown that this kind of crack is related to the diffusion of hydrogen during the welding process. During the welding process, hydrogen diffuses from the high temperature molten pool to the lower temperature heat-affected zone. The increase in hydrogen content increases the amount of TiH2 precipitated in this zone, which increases the brittleness of the heat-affected zone. In addition, the volume expansion during the precipitation of hydrides causes greater structural stress. , Coupled with the diffusion and accumulation of hydrogen atoms to the high-stress parts of the region, resulting in the formation of cracks. The method to prevent this kind of delayed cracking is mainly to reduce the source of hydrogen in the welded joint, and to perform vacuum annealing treatment if necessary.

3. Selection of welding method

GR2 titanium tube and tube sheet are designed with strength expansion and sealed welding. When welding titanium and titanium alloys, when the temperature is between 500°C and 700°C, it is easy to absorb oxygen, hydrogen and nitrogen in the air, which seriously affects the welding quality. The arc of argon arc welding is protected and cooled by the argon gas flow, the arc heat is relatively concentrated, the current density is high, the heat affected zone is small, and the welding quality is high. Due to the special physical and chemical properties of titanium, combined with our company's equipment and environmental conditions, this time we use automatic tungsten argon arc welding for the welding of the titanium heat exchange tube and the tube sheet.

4. Sample plate inspection

Visual inspection. The surface of the weld and heat-affected zone should be inspected 100% with a 10 times magnifying glass. The results of visual inspection and non-destructive testing should not have defects such as cracks, unfused, pores, undercuts, arc craters, inclusions, and spatters. There should be no defects outside the weld There are arcing points. There should be no oxidized color on the surface (only silver white and light yellow are allowed). After inspection, no cracks, pores, lack of fusion and other defects were found on the surface of the weld, the surface color was silver-white, and the appearance inspection was qualified.

5. Sealed room for welding

Because of the special nature of titanium, the influence of wind speed, temperature, humidity, dust and other environmental factors on welding quality should not be underestimated. In order to ensure the welding environment, we have built a 30m2 (5mx6m) sealed welding room that is relatively isolated from the outside world. Keep the temperature around 25°C and the relative humidity less than 60%. The welding qualification of the test piece and the welding of the product are carried out in this wind-proof and dust-proof sealed room. (1) Air conditioners, dehumidifiers, vacuum cleaners and exhaust fans should be installed in the sealed room. The indoor light should be sufficient, and the ground should be flat, clean and clean; (2) Fire prevention measures and safe passages should be considered in the sealed room; (3) Welders during welding Wear clean work clothes and wear degreasing pure polyester gloves.

6. Basic requirements for people, machines and materials in welding

6.1. Personnel requirements. (1) Welding technicians, welding quality inspectors and welding inspection and testing personnel should have corresponding qualifications; (2) All welders who participate in welding should go through the theory and practical operation of mobile welding of any grade of titanium and titanium alloy Only after skill training and qualified examinations can the welding of titanium heat exchange tubes and tube sheets be carried out. And need to master the following basic knowledge: a. Basic knowledge of titanium metal materials; b. Basic knowledge of welding materials (argon) and its use; c. Basic knowledge of welding process and professional knowledge of titanium welding; d. Common Causes, hazards, prevention and treatment measures of welding defects; e. Basic knowledge of the types, use and maintenance of tube sheet welding equipment and measuring instruments.

6.2. Requirements for automatic tungsten argon arc welding machine for tube sheet. (1) The arc characteristic is stable; (2) The current adjustment is flexible and convenient, and has good process repeatability; (3) The mechanical actuator is flexible in operation and easy to install; (4) It has advanced air supply, delayed air stop, pulse, Non-contact arc ignition and current attenuation function.

7. Preparation before welding tube sheet

(1) Tube-sheet welders should strictly follow the welding process regulations; (2) The gap between the size of the welder's center positioning rod and the inner diameter of the expanded tube hole is 0.1 ~ 0.15mm, and the welding process does not shake; (3) Tungsten electrode processing shape , Welding joint form, setting of tungsten electrode. (4) When installing the tungsten electrode, the tungsten electrode should be in the center of the nozzle of the welding gun and must not be skewed; (5) When welding, always pay attention to the shape of the tip of the tungsten rod. If the tip of the tungsten electrode becomes round, the direction of the arc will change. The welding seam is not smooth and has burrs, and the tungsten electrode should be replaced at this time; (6) It is strictly forbidden to expand or cut the pipe when welding on one side and the other side to ensure the welding quality. When welding on both sides, it is not allowed to weld a titanium tube at the same time. (7) Jump welding should be used when welding, and the welding should be row by row from bottom to top. (8) The surface of the weld should be uniform, beautiful and fish-scale. The weld reinforcement should not be greater than 0.5mm, and the weld width should be 2～2.5mm; (9) The surface of the weld is not allowed to have defects such as cracks, pores, lack of fusion, and offset; (10) The surface of the weld should be silver. White or light yellow, purple, blue, gray, etc. are not allowed.

8. Non-destructive testing of tube sheet weld surface

After the weld surface has passed the visual inspection, the coloring inspection shall be carried out.

9. hydrostatic test

After the non-destructive testing meets the quality requirements, the assembly is carried out. After the assembly is completed, the hydraulic test is carried out. The weld of the heat exchange tube and the tube sheet is not found to leak in the hydraulic test.

Titanium Welds In Heat Exchanger

Titanium welding products are widely used in heat exchangers due to their excellent corrosion resistance, high strength-to-weight ratio, and ability to withstand high temperatures. Here are some examples :

1. Titanium tubes: Titanium tubes are commonly used in heat exchangers due to their excellent corrosion resistance, even in highly aggressive environments. The high strength-to-weight ratio of titanium also makes it an ideal material for use in heat exchanger tubes.

2. Titanium plates: Titanium plates are used in plate heat exchangers, which are highly efficient heat exchangers that use a series of thin, corrugated plates to transfer heat between two fluids. The corrugations in the plates increase the surface area, which enhances the heat transfer process.

3. Titanium welds: Titanium welds are used to join the various components of a heat exchanger, including the tubes, plates, and fittings. The specialized welding techniques used to weld titanium ensure the integrity of the weld and the corrosion resistance of the joint.

4. Titanium fittings: Titanium fittings are used to connect the various components of a heat exchanger, including the tubes and plates. The corrosion resistance of titanium ensures the durability and longevity of the fittings, even in highly corrosive environments.

Overall, the use of titanium welding products in heat exchangers offers many benefits, including improved corrosion resistance, increased efficiency, and reduced maintenance costs. These advantages make titanium an ideal material for use in heat exchangers in various industries, including chemical processing, oil and gas, and power generation.

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