Steps and key points for long time welding of heat exchanger tube

In heat exchanger manufacturing and maintenance, the welding quality of tubes and tube sheets directly affects the performance and service life of the equipment. The following are the key steps and considerations for long-time welding of heat exchanger tubes to ensure stable and reliable welding quality.

Preparation Before Welding

- Tube and Tube Sheet Cleaning
Thoroughly clean the surfaces of the heat exchanger tubes and tube sheet before welding to remove oxides, oil, and contaminants.

- Use a wire brush or a chemical cleaner suitable for the material.

For stainless steel tubes and tube sheets, an acid pickling solution can be used to clean and passivate the surface, ensuring better welding quality.

- Tube Alignment
Ensure that the tube is properly aligned with the hole in the tube sheet to achieve good weld penetration and joint quality.

Use alignment tools such as tube expanders or clamps to hold the tube in the correct position.

Maintain a consistent gap between the tube and the tube sheet to prevent misalignment during welding.

Welding Method Selection

- TIG (Tungsten Inert Gas) Welding
TIG welding is commonly used for heat exchanger tubes because it provides precise heat control and produces high-quality, clean welds. It is suitable for materials such as stainless steel and titanium.

The welding torch should be held at an appropriate angle (typically around 70°-80° to the tube sheet surface) to ensure proper fusion.

Use high-purity argon shielding gas to protect the weld pool from oxidation, which is essential for achieving corrosion resistance and weld integrity.

- MIG (Metal Inert Gas) Welding
MIG welding is an alternative method, especially for thick-walled tubes or when higher welding speeds are required. However, it may generate more spatter than TIG welding.

Set the appropriate wire feed speed and welding voltage based on the tube material and thickness.

For example, when welding carbon steel tubes, trial welds may be necessary to determine the best parameter settings for achieving proper penetration and weld bead formation.

Welding Process

- Weld Penetration
Proper weld penetration ensures a strong joint. The penetration should reach the full thickness of the tube sheet without excessive burn-through.

Carefully control the welding current and speed.

For instance, when TIG welding thin-walled stainless steel tubes, a welding current of about 80-120A may be used, depending on the tube diameter and thickness.

Use of Filler Metal
If filler metal is required (such as when welding dissimilar materials or reinforcing the weld bead), select a filler material compatible with both the tube and tube sheet.

The filler metal should have similar mechanical and chemical properties to ensure good weld strength and corrosion resistance.

For example, when welding a nickel alloy tube to a carbon steel tube sheet, a nickel-based filler metal may be used to prevent galvanic corrosion and ensure a strong bond.

- Weld Bead Formation
The weld bead should be smooth and free of cracks, porosity, and undercuts.

The welder should maintain a consistent welding speed to form a uniform bead.

In TIG welding, filler metal (if used) should be added in a controlled manner to achieve the desired bead shape.

The width and height of the weld bead should meet the design and quality requirements of the heat exchanger.

Post-Weld Inspection and Quality Control

- Visual Inspection
After welding, perform a visual inspection of the weld to check for surface defects such as cracks, porosity, lack of fusion, or spatter.

The weld should have a consistent color and appearance.

Any visible defects should be repaired or re-welded as needed.

- Nondestructive Testing (NDT)
Detect internal weld defects using nondestructive testing methods such as dye penetrant testing, magnetic particle testing (for ferromagnetic materials), or ultrasonic testing.

For example, in dye penetrant testing, a colored penetrant is applied to the weld surface. After a specified dwell time, excess penetrant is removed, and a developer is applied. The penetrant will seep into any defects and become visible, allowing detection of cracks or porosity.

- Hydrostatic or Pneumatic Testing
Perform hydrostatic or pneumatic testing to verify the integrity of welded tubes.

Hydrostatic Testing: Fill the heat exchanger with water (or a suitable liquid) and pressurize it above the operating pressure. Observe for any leaks in the welds or pipes.

Pneumatic Testing: Use compressed air or an inert gas for testing. Since pressurized gas stores more energy, additional safety precautions are required to prevent hazards.

By carefully controlling welding parameters and performing thorough quality inspections, the long-term reliability of heat exchanger tube welding can be significantly improved, ensuring safe and efficient operation.