In the installation and maintenance of flue system, welding is one of the most common ways to connect the expansion joint with the flue. However, it is not entirely clear to many field welders and technicians: How are flue expansion joints welded? Irregular welding operation may lead to serious quality problems such as expansion joint deformation, bellows burn-through, weld cracking, etc. This paper will systematically explain the technical requirements, operation steps, quality control and common problems of flue expansion joint welding, and provide comprehensive guidance for on-site welding operation.
First, the basic concept of flue expansion joint welding
Before answering "How are the expansion joints of flue welded", it needs to be clear first: not all expansion joints are welded. There are two main ways of connecting the expansion joint with the flue:
| Connection mode | Applicability | peculiarity |
|---|---|---|
| Flanged connection | Non-metallic expansion joints, small and medium diameter metal expansion joints, occasions that need to be replaced regularly | It is easy to disassemble, but has many sealing surfaces and bolts are easy to corrode |
| welded connection | Large diameter metal expansion joints, high temperature and high pressure working conditions, permanent installation | Good sealing performance and high strength, but difficult to disassemble |
This paper focuses on the welding connection mode, that is, the interface end of the expansion joint is directly connected with the butt welding of the flue nozzle.
2. Preparation before welding
Proper preparation is half the battle. The following work must be completed before welding:
1. Welding process evaluation and welding process specifications
- According to the material of the expansion joint, the material of the flue and the wall thickness, the welding process specification shall be formulated
- Welding process evaluation shall be completed for important parts to confirm the feasibility of process parameters
2. Welder qualification confirmation
- Welders must hold the special equipment welding operation qualification certificate of the corresponding project
- The certificate items shall cover: base metal material, welding method, welding position (horizontal welding, vertical welding, overhead welding, etc.)
3. Welding Material Preparation
| Base metal combination | Recommended Welding Materials |
|---|---|
| Carbon steel flue + carbon steel expansion joint | E4315/E5015 (J422/J507) |
| Carbon steel flue + stainless steel expansion joint | A302/A307 stainless steel welding electrode (transition layer) |
| Stainless steel flue + stainless steel expansion joint | A102/A132 (E308/E347) |
| Different steel welding | Nickel-based welding materials (e.g. ENiCrFe-3) |
4. Bevel processing and cleaning
- Groove form: V-shaped or U-shaped groove, angle 60° ~70°, blunt edge 1~2mm
- Cleaning scope: Oil, rust, moisture and paint within 20mm of each side of the groove must be removed
- Cleaning Method: Angle grinder grinding or acetone wiping
5. Expansion joint protection measures
This is the most overlooked link in how to weld the expansion joint of the flue:
- The surface of the bellows must be covered with asbestos cloth or metal protective cover to prevent welding slag from splashing and damaging
- The welding ground wire shall be clamped directly on the flue pipe nozzle, and it is strictly prohibited to be clamped on the expansion joint bellows
- The ground circuit of the welder shall avoid passing through the expansion joint body
3. Welding process parameters and operation points
1. Selection of welding method
| Welding method | Applicability | advantage | shortcoming |
|---|---|---|---|
| Manual arc welding | Ordinary carbon steel, wall thickness ≥4mm | Simple equipment and flexible operation | Large spatter, high heat input |
| Argon arc welding | Stainless steel, thin wall (≤3mm) | Small heat input, high weld quality | High cost, slow speed |
| CO₂ gas-shielded welding | Plate carbon steel | High efficiency, large penetration depth | Large splash |
| Argon combined welding | Stainless steel medium-thick wall | Bottom layer argon arc welding + filled cover electrode | Good comprehensive performance |
2. Key process parameters (taking manual arc welding and carbon steel as examples)
| parameter | Recommended value | Description |
|---|---|---|
| Electrode diameter | Bottom layer φ 3.2mm, filling cover φ 4.0mm | Small diameter for thin wall |
| Welding current | φ 3.2: 90-120 A; φ 4.0: 130-160A | Excessive current will burn through the bellows |
| Arc voltage | 22~26V | Short arc operation |
| welding speed | 100~150mm/min | Moving at uniform speed |
| Interlayer temperature | ≤150℃ (stainless steel ≤100℃) | Controlling heat input |
3. Operation sequence and technique
Step 1: Positioning Welding
- Symmetrical spot welding 4~6 points along the circumference
- The length of positioning weld is 10~15mm, and the thickness shall not exceed 1/2 of the formal weld
Step 2: Bottom layer welding
- Adopt segmented desoldering or symmetric welding
- The length of each section is 150~200mm, starting from the bottom and alternating to both sides
- Must be soldered through to ensure that the back is well formed
Step 3: Fill layer welding
- Removing bottom weld slag
- Continue to weld in segments symmetrically, each layer thickness ≤3mm
- Take care to control the interlayer temperature
Step 4: Cover surface layer welding
- The last weld should be 1~2mm higher than the base metal
- Smooth transition with base metal, no biting edge
4. Special requirements for expansion joint welding
How to weld the expansion joint of the flue is significantly different from ordinary pipe welding, and the following special requirements must be observed:
1. Symmetrical welding to prevent bellows deformation
The bellows of the expansion joint has thin walls (usually 1~3mm) and low stiffness. Continuous welding on one side creates an uneven heat input that causes the bellows to shrink and deform or create additional stresses.
Correct way to do this:
- Two people simultaneously welded symmetrically on both sides
- Or one person adopts jump-type segment welding, and immediately turns to a symmetrical position after each segment is welded
2. Narrow bead, low current, fast welding
- Multi-layer multi-pass welding is adopted, and the width of each weld is ≤3 times the diameter of the welding electrode
- Control heat input to avoid coarse or sensitized grains in the heat affected zone of bellows (stainless steel)
3. Use argon arc welding as much as possible
For stainless steel expansion joints, argon arc welding primer can:
- Ensure that the back surface is formed smoothly and without welding nodules
- Reduces oxidation and prevents "burning through" bellows
- Reducing stress concentration at weld root
4. Post-weld heat treatment (depending on the material)
| Material | Whether post-weld heat treatment is required | Treatment process |
|---|---|---|
| Carbon steel (wall thickness ≥20mm) | Needed | 600~650℃ heat preservation to relieve welding stress |
| Carbon steel (wall thickness | Generally not required | — |
| Austenitic stainless steel | Not required (but interlayer temperature controlled) | — |
| Different steel welding | Needed | Determined by higher side material |
V. Welding quality inspection and acceptance
After the welding is completed, how to weld the expansion joint of the flue must pass the quality acceptance to close the loop.
1. Appearance inspection (100%)
- Weld seam forming: smooth transition with base metal, no biting edge, crater, crack, air hole
- Size of weld: Surplus height of weld ≤3mm, not lower than base metal
- Surface defects: no cracks, unfused, surface pores
2. Non-destructive testing (according to design requirements)
| Detection Methods | Proportion of detection | Qualification Grade |
|---|---|---|
| Penetration detection | Important welds 100% | Crack-free, nonlinear display |
| Magnetic particle detection | Ferromagnetic material optional | crack-free |
| Radiographic detection | On special request | Class II qualified |
| Ultrasonic testing | Thick-walled weld | Class I or II |
3. Weld Hardness Testing
- Austenitic stainless steel weld hardness ≤220HB
- If the hardness is too high, the heat input is too large or the cooling is too fast
VI. Common welding problems and treatment
| Question | Causes | Treatment method |
|---|---|---|
| Bellows burn through | Excessive current, slow welding speed, too thin wall thickness | Grinding and repair welding, using argon arc welding to reduce current |
| Welding deformation | Unilateral continuous welding and positioning welding are insufficient | Correction of deformation, replacement of expansion joint in severe cases |
| Bite edge | Excessive current, improper angle of welding electrode | Repair welding, adjusting welding parameters |
| Weld crack | Large restraint stress and mismatch of welding materials | Carbon planing to clear cracks, re-welding, preheating if necessary |
| Damage to bellows by welding slag spatter | Unprotected | Clean up splashes, inspect bellows, and repair if necessary |
| Weld porosity | The groove is unclean and the welding electrode is damp | Sand and remove, dry welding electrode, re-weld |
7. Safety points of welding operation
- Fire-and explosion-proof: combustible gas may remain in the flue, and gas detection must be carried out before welding (especially gas flue)
- Ventilation: Welding in confined spaces must be forcibly ventilated, and air-supply breathing mask must be worn
- Anti-scald: the thin wall of the bellows is easy to conduct heat, and the surrounding personnel should wear heat-resistant gloves during welding
- Grounding safety: The ground wire of the welding machine must be directly connected to the welding workpiece, away from instruments and cables
VIII. Summary
How the expansion joint of the flue is welded-This is a comprehensive problem involving the welding process, materials science, and field operation. Through the above systematic explanation, the core points can be summarized as follows:
- Preparation before welding is the foundation: groove cleaning, protective covering and correct connection of ground wire are indispensable. Especially the protection of bellows directly determines the success or failure of welding
- Symmetric segmented welding is the core: the expansion joint has thin wall and poor rigidity, so it is necessary to adopt symmetric, segmented and jump welding technology to prevent thermal deformation
- Small currents, narrow beads, fast welding: controlling the heat input is key. Argon arc welding bottoming + manual arc welding filling cover is the most mature process combination
- Welding material matching by material: carbon steel with carbon steel welding electrode, stainless steel with stainless steel welding electrode, different steel with nickel-based welding material
- Quality control is not relaxed: the appearance inspection is 100% covered, and important welds are subjected to non-destructive testing (penetration/magnetic powder) to ensure no cracks and no biting edges i>
- Safety always comes first: gas detection, ventilation, fire prevention, scald prevention-the safety risk of welding expansion joints is higher than that of ordinary pipes
In practical engineering, it is suggested that the above process requirements should be compiled into the welding instruction book, and special training should be carried out for welders. Only by strictly adhering to the standard process can we ensure the reliable strength of the welded joint, no air leakage, and at the same time do not damage the compensation function of the bellows. Remember one principle: when welding the expansion joint, it is better to slow down and divide into several sections than to burn it to the end at once-this is the correct answer of "how to weld the expansion joint of the flue".