I. Basic overview of flue stainless steel expansion joint welding

Flue stainless steel expansion joint welding is a crucial manufacturing and installation process in industrial piping systems. As a key component to compensate pipeline thermal displacement, absorb vibration and reduce noise, the welding quality of expansion joint is directly related to the safe operation and service life of the whole pipeline system. Stainless steel has become the first choice material for flue gas pipe expansion joints because of its excellent high temperature resistance and corrosion resistance. However, the welding characteristics of stainless steel are significantly different from those of ordinary carbon steel, and specialized process points and operating specifications are required to ensure the strength, sealability and durability of welded joints.

2. Material preparation for welding of flue stainless steel expansion joint

2.1 Selection of stainless steel base metal

Before welding the flue stainless steel expansion joint, you need to confirm the base metal grade first. Common flue gas pipe expansion joints mostly use 304, 304L, 316 or 316L austenitic stainless steel. Among them, 316L has better intergranular corrosion resistance in sulfur-containing flue gas environment due to the addition of molybdenum element. When selecting materials, it is necessary to check the material certification documents, and observe whether there are cracks, delamination, oxide scale and other defects on the surface.

2.2 Matching of welding materials

The selection of welding materials should follow the principle of "equal strength and equal composition". For 304 base metal, ER308 or ER308L welding wire is usually selected; For 316 base metal, use ER316 or ER316L wire. The integrity of the packaging should be checked before the use of welding materials, and the use of welding materials that are damp or rusty is strictly prohibited. During manual arc welding, the welding electrode should be dried at the specified temperature and kept warm.

2.3 Design of welding grooves

Reasonable groove design is the prerequisite to ensure welding quality. Flue stainless steel expansion joint welding usually adopts V-shaped or U-shaped groove, the single-side groove angle is controlled at 60°-70°, the blunt edge height is 1-2mm, and the pair clearance is 2-3mm. The groove surface should be flat and smooth, free of burrs and oil stains, and can be prepared by mechanical processing or grinding after plasma cutting.

3. Core technology of flue stainless steel expansion joint welding

3.1 Selection of Welding Method

According to the structural characteristics and field conditions of the expansion joint, a variety of technical methods can be adopted for welding the flue stainless steel expansion joint:

• Manual Tungsten Arc Welding (GTAW/TIG): Suitable for thin-walled bellows and bottoming welding, which can obtain excellent forming and sealing properties
• Manual arc welding (SMAW): suitable for medium and heavy plate construction and field installation
• Fused Electrode Gas Shielded Welding (GMAW/MIG): Suitable for long straight welds and mass production

It is recommended that the connection between bellows and flange adopt the combination process of TIG welding and SMAW filling cover.

3.2 Control of welding parameters

Stainless steel has poor thermal conductivity and large linear expansion coefficient, so the welding parameters need to be strictly controlled:

The principles of small current, fast welding speed and narrow welding bead are adopted to avoid coarse grains or carbide precipitation caused by overheating.

3.3 Back Gas Protection

In the case of single-sided welding and double-sided molding, shielding gas must be introduced into the back of the weld seam to prevent root oxidation. The flow rate of argon gas should be 6-10 L/min, and the ventilation time should start 2-3 minutes before arc ignition. After arc quenching, the ventilation should be continued until the weld seam is cooled to room temperature.

4. Welding deformation control and prevention

4.1 Causes of welding deformation of stainless steel

The most prominent problem in the welding process of flue stainless steel expansion joints is hot deformation. The thermal conductivity of stainless steel is about 1/3 of that of carbon steel, but the thermal expansion coefficient is 1.5 times that of carbon steel, which leads to large welding stress and angular deformation during heat concentration and cooling shrinkage.

4.2 Anti-deformation measures

• Rigid fixing method: Use special fixtures to fix the workpiece on the platform to limit the degree of freedom
• Symmetric welding: Sequence of segmented jump welding and symmetric welding is adopted to disperse heat input
• Reserved anti-deformation amount: According to experience, the anti-deformation angle of 1°-3° is preset when pairing
• Control wire energy: The wire energy should not exceed 15 kJ/cm. If necessary, use multi-layer multi-pass welding instead of wide pendulum welding

4.3 Post-weld correction

When the deformation exceeds the standard, mechanical correction (press flattening) or local heating correction (heating temperature controlled at 650-850℃, assisted by water cooling) can be used. Care should be taken not to damage the effective working part of the bellows during flame correction.

V. Welding quality inspection standards

5.1 Appearance inspection

Visual inspection shall be performed after the welding process of each flue stainless steel expansion joint is completed:

• Weld formed well, smooth transition with base metal
• No cracks, unfused, bitten edges (bitten edge depth ≤0.5mm), air holes (diameter ≤1mm and no more than 2 welds per 50mm)
• Residual height ≤3mm, uniform width

5.2 Non-destructive testing

Select the detection method according to the pressure level and operating conditions:

• Permeability testing (PT): routine testing of surface opening defects with qualified grade not less than Class I
• Radiographic inspection (RT): internal quality inspection of butt joint, qualified grade not less than Class II
• Air tightness test: Check the weld seam by charging with 0.2-0.5MPa compressed air and applying soapy water

5.3 Pressure test

After the expansion joint is assembled, the whole hydraulic pressure test is carried out, the test pressure is 1.5 times of the design pressure, and the pressure is stabilized for 10 minutes without pressure drop and leakage. The test water temperature shall not be less than 5℃, and the chloride ion content shall be ≤25mg/L during the hydraulic test of austenitic stainless steel to prevent stress corrosion cracking.

VI. Common welding defects and countermeasures

VII. SUMMARY

Flue stainless steel expansion joint welding is a professional technical work with high requirements for process control. In order to obtain high-quality welded joints, we must strictly control the whole process from material matching, groove preparation, welding parameter selection, deformation control to quality inspection. The core points can be summarized as: "small current, fast welding speed, narrow welding bead, layer temperature control, back gas protection and anti-deformation". In practice, according to the structural form, wall thickness dimension and service environment of the expansion joint, targeted welding process specification (WPS) should be formulated and implemented by welders with corresponding qualifications. Only by ensuring the reliability of each weld can the stainless steel expansion joint run stably in high temperature flue gas pipeline for a long time, and effectively play its functions of absorbing heat displacement and absorbing shock and noise. Regular inspection and maintenance of expansion joint welds that have been put into use is also an important link to prolong the life of equipment and ensure the safety of system.