What issues need to be paid attention to during the welding process of stainless steel pipes used

When welding stainless steel pipes for industrial fluid transportation, the following key issues should be noted:

1. Selection of welding process

Avoid using high-temperature processes such as oxyacetylene welding to prevent the loss of alloy elements and the precipitation of carbides (such as in the sensitization temperature range of 450~850℃). 

2. Pre-weld Preparation

Groove Treatment: Polish the groove to a metallic luster, remove oil and oxide scale, and use mechanical processing or plasma cutting (avoiding carbon contamination). 

Argon filling protection: Before welding, fill argon gas inside the pipeline (purity ≥ 99.99%) at a flow rate of 5 to 15 L/min until the oxygen content is less than 50 ppm. This prevents the backside of the weld from oxidizing and forming a "slag shell", thereby reducing the corrosion resistance. 

Workpiece fixation: Use fixture for rigid fixation to reduce welding deformation and avoid stress caused by forced alignment. 

3. Welding material matching

The welding rods/swarfs must match the composition of the base material:

For 304 base material, ER308L welding wire (low carbon) is selected; for 316L, ER316L is chosen. Avoid corrosion caused by mismatch in carbon content or alloy elements. 

For welding of different types of steel (such as 304 and 316L), high-alloy welding wires (such as ER317L) are selected to meet the composition requirements of both materials. 

4. Welding Parameter Control

Current and Speed: Use a low current (about 20% lower than that for carbon steel) and a fast welding speed to reduce the heat input, control the interlayer temperature to be ≤ 150℃, and prevent grain coarsening in the heat-affected zone. 

Welding gun angle: During argon arc welding, the angle between the welding gun and the workpiece should be maintained at 70° to 80°, and the arc length should be 2 to 4 mm. Ensure that the molten pool is adequately protected. 

Number of weld layers: For thick-walled pipes, welding is carried out in layers. Each layer should have a thickness of no more than 3mm. The slag and spatter between layers should be removed to avoid slag inclusion and incomplete fusion. 

5. Corrosion Prevention and Stress Control

Prevent iron ion contamination: Welding tools (such as wire brushes, fixtures) should be made of dedicated stainless steel to avoid carbon steel contact, which could lead to "iron penetration" and electrochemical corrosion. 

Post-weld treatment:

Acid pickling and passivation: Use a solution of nitric acid and hydrofluoric acid to remove welding slag and oxide scale, forming a uniform passivation film (Cr₂O₃), which enhances corrosion resistance. 

Stress relief: For high-pressure or large-diameter pipelines, after welding, perform solution treatment (water cooling at 1050~1100℃) or low-temperature annealing (holding at 300~400℃), to reduce welding stress and prevent cracking. 

6. Special Medium and Environmental Response

Chloride-sensitive scenarios: When transporting media such as seawater and salt solutions, strictly control welding spatter (containing Cl⁻ impurities), thoroughly clean and test the surface Cl⁻ residue (≤ 25 ppm) after welding. 

High-temperature / Low-temperature pipelines: For high-temperature (such as steam) pipelines, high-temperature welding rods (such as ER310) should be used; for low-temperature (such as liquefied gas) pipelines, the welds must have low-temperature impact toughness (nickel-based welding rods should be selected). 

7. Quality Inspection and Acceptance

Visual inspection: The surface of the weld should be smooth without cracks, pores, or undercut (depth ≤ 0.5mm, length ≤ 10% of the total length of the weld). 

Non-destructive testing:

For high-pressure pipelines (≥ 10 MPa) or pipelines carrying toxic media, 100% radiographic (RT) or ultrasonic (UT) testing is required;

For general pipelines, sampling tests are conducted according to design requirements, with a focus on checking the heat-affected zones and the arc termination points.

Pressure test: After welding is completed, hydraulic/air pressure tests are carried out in accordance with the specifications. For austenitic stainless steel pipelines, the water quality Cl⁻ should be controlled at ≤ 25 ppm to prevent stress corrosion. 

Summary: The welding process needs to be strictly controlled throughout the entire process, including technology, materials, parameters, anti-corrosion measures, and testing. The core objective is to minimize thermal damage, avoid corrosion risks, and ensure structural strength, thereby ensuring the safe and reliable operation of stainless steel pipelines in industrial fluid transportation.