Steel Pipe Welding Techniques for Connecting Steel Pipes and Flanges

**Steel Pipe Welding Techniques for Connecting Steel Pipes and Flanges**

 

### (1) Steel Pipe Butt Welding and Splicing

1. **Steel Pipe Butt Welds**

– **Between any two adjacent nodes, there must not be two splicing points in the steel pipe butt welds, and the minimum length of the connecting pipe should be greater than 800 mm.**

2. **Welder Qualification**

– **Welders performing steel pipe butt welds must pass examinations and obtain a “Qualification Level Certificate” issued by a nationally recognized institution. They must carry the certificate when on duty.**

3. **Inspection Standards**

– **Steel pipe butt welds are classified as first-class welds according to the national “Code for Construction and Acceptance of Steel Structure Engineering.” Visual inspection is conducted according to first-class quality standards; 100% ultrasonic testing is required, and 2% of the length is subjected to X-ray inspection. For the weld connecting the flange and the steel pipe, visual inspection is conducted according to first-class quality standards, and magnetic particle testing is conducted for 25% of the length.**

4. **Welding Preparation**

– **Steel pipe butt welds need to have beveled edges; the angle of the double-sided V-groove should be between 60° and 90°. The weld height should be greater than or equal to 0.8 t (where t is the pipe wall thickness). If welding with a backing ring, the root width of the weld \( b \) must be ≥ 4 mm. The backing thickness \( \delta \) must be ≥ 0.5 t.**

5. **Straightness Tolerance**

– **After butt welding and splicing of steel pipes, the straightness tolerance must not exceed L⁄1000 of the nominal length L.**

6. **Preheating Requirements**

– **When butt welding steel pipes, the ambient temperature at the worksite should be above 0 °C. For ordinary carbon steel pipes with wall thickness ≥ 50 mm and low-alloy steel pipes with wall thickness ≥ 36 mm, preheating is required. The preheating temperature and interpass temperature should be controlled between 100 °C and 150 °C, with the preheating zone extending 80–100 mm on each side of the weld area. When the worksite temperature is below 0 °C, the preheating temperature should be determined through testing.**

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### (2) Oblique Overlap of Steel Pipes

1. **Fit-Up Requirements**

– **For the oblique overlap of steel pipes, the fit-up rate of the intersection line should be greater than or equal to 85%. Any local gaps where there is no contact should be less than or equal to 4 mm.**

2. **Welding Preparation**

– **The slanted welds at the intersection lines of steel pipes also need to have beveled edges. The slope angle should be greater than or equal to 45°, and the weld height should be greater than or equal to 0.6 t (where t is the wall thickness of the branch pipe). See Figure 23.**

3. **Welding Technique**

– **The weld connecting the intersection lines of steel pipes is a fillet weld. When welding in multiple layers, continuous welding should be carried out. The surface of each weld pass should be cleaned promptly. If quality defects are found, they should be removed and re-welded.**

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### (3) Flange Processing and Connection

1. **Flange Surface Machining**

– **The pressure-bearing contact surface of the flange should be machined. To prevent welding deformation, the contact surface of the flange should be recessed inward to a depth of \( h = (0.10\text{ – }0.20)\,t \) (where t is the flange thickness) to provide a predetermined deformation allowance. See Figure 24.**

2. **Welding Technique**

– **The weld connecting the intersection lines of steel pipes is a fillet weld. When welding in multiple layers, continuous welding should be carried out. The surface of each weld pass should be cleaned promptly. If quality defects are found, they should be removed and re-welded.**

3. **Flatness Requirement**

– **The flatness deviation of the pressure-bearing contact surface of the flange should be less than or equal to ±0.20 mm. To ensure that after flange connection, the fit-up rate of the contact surface is greater than or equal to 75%, a 0.4 mm feeler gauge should be used for inspection. The total area where the feeler gauge can be inserted should not exceed 25% of the total area, and the maximum connection gap should not exceed 0.8 mm.**

4. **Dimensional Tolerances**

– **The ovality of the inner and outer diameters after flange machining should be less than or equal to 0.03–0.1 mm (according to grade 11 tolerance accuracy). Considering the impact of steel pipe ovality tolerance, the machining enlargement allowance for the flange inner diameter is +0.4–1.0 mm. The bevel angle for the inner diameter fillet weld should be greater than or equal to 45°, and the bevel height \( h \) should be ≥ 0.7 t (see Figure 25).**

– **For the enlargement of flange bolt hole diameters: when the bolt size is ≤ M16, the enlargement is 1–1.5 mm; when greater than M16, it is 2 mm. The bolt hole pitch tolerance should be less than ±0.2 mm to ensure a 100% fit rate.**

– **Flange reinforcement rib plates are to be fabricated at the butt weld between the steel pipe and the flange. The rib plates should be chamfered; when the chamfer angle is 45°, the chamfer width should be 1.2 hₑ (where \( hₑ \) is the weld height), meaning the chamfer is 1.2 times the weld leg height.**

– **Flange connections use ordinary bolts or high-strength bolts. For ordinary bolt connections, a single nut with a spring washer is used for locking.**