How To Assess The Quality Of Steel Pipe Welds

Theoretical basis and standard system

The quality assessment of steel pipe welds is mainly based on national standards and industry regulations. The core standards include:
• GB/T 40791-2021 "Non-destructive Testing of Welded Joints"
• GB/T 12437-2008 "Welding Joint Ultrasonic Testing Method"
• GB/T 3323-2008 "Radiographic Testing Methods for Welded Joints"
• NB/T 47013 Series "Non-destructive Testing of Pressure Equipment"

The weld inspection levels are divided into three grades:
• Grade 1 weld: 100% ultrasonic testing
• Secondary weld: 20% ultrasonic testing
• Grade 3 welds: The sampling ratio is relatively low, but they must meet the basic testing requirements.

Main detection methods (theoretical level)

Key points of practical experience

1. Start with the visual inspection
• The surface must not have any defects such as cracks, incomplete fusion, pores, slag inclusions, or arc pits.
The edge-biting depth should be no more than 0.5mm, and the continuous length should not exceed 10% of the total length of the weld.
• Weld bead height: 0 - 3mm for the outer wall, 0 - 4mm for the inner wall. Any excessive height requires grinding.
The weld width should be 1 to 2 millimeters wider than the surface of the groove.

2. Defect Identification Experience
• Cracks: The most dangerous defect. It is strictly prohibited from existing under any circumstances.
• Pore: Single diameter ≤ 3.0mm or T/3 (take the smaller value), cumulative diameter within 150mm weld length ≤ 6.0mm or 0.5T
• Bar-shaped slag inclusion: Single length ≤ 12.0mm or T (take the smaller value), width ≤ 1.5mm
• Lack of penetration: Applicable only to single-sided welds that require full penetration. h ≤ 0.2t and maximum 2mm.

3. Key Points of Operating Experience
• The ambient temperature should be ≥ 5℃. If it is lower than 5℃, preheating is required.
• Relative humidity should be no more than 90%, and the welding rods need to be kept warm and dried.
• Wind speed control: Manual arc welding ≤ 8 m/s, Gas shielded welding ≤ 2 m/s
The interlayer temperature is controlled within the range of 150 - 250℃.

1. Appearance quality requirements
According to the SL/T 432-2024 standard, the appearance quality of weld seams is classified into three categories:

• Defect Name: Class 1 Weld Seam, Class 2 Weld Seam, Class 3 Weld Seam
• Crack Not allowed Not allowed Not allowed
• Surface pores: d ≤ 0.2s is not allowed. Maximum 2mm. d ≤ 0.3s is not allowed. Maximum 3mm.
• Edge biting: h ≤ 0.05t, maximum 0.5mm; h ≤ 0.1t, maximum 0.5mm; h ≤ 0.2t, maximum 1mm.
• Weld height: h ≤ 0.1b + 1 (maximum 5mm) h ≤ 0.15b + 1 (maximum 7mm) h ≤ 0.25b + 1 (maximum 10mm)

2. Dimensions and Geometric Requirements
• Deviation amount: ≤ 10% of the wall thickness, and ≤ 2mm
• Weld spacing:
The distance between the two rings of the straight section shall be no less than 100mm and shall also be no less than the outer diameter of the pipe.
When the nominal diameter is 150mm or more, the weld spacing shall be no less than 150mm.
When the nominal diameter is less than 150mm, it shall be no less than the outer diameter of the pipe.
• Welding position: It must not be within the pipe support area. The distance from the starting point of the bent pipe should be no less than 100mm and no less than the outer diameter of the pipe.

3. Requirements for Non-Destructive Testing
• Critical pipelines: 100% radiographic testing or ultrasonic testing
• Testing standards: NB/T 47013.2 (Radiation), NB/T 47013.3 (Ultrasonics)
• Qualification level: Radiographic testing is qualified at level II, and ultrasonic testing is qualified at level I.

4. Welding process requirements
• Groove processing: For walls with a thickness greater than 4mm, a groove (V-shaped, J-shaped or U-shaped) must be machined.
• Welding material matching: It must be compatible with the base material. For example, E4303 welding rods should be used for Q235B.
• Preheating requirements: For carbon steel with a wall thickness greater than 26mm or when the ambient temperature is less than 5℃, preheat to 100-150℃.

5. Post-welding treatment
• Welding seam cleaning: Remove welding slag and spatter.
• Acid pickling and passivation: The weld seams of stainless steel need to undergo acid pickling and passivation. The surface roughness should be less than or equal to 0.8 μm.
• Hydrogen removal treatment: High-grade steel pipelines (X70/X80) require a hydrogen removal treatment at 250°C for 2 hours.

6. Special Requirements
• Pressure pipeline: A water pressure test is required. The test pressure shall be 1.5 times the design pressure.
• Hydraulic Engineering: Follow SL/T 432-2024. Class I welds require the strictest requirements.
• Petrochemical industry: Follow SH 3501 standards. For pipelines carrying toxic and flammable media, even higher standards are required.

Judging the quality of steel pipe welds requires the combination of theoretical standards and practical experience. It involves multi-dimensional evaluations such as visual inspection, non-destructive testing, and dimension measurement. Generally, it covers aspects like appearance quality, dimensional tolerance, non-destructive testing ratio, and welding process control. Specific standards should be selected based on the application of the pipeline (pressure pipelines, water conservancy projects, petrochemicals, etc.).