GB/T 3091-2015 Standard - Welded Steel Pipes for Low-pressure Fluid Conveyance

 1. Scope  

Applies to welded steel pipes for low-pressure fluid transport, typically with a nominal diameter ≤150 mm and wall thickness ≤12.5 mm.

 2. Ordering Information  

Buyers should specify in contracts:  

- Standard number (GB/T 3091-2015)  

- Steel grade (e.g., Q195, Q215A/B, Q235A/B, Q265A/B, Q295A/B, Q345A/B)  

- Dimensions (outer diameter, wall thickness, length)  

- Surface treatment (galvanized or black)  

- Special requirements (e.g., hydrostatic testing, non-destructive testing).  

 3. Main Steel Grades & Chemical/Mechanical Properties  

 Steel Grades:  

- Non-galvanized pipes: Q195, Q215A/B, Q235A/B, Q265A/B, Q295A/B, Q345A/B.  

- Galvanized pipes: Typically use Q195–Q345 grades.  

 Key Requirements:  

- Chemical composition: Limits for C, Mn, Si, S, P, etc., vary by grade.  

- Mechanical properties: Tensile strength, yield strength, and elongation comply with grade-specific values (e.g., Q235: yield strength ≥235 MPa).  

 4. Manufacturing Process  

- Pipes are welded via high-frequency resistance welding (ERW) or other welding methods.  

- Galvanized pipes undergo hot-dip galvanizing (zinc coating ≥500 g/m² for DN≥100 mm).  

 5. Inspection Requirements  

 (1) Dimensional Tolerances:  

- Outer diameter, wall thickness, and length must meet specified limits.  

 (2) Mechanical Tests:  

- Tensile test, flattening test, bending test (for pipes ≤DN50 mm), and hydrostatic test (or non-destructive testing).  

 (3) Surface Quality:  

- No cracks, laps, or harmful defects. Galvanized pipes must have a continuous, adherent zinc layer.  

 (4) Hydrostatic Test:  

- Test pressure = 5 MPa (or 2.5 MPa for galvanized pipes); held for ≥5 sec without leakage.  

 (5) Non-Destructive Testing (NDT):  

- Alternative to hydrostatic testing; methods include eddy current or ultrasonic testing.  

 6. Marking & Certification  

- Each pipe must be marked with: manufacturer’s name, steel grade, size, standard number, and inspection mark.  

- A quality certificate must accompany deliveries, including test results.  

 7. Comparison with Previous Versions  

GB/T 3091-2015 replaces the 2008 version, with updates to:  

- Expanded steel grades (added Q265, Q295, Q345).  

- Clarified inspection rules for galvanized pipes.  

- Enhanced NDT requirements.  

 Key Applications  

Widely used in construction, plumbing, firefighting, and low-pressure gas/water supply systems.  

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