I. Emergency Response: Ensuring On-Site Safety
1. Immediate Shutdown and Isolation
Upon discovery of a crack, immediately shut off the medium (e.g., oil, gas, water), depressurize, and isolate the cracked section to prevent it from expanding under pressure or vibration and causing an accident.
2. Marking and Recording
Mark the location, direction, and length of the crack with a marker and take photos for future analysis and tracing.
3. Preliminary Assessment of Hazard Level
If the crack penetrates the pipe wall or is located in high-stress areas such as welds or elbows, it is a serious defect and must be replaced. If it is a shallow surface crack that has not expanded, an assessment can be conducted to determine whether to repair or downgrade the pipe.
II. Professional Inspection to Confirm Crack Status
1. Magnetic Particle Inspection (MT)
Suitable for ferromagnetic materials such as carbon steel and alloy steel, it can efficiently detect surface and near-surface microcracks (sensitivity up to 0.1μm). During operation, ensure the magnetization direction is perpendicular to the crack to avoid missed detections.
2. Ultrasonic Testing (UT): Capable of penetrating and detecting the depth, length, and propagation direction of internal cracks, it is a core method for assessing structural integrity. It should comply with GB/T 5777-2019 standard and is particularly suitable for high-pressure pipelines.
3. Penetrant Testing (PT): Applicable to non-ferromagnetic materials such as stainless steel. It identifies open cracks through penetrant imaging. Operation is simple but limited to surface defects.
4. Eddy Current Testing (ET): Suitable for rapid online screening. It is sensitive to surface cracks, folds, and other defects and is commonly used for quality monitoring in production processes.
✅ It is recommended to use MT + UT in combination to achieve full coverage detection of both surface and internal defects.
III. Crack Cause Analysis (Preventing Recurrence)
Cracks may originate from raw material defects, uneven heating, rolling stress concentration, or improper heat treatment. For example:
Cracks originating from inclusions or segregation zones indicate billet quality problems;
Cracks distributed along temperature difference interfaces reflect uneven heating;
Cracks frequently appearing at locations of abrupt changes in wall thickness indicate process parameter mismatch;
Microstructural analysis showing coarse martensite indicates stress cracking due to excessively rapid quenching and cooling;
Fatigue streaks observed under electron microscopy on the fracture surface indicate fatigue propagation under long-term load.
🔍 If it is a batch of cracks, trace back to the same batch of billets, heat treatment parameters, and equipment operation logs.
IV. Handling Methods and Decisions
1. Situations requiring replacement
Cracks penetrating the pipe wall, length > 50mm, located in critical pressure-bearing areas; Multiple intersecting or branching cracks;
→ Replace the entire section; welding repair is strictly prohibited.
2. Temporary emergency treatment (limited to low-pressure non-critical systems)
Use metal repair adhesive or mechanical sleeves for reinforcement; This is only a transitional measure and replacement should be arranged as soon as possible.
3. Permissible Repair Situations
For shallow cracks, local grinding can be used for a smooth transition (R angle ≥ 3mm); After repair, MT or PT testing must be performed again to ensure no residual defects.
V. Systemic Prevention Recommendations
1. Source Control: Select high-purity billets, control S and P content < 0.025%, and inclusion level ≤ 2;
2. Process Optimization: Ensure uniform heating (within ±10℃), and rationally set piercing and rolling parameters;
3. Heat Treatment Control: Complete tempering within 2 hours after quenching to prevent delayed cracking;
4. Equipment Maintenance: Regularly calibrate the mill centerline and replace worn parts;
5. Closed-Loop Inspection: Perform hydrostatic testing + UT + MT triple testing on each steel pipe before leaving the factory.
✅ In critical scenarios such as high-pressure boilers and oil and gas transportation, prioritize products from manufacturers with API 5CT and GB 5310 certifications to ensure process compliance from the source.
