How to prevent the risk of cracking in Q355NH during processing?

1. Thermal Cutting (Plasma, Laser, Flame Cutting)

Risks:

Heat-affected zone (HAZ) cracking due to rapid cooling and hardened microstructures.

Stress concentration from uneven heating.

Prevention Measures:

Preheating:

Preheat to 100–150°C for thicknesses >20 mm (use temperature-indicating sticks).

Maintain interpass temperature during prolonged cutting.

Cutting Parameters:

Use lower heat input (e.g., reduced plasma gas flow) to minimize HAZ size.

For flame cutting, avoid excess oxygen (neutral flame preferred).

Post-Cutting Inspection:

Grind cut edges to remove microcracks.

Perform dye penetrant testing (DPT) for critical components.

2. Cold Working (Bending, Punching, Shearing)

Risks:

Edge cracking due to high yield strength (≥355 MPa) and reduced ductility.

Notch sensitivity from phosphorus (P) content.

Prevention Measures:

Bending:

Use a minimum bend radius of 2× thickness (e.g., 20 mm radius for 10 mm plate).

For tight bends, anneal at 600–650°C to restore ductility.

Punching/Shearing:

Ensure tools are sharp and clearance is ≤10% of material thickness.

Avoid repeated deformation at the same location.

3. Welding

Risks:

Hydrogen-induced cracking (HIC) in the HAZ.

Residual stresses from thermal cycles.

Prevention Measures:

Preheating:

Preheat to 100–120°C for thicknesses >15 mm (even if ambient temperature is high).

Electrode Selection:

Use low-hydrogen electrodes (e.g., E5015-G for SMAW, ER50-6 for GMAW).

Dry electrodes at 300–350°C for 1 hour before use.

Welding Technique:

Control interpass temperature (≤200°C).

Use multi-pass welds to reduce stress concentration.

Post-Weld Stress Relief:

For critical structures, perform stress relief annealing at 550–600°C (optional but recommended).

4. Environmental Protection

Interim Corrosion Prevention:

Apply temporary rust inhibitors or organic coatings on cut edges during storage/fabrication to prevent pitting (which can initiate cracks).

Avoid Moisture:

Store steel in dry conditions; cover during rain.

5. Design and Fabrication Best Practices

Avoid Notches:

Use rounded corners in designs to reduce stress concentration.

Sequential Processing:

Perform cold working before thermal cutting to minimize residual stress叠加.

Quality Verification:

Conduct ultrasonic testing (UT) or magnetic particle inspection (MPI) for critical welds and cut edges.

6. Special Case: Thick Sections (>40 mm)

Risks: Higher residual stress and cracking susceptibility.

Solutions:

Preheating to 150°C for thermal cutting and welding.

Use normalizing (heating to 890–920°C followed by air cooling) post-fabrication to refine grain structure.

7. Summary Table: Key Prevention Steps