If cold cracks are found in the hot runner welding, the system must be stopped immediately and repaired to prevent crack propagation from causing sudden breakage, leakage, or mold damage.
1. Emergency Measures
Immediate shutdown: Once a cold crack is confirmed, the hot runner system should be stopped immediately to prevent rapid crack propagation under high temperature and pressure.
Pressure relief and cooling: Safely release the pressure of the heat transfer medium in the system and gradually cool it to room temperature to prevent secondary thermal stress.
Identification and isolation: Mark the defective parts and isolate them from the production process to prevent misuse.
Cold cracks have high propagation potential; even small cracks should not be allowed to propagate, otherwise, serious accidents may occur within hours.
2. Repair Procedures
(1) Crack removal
Grinding and grooving: Use an angle grinder or milling equipment to completely remove the cracked area, forming a U-shaped bevel to ensure no crack residue remains. Dye Penetrant Testing (PT) Verification: Perform penetrant testing after polishing to confirm complete crack removal.
It is recommended to remove the area 5–10 mm beyond both ends of the visible crack to prevent hidden propagation.
(2) Pre-welding Preparation
Thorough Cleaning: Remove oil, moisture, scale, and other impurities to prevent the introduction of hydrogen sources.
Preheating Treatment: Preheat low-alloy steel or thick-walled components (>25 mm) to 100–200°C to reduce the cooling rate and inhibit martensite formation.
Environmental Control: Operate in a dry environment. Given the current high humidity in Wuhan (April–June), it is recommended to use dehumidifiers to control humidity to ≤60%.
(3) Welding Repair
Welding Material Selection: Use low-hydrogen welding materials (such as E5015, ER70S-6, ERNiCr-3), and ensure that the welding rods are dried at 350–400℃ for 2 hours and then stored at this temperature.
Process Control: Use low heat input and multi-layer, multi-pass welding to avoid single-pass overheating.
Protection Measures: When welding TIG or MIG, ensure that the argon purity is ≥99.99% and the flow rate is 18–22 L/min.
(4) Post-Welding Treatment
Slow Cooling and Insulation: Immediately after welding, cover the joint with a ceramic insulation cover or asbestos cloth and allow it to cool slowly for at least 12 hours to promote hydrogen escape.
Stress Relief Treatment (Optional): For high-restraint joints, it is recommended to perform tempering treatment at 600–650℃ to eliminate residual stress. 3. Post-Repair Inspection and Acceptance
Wait 24 hours: Ensure potential delayed cracks are fully visible;
Non-destructive Testing (NDT) combination:
MT: Inspect surface/near-surface cracks;
UT (PAUT/TOFD): Detect internal defects;
PT: Assist in verifying non-ferromagnetic materials;
Hardness testing: Heat-affected zone HV ≤ 350HV; exceeding this limit requires reprocessing;
Metallographic analysis: Sampling is necessary to verify microstructure homogeneity and fusion quality.
Final judgment: Only after all test items pass can the material be put back into use.
