Crack detection in continuous casting copper mold plate is a critical step in ensuring continuous casting production safety. Failure to detect cracks in a timely manner can lead to serious accidents such as cooling water leaks and steel breakouts. The following are common methods for detecting cracks in copper plates, covering both online rapid testing and offline precision testing. The method can be selected based on production needs and crack type:
Online Rapid Testing (In-Process or During Short Downtime)
This method is suitable for detecting obvious cracks or emergency troubleshooting without disrupting production flow. It primarily relies on visual inspection and simple tools.
Visual Inspection Method
Procedure: After shutting down the machine, remove mold slag, oxide scale, and other debris from the copper plate surface. Illuminate the copper plate surface with a strong light (such as a flashlight or industrial searchlight) and observe for linear depressions, cracks, or discoloration (impurities tend to accumulate at cracks, resulting in dark lines).
Applications: Detecting open surface cracks (especially visible cracks >0.2mm wide) or cracks accompanied by localized wear or deformation.
Advantages: Fast and convenient, no specialized equipment required;
Limitations: Unable to detect deep (subsurface) cracks or micro-cracks (<0.1mm), relying on the inspector’s experience.
Penetrant Testing (Simplified)
Procedure: Spray a portable colored penetrant (such as red penetrant) onto the copper plate surface. Allow it to sit for 5-10 minutes (allowing the penetrant to penetrate the cracks). Clean the surface with a cleaning agent, then spray with a developer (such as white powder). If cracks are present, the penetrant will be drawn out, revealing a red line.
Applications: Online detection of open surface cracks (including micro-cracks), particularly suitable for shallow cracks beneath copper plating layers (such as Cr layers).
Precautions: Clean the surface of the copper plate of any oil or scale to ensure adequate penetration of the penetrant. After testing, thoroughly remove any residual penetrant to avoid corrosion of the copper plate.
Offline Precision Inspection (Disassembly and Inspection after Shutdown, Suitable for Major Repairs or Suspected Internal Cracks)
Used to comprehensively assess the condition of the copper plate and detect deep or hidden cracks, providing a basis for repair or replacement.
Used for Ultrasonic Testing
Principle: Ultrasonic waves (usually with a frequency of 2-5 MHz) propagate through the copper plate and reflect at the crack interface. The probe receives the reflected signal and determines the crack’s location, depth, and length.
Operation:
Clean the copper plate surface and apply a coupling agent (such as engine oil) to ensure close contact between the probe and the plate.
Use a straight probe to detect internal cracks perpendicular to the surface, and an angled probe to detect oblique cracks at or near the surface.
Analyze waveforms (such as A-scans): Normal areas will show a single backscattered wave, while cracks will produce additional reflected waves, which attenuate or disappear.
Applications: Detect internal cracks in copper plates (such as subcutaneous cracks and weld cracks) or surface cracks greater than 0.5 mm in depth, with an accuracy of up to 0.1 mm. Advantages: Can detect non-open cracks and quantitatively analyze crack size. Suitable for inspecting copper plates (such as welds between copper plates and water jackets).
Magnetic Particle Testing
Principle: Magnetize the copper plate (using DC or AC magnetization). If cracks are present, a leakage magnetic field is generated at the crack site, attracting applied magnetic powder (black or fluorescent), forming visible magnetic traces.
Operation:
Magnetize the entire copper plate (e.g., using a current method to generate a longitudinal magnetic field for transverse cracks; using a wire winding method to generate a circumferential magnetic field for longitudinal cracks).
Spray magnetic powder (dry or wet suspension) and observe the magnetic trace morphology under strong light or ultraviolet light (linear traces are often cracks).
Applications: Detect surface or near-surface cracks (depth ≤ 2mm) on ferromagnetic copper plates (such as copper and brass). It is particularly suitable for detecting cracks in stress-concentrated areas such as corners and bolt holes.
Precautions: Not suitable for non-ferromagnetic materials (such as certain alloy copper plates). Demagnetization is required after testing (residual magnetization < 0.3mT) to avoid attracting iron filings. Penetrant Testing (Precision)
Difference from the online simple type: Using a more sensitive penetrant (such as a fluorescent penetrant) combined with UV illumination, it can detect micro-cracks <0.1mm and clearly display the crack outline.
Operational steps: Pre-cleaning → Penetration (10-20 minutes) → Emulsification → Cleaning → Development (10-30 minutes) → Inspection (observe green fluorescent lines under UV light).
Applications: Detecting micro-cracks on the surface, especially debonding cracks at the junction of the coating (such as Cr and Ni-Co alloy) and the copper substrate.
Eddy Current Testing
Principle: Eddy currents generated by alternating current are induced on the copper surface. Cracks cause the eddy current distribution to distort, and the probe detects impedance changes, indicating the presence of a crack.
Features: Non-contact testing allows for rapid scanning of large copper surfaces, making it suitable for automated inspection (e.g., with a robotic arm for batch inspection). Sensitive to surface or near-surface cracks, but limited in its ability to detect deep cracks.
Targeted Inspection of Key Areas
Copper plate cracks are prone to developing in areas of stress concentration or high-temperature fatigue. Inspection should focus on the following areas:
Copper plate corners: The corners of the billet or slab mold are prone to cracking due to uneven cooling and mechanical stress.
Copper plate and water jacket weld area: The heat-affected zone (HAZ) is prone to weld cracking and requires ultrasonic testing.
Coating damage area: The copper plate substrate where the coating has fallen off is susceptible to corrosion and wear, potentially resulting in microcracks.
Vibrating contact area: The area of friction with the ingot shell (such as the center of the copper plate) is prone to fatigue cracking due to long-term vibration.
Inspection Cycle and Standards
Online Inspection: Visual inspection and simple penetrant testing are performed after each casting (or every 24 hours).
Offline Inspection: Ultrasonic and magnetic particle testing are performed after every 1,000-3,000 heats (or based on wear).
Scrap Criteria: Cracks >50mm in length and >1mm in depth (or exceeding 10% of the copper plate thickness), or located in critical stress areas (such as corners), require copper plate replacement. Shallow cracks (<0.5mm in depth) can be repaired by grinding and retesting.
By combining online rapid screening with offline precision testing, comprehensive copper plate crack identification can be achieved, preventing damage to the mold and ensuring mold cooling efficiency and ingot quality. The appropriate combination of inspection methods should be selected based on the copper plate material, operating conditions, and crack risk level.
