CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Tundish Chains in Continuous Casters
Literature Overview
Tundish chains (also referred to as ingot chains or starter bar chains) in continuous casting machines serve as the initial solidification starter for the cast strand. These chains are subjected to extreme operating conditions including contact with molten steel at temperatures exceeding 1500 °C, mechanical loading from strand weight and pulling force, and corrosive attack from molten flux. Wear, corrosion, and deformation of the chain surface and engagement teeth necessitate periodic repair or replacement. Weld overlay repair offers a cost-effective alternative to complete chain replacement, extending service life while minimizing downtime.
Operating Conditions and Failure Modes
Service Environment
| Parameter | Typical Value | Impact on Chain Material |
|---|---|---|
| Molten steel temperature | 1500–1600 °C | Thermal degradation, oxidation |
| Strand pulling speed | 0.5–2.5 m/min | Mechanical wear, fatigue |
| Flux temperature | 1300–1400 °C | Chemical attack, spalling |
| Thermal cycling frequency | 1 cycle per cast | Thermal fatigue cracking |
| Cast duration | 8–12 hours | Cumulative damage accumulation |
Primary Failure Modes
- Surface wear: Gradual material removal from the chain engagement surface due to sliding contact with the starter bar
- Thermal erosion: Material loss from the chain surface due to molten steel and flux impingement
- Corrosion spalling: Oxidation and subsequent spalling of the surface layer due to thermal cycling
- Mechanical deformation: Plastic deformation of chain teeth under high contact stress
- Cracking: Thermal fatigue cracks initiating at surface defects or stress concentration sites
Weld Overlay Repair Technology
Material Selection for Overlay
The overlay material must provide a combination of high-temperature strength, oxidation resistance, and wear resistance while maintaining adequate bond strength to the base chain material (typically carbon steel or low-alloy steel).
| Overlay Material | Hardness (HV) | Service Temperature Limit | Application Scenario |
|---|---|---|---|
| Cr12MoV (high carbon steel) | 500–550 | 600 °C | General wear repair |
| H13 (hot work die steel) | 400–450 | 600 °C | Moderate thermal stress |
| 410H (martensitic SS) | 350–400 | 800 °C | Higher temperature service |
| Stellite 6 (Co-Cr alloy) | 400–450 | 1000 °C | Severe wear and corrosion |
| Ni-based alloy (Inconel 625) | 250–300 | 900 °C | Severe corrosion environment |
Welding Process Selection
The choice of welding process depends on the chain geometry, repair area size, and available equipment:
| Process | Deposition Rate | Penetration | Suitability for Chain Repair |
|---|---|---|---|
| Submerged Arc Welding (SAW) | High | Deep | Large flat surfaces, thick buildup |
| Gas Metal Arc Welding (GMAW) | Medium | Moderate | Versatile, good for complex geometry |
| Electroslag Welding (ESW) | Very High | Very Deep | Large areas, thick overlay |
| Flux-Cored Arc Welding (FCAW) | Medium-High | Moderate | Field repair, good deposition rate |
| Plasma Transferred Arc (PTA) | Medium | Shallow | Precision repair, low dilution |
Repair Procedure and Process Design
Pre-Repair Preparation
- Inspection and assessment: Determine the extent of wear, identify cracks (using magnetic particle testing or ultrasonic testing), and measure remaining material thickness
- Surface preparation: Remove worn material, oxidation, and flux residue using grinding or cutting; ensure a clean, sound base for overlay
- Crack repair: If cracks are present, remove them completely by machining or grinding, and apply a stress-relieving groove preparation
- Preheating: Apply uniform preheating at 150–250 °C to reduce thermal stresses and prevent hydrogen-induced cracking
Overlay Application Strategy
The repair overlay is typically applied in multiple passes to achieve the required thickness and ensure proper metallurgical bonding:
- First pass (transition layer): Applied with a weld metal composition designed for good wetting and bonding to the base material; this pass may use a lower-alloy wire to ensure complete fusion
- Intermediate passes: Build up the required thickness while gradually transitioning to the final overlay composition
- Final pass: Applied with the specified overlay material to achieve the target surface properties
Post-Weld Treatment
| Treatment | Purpose | Typical Parameters |
|---|---|---|
| Stress relief | Reduce residual stresses | 550–650 °C, 1–2 hours |
| Heat treatment (if applicable) | Achieve target hardness and toughness | Quench and temper per material specification |
| Surface finishing | Remove surface irregularities | Grinding to specified surface roughness |
| Dimensional verification | Ensure geometry meets specifications | CMM or manual measurement |
Quality Control and Inspection
Non-Destructive Testing Requirements
| NDT Method | Application | Acceptance Criteria |
|---|---|---|
| Magnetic Particle Testing (MT) | Surface and near-surface cracks | No linear indications in overlay or HAZ |
| Ultrasonic Testing (UT) | Bond strength, internal defects | No indications per relevant standard |
| Visual Testing (VT) | Surface quality, geometry | No porosity, undercut, or excessive reinforcement |
Mechanical Property Verification
- Hardness testing: Verify hardness profile through the overlay thickness; hardness should be uniform within ±30 HV across the overlay layer
- Bond strength testing: Transverse tensile or peel test specimens machined from test coupons welded under identical conditions
- Metallographic examination: Verify absence of cracks, pores, and inclusions; assess microstructure quality
Engineering Practice Considerations
Economic Analysis
The economic case for weld overlay repair versus complete chain replacement must consider:
- Direct material cost of overlay wire versus complete chain cost
- Labor cost for repair versus replacement
- Downtime cost (production loss during repair/replacement)
- Extended service life achieved through repair
- Quality and reliability of repaired versus new chains
Typically, weld overlay repair costs 30–50% of complete replacement, with the added benefit of potentially restoring the chain to better-than-original condition through the use of superior overlay materials.
Field Implementation Challenges
- Access limitations: Chains in service may have limited access for welding equipment and operator positioning
- Geometry complexity: Chain teeth and engagement surfaces have complex geometries that may be difficult to weld
- Base metal condition: Repeated thermal cycling may have degraded the base metal properties, affecting weldability
- Dimensional accuracy: Repair must restore original dimensions to ensure proper engagement with starter bars
Key Technical Insights and Reflections
The most important lesson from studying tundish chain repair is that the overlay repair strategy must be tailored to the specific failure mode rather than applying a generic repair approach. A chain suffering primarily from thermal erosion requires a different overlay material and process than one suffering from mechanical wear. Failure to diagnose the root cause of degradation and select the appropriate repair strategy is the most common cause of premature repair failure.
Another critical insight is the importance of the transition layer in the multi-pass repair strategy. The first pass serves as the metallurgical bridge between the base material and the overlay material, and its quality directly determines the long-term reliability of the repair. In practice, the transition layer composition should be carefully selected to ensure complete fusion with the potentially degraded base metal while providing a suitable substrate for the subsequent overlay passes.
The practical experience also highlights the importance of post-weld stress relief. The thermal stresses induced during the welding repair process can be comparable to those experienced during normal service, and without proper stress relief, these stresses can initiate premature cracking during subsequent thermal cycling. The stress relief treatment parameters must be carefully controlled to avoid softening the overlay layer while effectively relieving residual stresses.
Conclusion
Weld overlay repair of tundish chains is a well-established technology that offers significant economic and operational benefits when properly implemented. The key to successful repair lies in accurate failure diagnosis, appropriate material and process selection, meticulous execution of the repair procedure, and thorough quality verification. Engineers must approach each repair with a systematic methodology that considers the specific service conditions, failure history, and available resources to achieve optimal repair outcomes.