Weld Overlay Repair of Continuous Casting Machine Tundish Chains
1. Definition and Technical Principles
The tundish chain (also referred to as the pull rod chain or caster chain) is a critical mechanical component in continuous casting machines that grips the solidified steel shell and pulls it through the casting mold and secondary cooling zones. These chains operate under extreme conditions: repeated thermal cycling from molten steel exposure (temperatures up to 1,500°C at the mold exit), mechanical wear from shell contact, corrosion from secondary cooling water and flux residues, and significant tensile and bending loads. Over time, the chain links, pins, and structural surfaces develop wear grooves, cracks, and dimensional loss that compromise structural integrity and casting quality.
Weld overlay repair of tundish chains involves the application of a specialized consumable layer—typically a hardfacing alloy or corrosion-resistant overlay—onto worn or damaged surfaces using TIG (Tungsten Inert Gas) or MIG (Metal Inert Gas) welding processes. The fundamental principle relies on depositing a metallurgically compatible, wear-resistant, and thermally stable layer that restores original dimensions and extends service life without requiring full component replacement. The overlay material is selected to resist thermal fatigue cracking, abrasive wear from the steel shell, and corrosion from the casting environment.
The repair process requires careful consideration of the base metal's thermal properties, residual stress management, and interpass temperature control to prevent cracking in the heat-affected zone (HAZ) and overlay weld itself. Given that tundish chains are typically fabricated from high-strength low-alloy (HSLA) steels or quenched and tempered steels, the repair welding procedure must account for the base metal's hardenability and susceptibility to cold cracking.
2. Category and Business Positioning
This capability falls squarely within the TIG/MIG Weld Overlay technology route of Cladding Technology Shanxi Co., Ltd. It represents a service-oriented, repair-focused application of the company's overlay welding expertise, distinguishing it from the production-oriented applications (such as clad plate fabrication or overlay pipe manufacturing) that constitute the company's core manufacturing business.
The business positioning of tundish chain weld overlay repair is as follows:
- Aftermarket Service Division: This capability positions the company as a value-added service provider to steel mills and casting equipment manufacturers, offering in-situ or shop-based repair services that reduce downtime and spare parts inventory costs for customers.
- Technical Demonstration Platform: Successful execution of tundish chain repairs serves as a technical reference case that demonstrates the company's proficiency in repair welding, overlay material selection, and process control—competencies directly transferable to more complex cladding applications.
- Entry Point to Customer Relationships: Repair welding engagements often serve as initial touchpoints with steel mill customers, creating opportunities to expand into larger cladding plate and overlay pipe supply contracts.
- WPS Qualification Building: Each repair procedure developed and executed contributes to the company's portfolio of qualified Welding Procedure Specifications (WPS), strengthening the qualification base for future projects.
3. Technical Purpose and Value
The primary technical purpose of weld overlay repair of tundish chains is to restore functional integrity to critical caster components while minimizing production downtime and associated costs. The value proposition encompasses multiple dimensions:
3.1 Economic Value
- Cost Reduction: Weld overlay repair typically costs 30–50% less than replacement with new chains, particularly for large-diameter chains where material costs are significant.
- Downtime Minimization: On-site or rapid-turnaround shop repairs reduce casting line shutdown time from days (for procurement and delivery of replacement chains) to hours.
- Inventory Optimization: Steel mills can reduce safety stock levels of expensive spare chains, knowing that qualified repair services are available.
3.2 Technical Value
- Performance Enhancement: The overlay layer can provide superior wear and corrosion resistance compared to the original base metal, effectively upgrading the component beyond its as-manufactured condition.
- Dimensional Restoration: Precision control of weld bead geometry enables accurate restoration of chain pin diameters, link clearances, and structural dimensions to manufacturer specifications.
- Process Knowledge Accumulation: Systematic documentation of repair parameters, materials used, and service performance builds institutional knowledge that improves future repair quality and reliability.
3.3 Customer Value
- Extended asset life and reduced total cost of ownership for continuous casting equipment
- Improved casting quality through restoration of proper chain-shell contact geometry
- Reduced risk of catastrophic chain failure during casting operations
- Technical partnership providing engineering support beyond simple parts replacement
4. Key Process and Implementation Points
4.1 Pre-Repair Assessment and Preparation
Before initiating any weld overlay repair, a comprehensive assessment of the damaged component must be conducted:
- Visual and Dimensional Inspection: Measure wear depth, assess dimensional loss against original specifications, and document crack locations using visual testing (VT) and, where necessary, magnetic particle testing (MT) or ultrasonic testing (UT).
- Base Metal Identification: Confirm the base metal grade through material certificates or spectroscopic analysis. Common tundish chain grades include Q345, Q460, 42CrMo, and similar HSLA or alloy steels.
- Damage Classification: Categorize damage as wear (surface), corrosion (chemical), thermal fatigue cracking (metallurgical), or mechanical damage (impact/fracture). Each damage type may require different repair strategies.
- Preparation: Grind or machine worn surfaces to remove all degraded material, creating a sound base for overlay deposition. Typically, a V-groove or U-groove preparation is required for deep wear areas to ensure adequate fusion and penetration.
4.2 Weld Overlay Process Parameters
The following table summarizes typical process parameters for tundish chain weld overlay repair:
| Parameter | TIG Overlay (GTAW) | MIG Overlay (GMAW) |
|---|---|---|
| Consumable Type | ER507, ER813, or Ni-based hardfacing electrode | ER506T, ER813T, or Ni-based hardfacing wire |
| Wire/Electrode Diameter | 2.0–3.2 mm | 1.2–1.6 mm |
| Current (A) | 120–220 | 150–280 |
| Voltage (V) | 12–18 | 18–24 |
| Travel Speed (mm/min) | 80–150 | 200–400 |
| Shielding Gas | Argon (99.99%) or Ar/CO₂ (80/20) | Argon (99.99%) or Ar/CO₂ (80/20) |
| Gas Flow Rate (L/min) | 10–15 | 12–20 |
| Preheat Temperature (°C) | 100–200 (per base metal) | 100–200 (per base metal) |
| Interpass Temperature (°C) | ≤150 | ≤200 |
| Post-Weld Heat Treatment | Stress relief at 550–650°C (if required) | Stress relief at 550–650°C (if required) |
| Typical Build-up Rate | 0.5–1.0 mm/pass | 1.0–2.0 mm/pass |
4.3 Overlay Material Selection
Material selection for tundish chain overlay repair depends on the specific service condition and failure mode:
| Failure Mode | Recommended Overlay Material | Key Properties | Applicable Standards |
|---|---|---|---|
| Abrasive wear (shell contact surfaces) | Cr-based hardfacing (e.g., ER507, ER506) | HRC 40–50, high hardness, good weldability | GB/T 12470, AWS A5.15 |
| Thermal fatigue cracking | Ni-based alloy (e.g., ERNiCrMo-3, Stellite 6) | Excellent thermal shock resistance, good ductility | ASTM B367, AWS A5.14 |
| Corrosion (cooling water zones) | Austenitic stainless (e.g., ER309L, ER316L) | Good corrosion resistance, low carbon to prevent sensitization | GB/T 17493, AWS A5.9 |
| Combined wear and corrosion | Multi-pass: Ni-based transition + Cr-based hardfacing | Combined properties, controlled dilution | Per WPS qualification |
| Dimensional restoration (structural) | Matching base metal consumable (e.g., ER50-6) | Matched strength, good toughness | GB/T 8110, AWS A5.18 |
4.4 Critical Implementation Steps
- Procedure Qualification: Develop and qualify a Welding Procedure Specification (WPS) specific to the tundish chain application, including all variables (base metal thickness, consumable type, preheat, interpass temperature, post-weld treatment) in accordance with applicable codes.
- Welder Qualification: Ensure welding operators are qualified on the relevant WPS, with qualification records maintained per applicable standards.
- Surface Preparation: Remove all contaminants (oil, rust, scale, flux residue) from the repair area to a minimum 12 mm beyond the weld preparation edge. Use mechanical grinding to a clean, bare metal finish.
- Groove Preparation: For wear depths exceeding 2 mm, machine a controlled groove (typically 60° V-groove or 30° included angle U-groove) to ensure proper weld fusion and reduce dilution effects.
- Preheat Application: Apply preheat uniformly to the entire repair zone using induction heating or oxy-fuel torches. Preheat temperature must be maintained throughout the welding operation.
- Weld Deposition: Execute the overlay weld in accordance with the qualified WPS, maintaining consistent travel speed, torch angle, and interpass temperature. For multi-pass builds, ensure adequate fusion between passes while controlling total heat input.
- Post-Weld Treatment: Where required by the WPS or base metal specifications, apply post-weld stress relief heat treatment. For high-strength chain steels, stress relief at 550–650°C for 2 hours per 25 mm of section thickness is typical.
- Post-Repair Inspection: Perform dimensional verification, visual inspection, and non-destructive testing (NDT) per the applicable acceptance criteria.
4.5 Heat Input Control
Heat input management is critical in tundish chain repair welding. Excessive heat input can cause:
- Tempering of the base metal HAZ, reducing hardness and strength
- Increased dilution of the overlay layer, degrading wear and corrosion resistance
- Residual stress buildup leading to distortion or cracking
- Grain coarsening in the HAZ, reducing toughness
Recommended heat input ranges: 0.5–2.0 kJ/mm for TIG overlay; 1.0–3.5 kJ/mm for MIG overlay. Multi-pass strategies with thinner individual passes are preferred over single thick deposits to minimize heat input per pass.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard Number | Title / Scope | Relevance |
|---|---|---|
| GB/T 12467 | Welding procedure specification—Qualification testing | WPS qualification for overlay welding procedures |
| GB/T 9452 | Welding procedure qualification test methods | Test methods for procedure qualification |
| NB/T 47014 | Qualification test and evaluation of welding procedures for pressure vessels | Reference for procedure qualification methodology (applicable by analogy) |
| ASME Section IX | Qualification of Welding Procedures and Welders | International reference for WPS and welder qualification |
| GB/T 3323 | Non-destructive testing of welds—Radiographic testing | RT acceptance for critical repairs |
| GB/T 15055 | Non-destructive testing of welds—Magnetic particle testing | MT acceptance for surface cracks |
| GB/T 11345 | Non-destructive testing of welds—Ultrasonic testing | UT acceptance for volumetric defects |
| GB/T 8110 | Welding consumables—Solid wire for arc welding | Consumable specification for structural steel overlay |
| GB/T 17493 | Welding consumables—Solid wire for arc welding of stainless steel | Stainless overlay consumable specification |
| GB/T 12470 | Welding consumables—Cast surfacing electrodes for hardfacing | Hardfacing consumable specification |
| AWS D10.12 | Overlay welding of castings and forgings | International standard for overlay welding practices |
| ISO 3834 | Quality requirements for fusion-welded products | Quality management framework for welding operations |
5.2 Acceptance Criteria
- Visual Inspection (VT): No surface cracks, undercut exceeding 0.5 mm, porosity clusters, or incomplete fusion visible on the overlay surface. Bead profile should be smooth and uniform with no excessive convexity or concavity.
- Magnetic Particle Testing (MT): Acceptance per Level 1 criteria of GB/T 15055—no linear indications (cracks, laps) permitted; rounded indications (slag inclusions, porosity) limited to 3 mm length maximum and not more than 3 per 100 mm of weld length.
- Ultrasonic Testing (UT): Where applicable (thicker sections), acceptance per GB/T 11345 Level 2—no indications above the relevant acceptance threshold for the section thickness.
- Dimensional Verification: Restored dimensions within ±0.5 mm of original chain specifications (pin diameter, link width, link height, chain pitch).
- Hardness Testing: Overlay surface hardness within the specified range for the selected consumable (e.g., HRC 40–50 for Cr-based hardfacing; HRC 35–45 for Ni-based alloys). Base metal hardness should show no more than 5 HRC reduction in the HAZ compared to original specification.
- Macrograph Examination (on test coupons): No lack of fusion, cracks, or excessive dilution at the overlay/base metal interface. Dilution should be controlled to ≤30% for hardfacing applications where composition is critical.
6. Common Risks and Controls
| Risk Category | Specific Risk | Consequence | Control Measures |
|---|---|---|---|
| Hydrogen-induced cracking | Diffusion hydrogen from welding arc or moisture in consumables | Delayed cracking in HAZ or weld metal, potential catastrophic failure | Control preheat temperature (≥150°C), use low-hydrogen consumables, bake electrodes/wire per manufacturer instructions, apply post-weld bake-out at 250–350°C for 2 hours |
| Thermal fatigue cracking | Excessive thermal cycling during service causes cracking in overlay or HAZ | Crack propagation leading to chain failure during casting | Select Ni-based or ductile austenitic overlay materials; control heat input to minimize HAZ embrittlement; ensure adequate overlay thickness (≥3 mm) to accommodate thermal strain |
| Excessive dilution | High heat input or improper technique causes excessive base metal mixing into overlay | Loss of overlay properties (hardness, corrosion resistance); overlay performance degraded | Use multi-pass technique with thin individual passes; select consumables with high alloy content to compensate for dilution; maintain low travel speed with appropriate current; use backing bars to improve root fusion control |
| Residual stress and distortion | Asymmetric welding sequence or excessive heat input causes component distortion | Dimensional non-conformance; chain misalignment in service; accelerated wear | Use symmetric welding sequence (weld from center outward); apply proper preheat and interpass temperature control; consider post-weld stress relief; use fixture clamping to control distortion |
| Incomplete repair assessment | Undetected cracks or subsurface damage not addressed before overlay | Crack propagation under service loads; premature repair failure | Perform thorough NDT (MT, UT) before repair; remove all cracked material; repair cracks by grinding to a sound base or by crack termination drilling per code requirements |
| Welder skill variability | Inconsistent technique leads to variable weld quality | Inconsistent overlay properties; dimensional non-conformance | Maintain welder qualification records; implement procedure audits; use automated or semi-automated welding where feasible; conduct first-piece inspection |
| Material incompatibility | Incorrect consumable selection for base metal or service condition | Poor weldability; inadequate overlay performance; cracking | Perform material identification before welding; consult consumable compatibility charts; qualify procedures with coupon testing before production application |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
Tundish chain repair is a direct application of the company's TIG/MIG weld overlay technology route. This route encompasses all arc-based overlay processes including:
- Manual TIG (GTAW): Preferred for precision repair of small areas, chain pins, and detailed surfaces where control of heat input and bead geometry is critical. Enables excellent visual inspection of each pass and precise positioning of the torch.
- Submerged Arc (SAW): Applicable for large-area build-up of heavily worn chain links where deposition rate is prioritized over visual access.
- Flux-Cored Arc (FCAW): Suitable for field repairs where shielding gas equipment is not available, using self-shielded flux-cored wire.
- Wire Feed MIG (GMAW): Preferred for larger surface areas and higher deposition rates, enabling efficient repair of extensively worn chain links.
The tundish chain repair application specifically leverages the following competencies developed through the company's overlay welding program:
- Multi-layer overlay design for wear/corrosion resistance
- Transition layer design between dissimilar materials (base metal to overlay)
- Heat input control for thick-section components
- WPS development and qualification for repair applications
- NDT integration into repair workflows
7.2 Hydraulic Explosive Bonding Route
While tundish chains are not typically candidates for hydraulic explosive bonding (which is used for permanent clad plate and pipe production), the metallurgical knowledge gained from chain repair welding directly supports the bonding route in the following ways:
- Interface Metallurgy Understanding: Understanding of base metal/overlay interface behavior in weld overlay repairs informs the design of bonded interfaces in hydraulic explosive cladding, particularly regarding interfacial shear strength and bond quality assessment.
- Material Compatibility Data: Experience with steel-to-Ni-alloy and steel-to-stainless combinations in repair welding provides practical data that supplements laboratory bond quality testing for explosive bonding applications.
- Post-Bond Repair Capabilities: For clad plates produced via hydraulic explosive bonding, the company's weld overlay expertise enables repair of damaged cladding layers or addition of functional overlay layers on bonded substrates.
7.3 Explosion Welding Route
Explosion welding produces clad plates and shapes through high-velocity impact bonding. The connection to tundish chain repair is primarily in the following areas:
- Clad Plate Applications in Casting Equipment: Explosion-welded clad plates (e.g., carbon steel base with Ni-alloy or stainless cladding) can be used in the manufacture of casting machine components where combined structural strength and surface resistance are required. The company's understanding of casting equipment service conditions—gained through chain repair work—enables more appropriate clad plate design for these applications.
- Repair of Exp-Welded Components: When explosion-welded clad components in casting equipment suffer damage to the cladding layer, the company's weld overlay expertise enables repair and restoration of the functional surface layer.
- Process Knowledge Transfer: Understanding of high-strain-rate bonding interfaces and their mechanical properties, developed through explosion welding, informs the selection of overlay materials and processes for applications requiring high interfacial strength.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
Each tundish chain repair project contributes to the company's qualification portfolio in the following ways:
- WPS Library Expansion: Each unique base metal/consumable/thickness combination generates a qualified WPS that can be referenced for future projects, reducing the need for repeated qualification testing.
- Welder Qualification Records: Maintaining welder qualification records across multiple procedures demonstrates organizational capability and meets customer audit requirements.
- Material Qualification Database: Accumulated data on consumable performance in casting equipment service conditions builds a proprietary database that differentiates the company from competitors.
- NDT Procedure Development: Development and validation of inspection procedures for overlay welds on curved chain surfaces contributes to the company's NDT capability matrix.
8.2 Product Delivery Enhancement
- Integrated Service Offering: The ability to offer both new clad product manufacture and in-service repair creates a comprehensive value proposition for steel mill customers, increasing customer retention and contract value.
- Technical Support Credibility: Practical repair experience provides the engineering insight necessary to deliver clad products that perform reliably in actual service conditions, reducing warranty claims and enhancing customer satisfaction.
- Supply Chain Resilience: Repair capability reduces customer dependence on long-lead-time spare parts procurement, making the company a more attractive partner for integrated cladding solutions.
8.3 Customer Value Demonstration
- Quantifiable ROI: Each successful chain repair generates documented data on service life extension, cost savings versus replacement, and downtime reduction—metrics that can be presented to prospective customers as evidence of value.
- Technical Consultancy: Repair experience enables the company to provide customers with preventive maintenance recommendations, overlay material selection guidance, and failure analysis services—high-value-added consultancy that strengthens long-term relationships.
- Quality Track Record: Documented service performance of repaired chains (measured in casting cycles, tonnage processed, or calendar time in service) provides empirical evidence of overlay welding quality that supports marketing claims for clad product offerings.
9. Best Practices and Lessons Learned
9.1 Documentation and Traceability
For every tundish chain repair, maintain complete documentation including: pre-repair inspection records, base metal identification, WPS reference number, consumable heat numbers, welder identification, preheat and interpass temperature logs, NDT results, dimensional verification records, and post-repair inspection reports. This traceability is essential for warranty purposes, customer audits, and continuous improvement.
9.2 Field Repair Considerations
When repairs are performed on-site at steel mills, additional considerations include:
- Environmental controls (wind protection for gas shielding, moisture management)
- Equipment portability (battery-powered TIG machines, portable preheat units)
- Safety coordination with mill operations (hot work permits, confined space entry, crane coordination)
- Power supply limitations (generator requirements for remote locations)
- Logistics for consumable storage and handling in industrial environments
9.3 Service Life Prediction
Based on accumulated repair experience, develop service life prediction models that correlate overlay material type, thickness, and service conditions with expected repair intervals. This enables proactive maintenance scheduling and reduces unplanned downtime for customers.
10. Conclusion
Weld overlay repair of continuous casting machine tundish chains represents a technically demanding, commercially valuable application that fully leverages the company's TIG/MIG weld overlay capabilities. It serves as a practical demonstration of the metallurgical expertise, process control discipline, and quality management systems that underpin the company's broader cladding technology offerings. By maintaining rigorous WPS qualification, welder certification, NDT integration, and documentation practices, the company ensures that every repair delivers reliable performance while building the qualification base and customer trust necessary for sustained business growth across all three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding.