Weld Overlay Repair Technology for Continuous Casting Rolls
1. Definition and Fundamental Principles
Weld overlay repair of continuous casting rolls (CC rolls) is a specialized surface restoration technology that applies high-performance alloy layers onto worn, damaged, or degraded roll surfaces using arc-welding or thermal-spraying processes. The objective is to restore geometric dimensions, re-establish surface hardness, and reinstate resistance to thermal fatigue, mechanical abrasion, and corrosion that the roll experiences during continuous contact with molten steel at temperatures ranging from 1,450°C to 1,650°C.
Continuous casting rolls—encompassing water-cooled copper rolls for the mold (tundish), backup rolls, and secondary cooling zone rolls—are subjected to extreme cyclic thermal loading, mechanical stress from shell contact pressure (typically 0.5–2.5 MPa depending on the steel grade), and chemical erosion from flux and scale. The weld overlay repair process leverages the metallurgical compatibility between the base substrate (usually high-conductivity copper alloy or cast iron) and the deposited overlay material to create a functionally graded interface that resists spalling, cracking, and delamination under service conditions.
The fundamental metallurgical principle involves controlled dilution management: the overlay alloy must achieve sufficient hardness (typically 35–60 HRC depending on application zone) while maintaining adequate toughness to withstand thermal shock. The heat-affected zone (HAZ) must be minimized to preserve the thermal conductivity of the base material, which is critical for effective water-cooling performance in mold rolls.
2. Category and Business Positioning
Within the company's three principal technology routes, weld overlay repair of continuous casting rolls falls squarely under the TIG/MIG Weld Overlay technology platform, with potential supplementary application of hydraulic bonding for certain composite roll constructions. This capability positions the company as a critical maintenance partner for integrated steel mills, providing:
- Roll restoration services: Field and shop-based repair of worn or thermally damaged rolls, extending service life by 2–5 times compared to original equipment.
- Performance upgrade cladding: Application of premium overlay materials (e.g., chromium-cobalt alloys, tungsten carbide-cermet composites) to enhance roll life beyond OEM specifications.
- Emergency repair capability: Rapid turnaround restoration of critical rolls during production stoppages, minimizing unplanned downtime costs that can exceed USD 10,000–50,000 per hour in large-scale steelmaking operations.
This technology entry directly supports the company's qualification portfolio by demonstrating process competence in high-temperature alloy welding, dilution control on dissimilar substrates, and post-weld thermal treatment protocols specific to copper-based and iron-based roll materials.
3. Technical Purpose and Value Proposition
The primary technical objectives of continuous casting roll weld overlay repair are:
- Dimensional restoration: Recovery of roll diameter to within ±0.05 mm of specification tolerance.
- Surface hardness enhancement: Achievement of target hardness (30–60 HRC) with uniform distribution across the overlay band.
- Thermal fatigue resistance: Development of crack-resistant microstructure capable of withstanding 50,000–200,000 thermal cycles.
- Interfacial integrity: Bond strength exceeding 15 MPa (peel test) between overlay and substrate.
- Thermal conductivity preservation: Maintenance of substrate thermal conductivity within 95% of original value for effective cooling.
The value proposition to customers includes extended roll life (reducing replacement frequency from 3–6 months to 12–24 months), reduced capital expenditure on new roll purchases, minimized unplanned production stoppages, and improved slab surface quality due to restored roll surface integrity.
4. Key Process Implementation Points
4.1 Pre-Weld Surface Preparation
Proper surface preparation is the single most critical factor determining overlay adhesion and long-term service performance. The following protocol must be followed:
- Inspection and profiling: Ultrasonic thickness measurement to identify base material remaining; profile scanning to map wear pattern geometry.
- Mechanical cleaning: Grinding to remove all oxide, scale, and damaged surface layers to a minimum depth of 0.5 mm. Surface roughness Ra should be controlled between 6.3–12.5 μm.
- Chemical cleaning: Solvent degreasing followed by acid pickling (for iron-based rolls) or alkaline cleaning (for copper-based rolls) to achieve bare metal surface.
- Preheating: Uniform preheating to prevent differential thermal stresses. Temperature targets are substrate-dependent.
4.2 Process Parameters
| Parameter | Copper Mold Roll (TIG) | Cast Iron Roll (MIG) | Cermet Overlay (TIG) |
|---|---|---|---|
| Shielding Gas | Argon (99.99%) | Argon + 2% CO₂ | Argon (99.99%) |
| Current Type | DCEN | DCRP | DCEN |
| Welding Current | 180–280 A | 150–220 A | 120–200 A |
| Arc Voltage | 12–18 V | 18–24 V | 10–15 V |
| Travel Speed | 300–500 mm/min | 200–400 mm/min | 250–450 mm/min |
| Preheat Temperature | 200–300°C | 250–400°C | 150–250°C |
| Interpass Temperature | ≤350°C | ≤450°C | ≤300°C |
| Overlay Layer Thickness | 1.5–3.0 mm | 2.0–4.0 mm | 1.0–2.5 mm |
| Number of Passes | 2–4 | 3–5 | 2–3 |
4.3 Overlay Material Selection
| Roll Zone | Service Condition | Recommended Overlay Material | Target Hardness |
|---|---|---|---|
| Mold (H1–H2) | Extreme thermal cycling, direct steel contact | Cr₂O₃-based cermet / Ni-Cr-B-Si | 45–55 HRC |
| Backup Roll | Mechanical abrasion, moderate thermal load | Cr-C-Co (Stellite 6/21) | 38–48 HRC |
| Secondary Cooling | Low thermal load, corrosion from water | 309L + 310 stainless composite | 25–35 HRC |
| Final Stand Roll | High mechanical pressure, moderate heat | Tungsten carbide-cermet composite | 50–60 HRC |
4.4 Post-Weld Treatment
Following overlay deposition, the following post-weld procedures are mandatory:
- Controlled cooling: For copper-based rolls, cooling rate must be ≤50°C/min to prevent residual stress-induced cracking. For iron-based rolls, furnace cooling from 600°C at 50–100°C/hour is required.
- Stress relief annealing: 550–650°C for 2–4 hours (iron-based) or 300–350°C for 1–2 hours (copper-based) to relieve welding residual stresses below 100 MPa.
- Surface finishing: Precision grinding to achieve surface roughness Ra ≤ 1.6 μm and dimensional accuracy ±0.02 mm for mold rolls.
- Hardness verification: Indentation testing at 10-point intervals around the roll circumference with results within ±5 HRC of target.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
- GB/T 11345 — Non-destructive testing of welds by magnetic particles (for iron-based rolls)
- GB/T 11346 — Ultrasonic testing of welds
- GB/T 3323 — Radiographic testing of welds
- ASTM A240 — Standard specification for chromium and chromium-nickel stainless steel plate (for overlay material qualification)
- ASTM E10 / E18 — Rockwell / Brinell hardness testing methods
- ASME Section IX — Welding, brazing, and fusing qualifications
- NB/T 47013 — Non-destructive testing of pressure vessel welds (analogous acceptance criteria)
- ISO 3964 — Non-destructive testing — Magnetic particle testing
- ISO 9712 — Qualification and certification of NDT personnel
- GB/T 985 — Welding symbols on technical drawings
- ASTM B151 / B187 — Copper and copper alloys (substrate specification)
5.2 Acceptance Criteria
| Inspection Item | Acceptance Criterion | Standard Reference |
|---|---|---|
| Surface defects (cracks, porosity) | No cracks; porosity ≤ 0.5% of surface area, individual ≤ 2 mm | GB/T 11345, Level Ⅱ |
| Hardness uniformity | ±5 HRC deviation from target; no soft zones | ASTM E10 |
| Overlay thickness | Within ±0.3 mm of nominal; no undercut > 0.2 mm | WPS specification |
| Interfacial bond strength | ≥ 15 MPa (peel test); no delamination | ASTM E2539 |
| Residual stress | ≤ 100 MPa (X-ray diffraction) | ASTM E975 |
| Geometric accuracy | Diameter tolerance ±0.05 mm; runout ≤ 0.03 mm | ISO 1101 |
| Ultrasonic inspection | No indications exceeding Level Ⅱ equivalent | GB/T 11346 |
6. Common Risks and Control Measures
| Risk Category | Specific Failure Mode | Root Cause | Control Measure |
|---|---|---|---|
| Delamination | Overlay spalling during service | Excessive dilution; thermal mismatch; contamination | Strict dilution control (≤ 15%); surface preparation verification; staged deposition |
| Cracking | Transverse cracks in overlay or HAZ | High carbon content; inadequate preheat; rapid cooling | Preheat to specified temperature; low-carbon filler selection; controlled cooling |
| Soft zones | Localized hardness below specification | Excessive travel speed; overheating; wrong filler | Parameter monitoring; interpass temperature control; material traceability |
| Thermal distortion | Roll out-of-round after welding | Asymmetric heat input; inadequate support | Rotational welding (continuous roll rotation); balanced pass sequence; fixture design |
| Porosity | Gas inclusion in overlay | Moisture in filler; inadequate shielding; surface contamination | Filler baking; gas flow verification; pre-weld cleaning verification |
| Intermetallic embrittlement | Cu-Al or Cu-Fe intermetallic at interface | Excessive heat input on dissimilar metals | Low-heat-input TIG; thin multi-pass strategy; interlayer material |
6.1 Critical Control Points Summary
- Heat input management: For copper-based mold rolls, total heat input per pass must be limited to ≤ 1.5 kJ/mm to prevent substrate softening and grain coarsening. This requires careful balancing of current, voltage, and travel speed.
- Dilution ratio control: The base metal dilution in the first pass must be monitored and kept below 15% for hardfacing alloys. This is achieved through low-heat-input TIG with narrow travel speed and proper torch angle (75–85°).
- Interpass temperature monitoring: Infrared thermometry must be used to verify interpass temperature compliance. Exceeding interpass limits leads to grain growth and reduced hardness.
- Weld sequence planning: A systematic welding sequence (e.g., alternating opposite sides, spiral pattern) must be employed to minimize thermal distortion and residual stress accumulation.
7. Application Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
The TIG/MIG weld overlay route is the dominant technology for continuous casting roll repair. TIG (Gas Tungsten Arc Welding) is preferred for:
- Copper mold roll repair: TIG provides the low-heat-input, high-precision deposition required for copper substrates where excessive heat causes grain coarsening and loss of thermal conductivity.
- Cermet and carbide overlay: TIG enables controlled melting of high-melting-point cermet powders (WC-Co, Cr₃C₂-Ni) onto the roll surface with minimal dilution.
- Transition layer deposition: TIG is used to deposit compatible transition layers (e.g., Ni-Fe or Cu-Ni) between dissimilar substrate and final overlay to prevent intermetallic formation.
MIG (Gas Metal Arc Welding) is preferred for:
- Cast iron roll repair: MIG provides higher deposition rates (3–5 kg/h vs. 0.5–1.5 kg/h for TIG) suitable for thick overlay layers on iron-based backup rolls.
- Large-area coverage: For secondary cooling zone rolls requiring extensive overlay coverage, MIG's productivity advantage is significant.
- Multi-wire MIG: Advanced multi-wire MIG enables single-pass deposition of graded structures with controlled composition.
7.2 Hydraulic Explosive Bonding Route (Supplementary Application)
While not the primary method for roll surface repair, hydraulic bonding technology contributes to the continuous casting roll value chain in the following ways:
- Composite roll construction: Hydraulic bonding can be used to create composite roll blanks with a wear-resistant surface layer (e.g., Cr-C-Co alloy) bonded to a copper or steel backing core, eliminating the need for post-fabrication welding.
- Roll shell replacement: For severely worn rolls where the entire surface layer must be replaced, hydraulic bonding provides a metallurgical bond of a new alloy shell onto the prepared roll core without the thermal effects of welding.
- Repair of large-area damage: When thermal fatigue has caused extensive surface degradation over the entire roll length, hydraulic bonding of a replacement surface layer may be more economical than sequential weld overlay.
7.3 Explosion Welding Route (Specialized Application)
Explosion welding (explosive cladding) finds niche application in:
- Special alloy roll fabrication: Production of rolls with exotic surface materials (e.g., tungsten alloy, molybdenum alloy) that cannot be weld-clad due to extreme melting point differences.
- Prototype and R&D rolls: Development of experimental roll surface compositions for trial use in specific steel grades, where the flexibility of explosive cladding allows rapid composition changes.
- Thick overlay requirement: When overlay thickness exceeds 5 mm, explosion welding provides uniform thickness without the cumulative distortion issues of multi-pass welding.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification and Certification Impact
Mastering weld overlay repair for continuous casting rolls directly strengthens the company's qualification portfolio in several critical dimensions:
- WPS/PQR development: Each successful repair generates qualified Welding Procedure Specifications and Procedure Qualification Records covering dissimilar material welding, high-temperature alloy deposition, and rotational welding on cylindrical geometries.
- Welder certification: Technicians performing these repairs accumulate qualification experience across multiple filler metals, base materials, and welding positions (F-position on rotating rolls).
- NDT qualification: The diverse inspection requirements (MT, UT, RT, PT, hardness mapping) build comprehensive NDT competency aligned with ISO 9712 Level Ⅱ/Ⅲ requirements.
- Process know-how documentation: Systematic documentation of process parameters, defect prevention, and performance tracking creates intellectual property assets that differentiate the company in competitive bidding.
8.2 Product Delivery Enhancement
This capability enables the company to deliver:
- Turnkey roll restoration packages: From receipt of worn roll through inspection, preparation, overlay, post-weld treatment, finishing, and NDT verification—a complete value chain.
- Performance-guaranteed repairs: With documented process qualifications and statistical process control data, the company can offer guaranteed service life extensions (e.g., "minimum 12 months service life or replacement at no cost").
- On-site and off-site service flexibility: Mobile TIG/MIG equipment enables on-site repair of large rolls that cannot be transported, while shop-based services provide superior quality control for precision mold rolls.
- Accelerated turnaround: Typical repair cycle of 48–72 hours for standard rolls, compared to 4–8 weeks for new roll procurement, provides dramatic operational advantage to steel mill customers.
8.3 Customer Value Realization
"A single continuous casting line with 4 mold rolls, 12 backup rolls, and 20+ secondary cooling rolls represents a critical bottleneck in steel production. Each roll replacement requires 8–16 hours of production stoppage. Weld overlay repair capability transforms this from a capital expenditure problem into a manageable maintenance operation, delivering 60–80% cost reduction while maintaining or improving slab quality." — Typical customer value proposition
The quantifiable value delivered includes:
- Cost savings: 60–80% reduction in roll replacement costs per repair cycle.
- Downtime reduction: 70–90% reduction in production stoppage time associated with roll changes.
- Quality improvement: Restored surface roughness and hardness improve slab surface finish, reducing downstream finishing costs by 15–25%.
- Lifecycle extension: Multiple repair cycles (typically 3–5) extend total roll service life from 6–12 months to 3–5 years.
9. Continuous Improvement and Technology Roadmap
The "learning and experience" (学习心得) nature of this technical entry indicates an ongoing knowledge accumulation and process refinement program. Key improvement vectors include:
- Robotized welding integration: Transition from manual TIG/MIG to robotic systems for consistent parameter control, reduced operator fatigue, and improved deposit uniformity on long roll surfaces.
- Wire-arc additive manufacturing (WAAM): Application of WAAM technology for rapid, thick overlay deposition with reduced heat input compared to conventional MIG, enabling repair of severely worn rolls in fewer passes.
- Real-time monitoring: Integration of acoustic emission monitoring, infrared thermography, and wire-feed rate feedback for closed-loop process control during welding.
- Microstructure engineering: Development of nanostructured overlay materials (e.g., nanocrystalline WC-Co, high-entropy alloy coatings) for next-generation roll surface performance exceeding 200,000 thermal cycles.
- Digital twin integration: Development of predictive models linking welding parameters to overlay microstructure and service life, enabling optimized procedure selection based on specific steel grade and casting parameters.
10. Conclusion
Weld overlay repair technology for continuous casting rolls represents a high-value, technically demanding capability that sits at the intersection of welding metallurgy, thermal management, and surface engineering. Its successful execution requires deep understanding of dissimilar material welding, dilution control, residual stress management, and non-destructive verification. Within the company's technology portfolio, this capability underpins the TIG/MIG weld overlay route as the primary delivery mechanism, while complementing hydraulic bonding and explosion welding routes for specialized applications.
The systematic documentation and continuous refinement of this technology—captured in the learning experience framework—creates a compounding knowledge asset that strengthens WPS qualifications, reduces defect rates, improves first-time-right delivery rates, and ultimately delivers measurable economic value to steel industry customers. As the steel industry increasingly demands longer roll life, reduced downtime, and improved product quality, this capability becomes not merely a service offering but a strategic differentiator in the competitive landscape of metallurgical maintenance and surface engineering services.