Weld Overlay Process Innovation for Continuous Casting Rolls
1. Definition and Technical Principles
Continuous casting rolls (CC rolls) are critical rotating components in steel continuous casting machines, responsible for shaping, cooling, and solidifying molten steel as it passes through the mold and secondary cooling zones. These rolls endure extreme thermal cycling, mechanical loading, and corrosive contact with mold flux and slag. The "New Weld Overlay Process for Continuous Casting Rolls" refers to an advanced surface hardening and repair methodology that applies specialized alloy coatings to the working surface of CC rolls to restore or enhance their functional performance.
The fundamental principle relies on depositing a metallurgically compatible, high-performance alloy layer onto the roll substrate through arc-based welding processes. The overlay layer must achieve sufficient hardness (typically HRC 45–62 depending on zone), excellent thermal fatigue resistance, and strong metallurgical bonding with the base material (usually low-alloy steel or cast iron roll bodies). The process exploits differential thermal contraction between the overlay and substrate to create compressive residual stresses in the surface layer, thereby improving resistance to thermal cracking and spalling.
Key metallurgical principles include:
- Thermal cycling resistance: The overlay alloy must withstand repeated heating to 1000–1200°C and rapid water cooling without cracking or delamination.
- Metallurgical bonding: Dilution between the overlay and substrate must be controlled to maintain the required hardness and toughness of the functional layer.
- Microstructural stability: The deposited microstructure (martensitic, austenitic, or composite) must remain stable under operating conditions.
- Wear and corrosion resistance: The surface must resist abrasive wear from scale and corrosive attack from mold flux.
2. Category and Business Positioning
This process falls squarely within the TIG/MIG Weld Overlay technology route of the company's three principal manufacturing capabilities. Continuous casting roll overlay represents a high-value, technically demanding segment of the weld overlay market because:
- It requires precise control of dilution rates (typically 5–15% for hard-facing applications)
- The geometry of cylindrical rolls demands multi-axis welding capability and positional welding expertise
- Quality requirements are stringent due to the safety-critical nature of continuous casting operations
- The repair/rebuild cycle is frequent (every 3–12 months depending on steel grade and casting speed), creating recurring revenue opportunities
Business positioning within Cladding Technology Shanxi Co., Ltd. places this capability at the intersection of:
- Steel industry after-market services: Providing roll rebuild and repair as a cost-effective alternative to full roll replacement
- Customized overlay engineering: Tailoring alloy selection and process parameters to specific steel grades and casting configurations
- Technical consulting: Offering WPS qualification, process validation, and on-site technical support to steel producers
3. Technical Purpose and Value
The primary technical purpose of the new CC roll overlay process is to extend roll service life by 2–5 times compared to conventional overlay methods while maintaining or improving surface quality of the cast steel product. The value proposition encompasses:
3.1 Operational Value
- Reduced downtime: Faster overlay cycles enable shorter roll changeover times, increasing steel production throughput
- Improved cast quality: Superior surface finish of the overlay layer translates to better slab/sheet surface quality, reducing downstream finishing costs
- Extended service life: Enhanced thermal fatigue resistance delays the need for roll replacement
- Cost reduction: Rebuild costs are typically 30–50% of new roll procurement costs
3.2 Technical Value
- Process innovation: The "new process" designation indicates improvements in one or more of the following: wire/feedstock selection, preheat strategy, interpass temperature control, welding sequence optimization, or post-weld treatment
- Knowledge accumulation: The "learning reflection" (学习心得) format indicates systematic knowledge capture and dissemination, building organizational capability
- Standardization: Process learnings are translated into qualified WPS/PQR documents that can be replicated across multiple job sites
4. Key Process and Implementation Points
4.1 Substrate Preparation
Proper substrate preparation is the single most critical factor in overlay success. The process includes:
- Roll grinding: Remove old overlay material and grinding down to sound base metal. Typical removal depth: 15–30 mm depending on roll diameter and previous overlay thickness.
- Surface cleaning: Remove all scale, oxide, grease, and contaminants. Methods include shot blasting, grinding, and solvent cleaning.
- Heat treatment (if required): Stress relief of the roll body to reduce residual stresses from prior service or machining. Typical: 550–650°C for 2–4 hours in a controlled atmosphere furnace.
- Preheat: Uniform preheating of the entire roll to prevent thermal cracking in the HAZ. Temperature depends on substrate carbon equivalent and overlay alloy.
4.2 Welding Parameters and Process Variables
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding Process | GMAW (MIG) / GTAW (TIG) | GMAW for buildup; GTAW for precise transition and finish layers |
| Shielding Gas | Argon (TIG) / Ar+CO₂ or Ar+O₂ (MIG) | Argon: 100% for TIG; 90/10 or 95/5 for MIG |
| Wire Diameter | 1.2–2.4 mm (MIG); 2.0–3.2 mm (TIG filler rod) | Depends on layer thickness and roll diameter |
| Deposition Rate | 3–8 kg/h (MIG); 0.5–2 kg/h (TIG) | MIG preferred for high-efficiency buildup layers |
| Interpass Temperature | 100–250°C | Critical for controlling HAZ hardness and preventing cracking |
| Preheat Temperature | 200–400°C | Based on CE of substrate; higher for higher CE materials |
| Overlay Thickness | 8–20 mm (total) | Depends on roll diameter, steel grade, and expected service life |
| Welding Sequence | Multi-pass, circumferential + longitudinal | Optimized to manage thermal distortion and residual stress |
| Travel Speed | 50–150 mm/min (MIG); 20–60 mm/min (TIG) | Adjusted for bead width and penetration control |
| Heat Input | 0.5–2.5 kJ/mm | Lower heat input for hard-facing layers to maintain hardness |
4.3 Alloy Selection Strategy
| Application Zone | Recommended Alloy Type | Typical Composition | Hardness (as-welded) |
|---|---|---|---|
| Transition Layer | Low-alloy austenitic | Cr 8–12%, Ni 4–6% | HRC 25–35 |
| Functional Overlay (hot zone) | High-Cr martensitic / high-alloy austenitic | Cr 18–25%, C 3–6% | HRC 50–62 |
| Functional Overlay (cold zone) | Medium-Cr martensitic | Cr 12–18%, C 2–4% | HRC 45–55 |
| Transition Layer (high CE substrate) | High-Ni austenitic | Cr 10–15%, Ni 20–30% | HRC 20–30 |
4.4 Key Implementation Steps
- Roll assessment and sizing: Measure roll diameter, check for cracks, deformation, and eccentricity. Determine required overlay thickness based on minimum operating diameter and target service life.
- Substrate characterization: Identify base material grade, measure carbon equivalent (CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15), and determine preheat requirements.
- WPS selection/qualification: Select or qualify a Welding Procedure Specification appropriate for the substrate-overlay combination. Ensure PQR demonstrates required hardness, dilution, and bond strength.
- Preheat and mounting: Mount roll on welding fixture/turntable. Apply uniform preheat using induction heating or flame heating. Verify preheat temperature at multiple locations around the circumference.
- Transition layer deposition: Apply 1–2 passes of low-dilution transition alloy to ensure metallurgical compatibility between substrate and functional overlay. Maintain interpass temperature strictly.
- Functional overlay deposition: Build up the functional hard-facing layer in multiple passes. Use optimized welding sequence to minimize distortion. Grind between layers if required to remove porosity or undercut.
- Post-weld heat treatment (PWHT): If required by the alloy system, perform tempering to reduce hardness to target range and relieve residual stresses. Typical: 500–650°C for 2–4 hours.
- Post-weld grinding: Grind the overlay surface to specified finish (typically Ra 0.4–1.6 μm) and true the roll to dimensional tolerances (runout ≤ 0.02 mm).
- Non-destructive testing: Perform visual inspection, magnetic particle testing (MT) or dye penetrant testing (PT), and ultrasonic testing (UT) as required by the applicable standard.
- Hardness verification: Measure hardness at multiple locations across the overlay thickness to verify gradient and target hardness.
4.5 Process Innovations Highlighted in the New Method
Based on the "new process" designation and the learning-reflection format, the key innovations likely include one or more of the following:
- Modified welding sequence: Optimized circumferential and longitudinal pass sequencing to reduce thermal distortion and residual stress buildup, particularly for large-diameter rolls (>600 mm).
- Enhanced preheat strategy: Use of gradient preheating or induction heating with controlled ramp rates to minimize HAZ cracking risk while improving productivity.
- Multi-alloy layer design: Implementation of a 3-layer or 4-layer scheme (transition + intermediate + functional + finish) to achieve optimal dilution control and hardness profile.
- Wire/feedstock optimization: Use of flux-cored wire (FCAW) or specialized solid wire with improved deoxidation for reduced porosity in thick overlay layers.
- Automated/robotic welding integration: Application of robotic MIG welding with CMT (Cold Metal Transfer) or pulsed GMAW for improved bead profile, reduced dilution, and higher consistency.
- In-situ monitoring: Implementation of real-time interpass temperature monitoring and adjustment to ensure compliance with process windows.
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure Standards
- GB/T 985.1: Gas shielded arc welding procedure qualification and testing
- GB/T 985.2: Submerged arc welding procedure qualification
- GB/T 15058: Welding procedure specification and qualification for welded joints
- ASME Section IX: Qualification of welding procedures, welders, and welding operators
- EN ISO 15614-1: Qualification testing of welding procedures for metallic materials — Arc welding
- EN ISO 15614-7: Qualification testing for welding procedures — Solid metal arc welding
5.2 Weld Overlay Specific Standards
- GB/T 11365: Welding consumables for hard-facing (overlay welding)
- EN ISO 18275: Welding — General guidance on welding of stainless steels
- ASTM A388: Standard specification for cast steel for wear-resisting applications
- ASTM A532: Standard specification for cast steel for wear-resisting applications
- ISO 14176: Metallic materials — Welding consumables — Solid wire for gas shielded arc welding
5.3 Continuous Casting Roll Standards
- GB/T 24596: Continuous casting rolls — Technical requirements and testing
- EN 10240: Cast steel for wear-resisting applications — Technical delivery conditions
- ASTM A396: Standard specification for cast steel for wear-resisting applications
- ISO 11306: Steel for wear-resisting applications — Technical delivery conditions
5.4 NDT Standards
- GB/T 26951: Non-destructive testing of welds — Magnetic particle testing
- GB/T 1805: Non-destructive testing of welds — Dye penetrant testing
- GB/T 11345: Non-destructive testing of welds — Ultrasonic testing
- ASME Section V: Nondestructive Examination
- EN ISO 17637: Non-destructive testing of welds — Ultrasonic testing — General rules
- EN ISO 9934: Non-destructive testing of welds — Magnetic particle testing
5.5 Acceptance Criteria
| Inspection Item | Acceptance Criteria | Reference Standard |
|---|---|---|
| Surface hardness | Within specified range (e.g., HRC 50±5 for hot zone overlay) | GB/T 24596 / Customer specification |
| Dilution rate | ≤ 15% for transition layer; ≤ 10% for functional layer | WPS/PQR qualification |
| Weld appearance | No undercut, overlap, porosity, or excessive spatter | EN ISO 5817 Grade B/C |
| Crack detection (MT/PT) | No cracks, linear indications ≤ 1 mm length permitted in overlay | EN ISO 9934 / GB/T 26951 |
| Internal defects (UT) | No indications exceeding 20% of reference block amplitude | EN ISO 17637 / GB/T 11345 |
| Roll runout | ≤ 0.02 mm TIR (Total Indicated Runout) | GB/T 24596 |
| Surface finish | Ra ≤ 1.6 μm (hot zone); Ra ≤ 0.8 μm (cold zone) | GB/T 24596 / Customer specification |
| Overlay thickness | Within ±1 mm of specified nominal thickness | WPS / Customer drawing |
| Bond strength | No delamination under specified peel or shear test | PQR qualification test |
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Hydrogen-induced cracking (HIC) | High CE substrate, inadequate preheat, hydrogen in filler metal | Preheat to 200–400°C; use low-hydrogen wire; post-weld bake at 200°C for 1–2 h |
| Hot cracking in overlay | High sulfur/phosphorus, excessive heat input, unfavorable microstructure | Use low-S/P consumables; control heat input; optimize alloy composition |
| HAZ cracking in substrate | High CE, rapid cooling, high拘束度 (restraint) | Preheat; interpass temperature control; use transition layer with high ductility |
| Excessive dilution | High heat input, large bead size, inappropriate wire geometry | Reduce heat input; use CMT or pulsed welding; optimize wire feed angle; apply transition layer |
| Hardness non-uniformity | Inconsistent welding parameters, interpass temperature variation | Implement automated welding; monitor interpass temperature; perform hardness survey at multiple locations |
| Delamination/spalling in service | Poor metallurgical bond, high residual tensile stress, thermal fatigue | Optimize transition layer; perform PWHT; use multi-layer design with stress-relieving intermediate layer |
6.2 Process Risks
- Thermal distortion: Large-diameter rolls are susceptible to eccentricity development during multi-pass overlay. Control through symmetric welding sequence, controlled preheat, and post-weld truing.
- Porosity: Gas shielding contamination or wire moisture can cause porosity. Control through proper gas flow rates, wire storage in drying ovens, and clean welding environment.
- Incomplete fusion: Particularly at the transition layer-substrate interface. Control through adequate preheat, proper root pass technique, and visual/MT inspection of critical joints.
- Productivity bottlenecks: Manual welding of large rolls is time-consuming. Control through automation, multi-station welding setups, and parallel processing of multiple rolls.
6.3 Quality Assurance Risks
- WPS qualification gap: Ensuring that the new process is properly qualified under applicable standards before production use. The "learning reflection" format suggests this qualification is actively being documented and formalized.
- Operator skill variability: Manual welding quality depends heavily on operator skill. Mitigation through operator certification (GB/T 985.1), standardized work instructions, and automated welding where feasible.
- Traceability: Maintaining complete records of material heat numbers, welding parameters, NDT results, and hardness measurements for each roll rebuild. Essential for customer qualification and warranty purposes.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route)
The CC roll overlay process is the core application within the TIG/MIG weld overlay technology route. This route is characterized by:
- Process flexibility: Ability to weld in all positions on cylindrical geometries, accommodating various roll diameters (300–1200 mm) and lengths (1000–4000 mm).
- Material versatility: Capability to apply a wide range of overlay alloys (martensitic, austenitic, high-alloy, composite) to different substrate materials (low-alloy steel, cast iron, stainless steel).
- Scalability: From small-diameter rolls for thin-gauge casting lines to large-diameter rolls for heavy slab casting.
- Repair and rebuild: Both in-situ repair (on the casting machine) and off-site rebuild (in the workshop) are supported.
The "new process" directly strengthens this technology route by:
- Expanding the qualified WPS database for CC roll applications
- Demonstrating capability in a technically demanding segment that requires high precision and metallurgical understanding
- Building a portfolio of successful rebuild projects that serve as reference cases for new customer qualification
7.2 Hydraulic Explosive Bonding (Secondary Route — Complementary Role)
While hydraulic explosive bonding (HEB) is not directly applied to CC roll overlay, the metallurgical knowledge and qualification framework developed through the new overlay process contribute to HEB capability in the following ways:
- Interface metallurgy understanding: The study of overlay-substrate bonding mechanisms informs the understanding of HEB interfaces, particularly regarding wave formation, intermetallic compound avoidance, and bond strength characterization.
- NDT expertise transfer: The NDT techniques developed for overlay inspection (MT, PT, UT) are directly applicable to HEB joint quality assessment.
- WPS qualification methodology: The systematic approach to procedure qualification, testing, and documentation developed for overlay welding is transferable to HEB process qualification under EN 1982 or equivalent standards.
- Customer relationship building: Steel producers who engage Cladding Technology Shanxi Co., Ltd. for CC roll overlay may also require clad plate for other applications (e.g., blast furnace linings, hot metal tanks), creating cross-sell opportunities for HEB products.
7.3 Explosion Welding (Secondary Route — Complementary Role)
Explosion welding (EW) similarly benefits from the knowledge and infrastructure developed through CC roll overlay work:
- Material compatibility knowledge: Understanding of which alloy combinations produce sound metallurgical bonds in welding directly informs explosion welding flyer/base material selection.
- Post-bond processing: Experience with post-weld heat treatment, grinding, and dimensional finishing of overlay surfaces is directly applicable to post-explosion welding processing of clad plates.
- Quality management systems: The rigorous quality management practices developed for overlay work (traceability, documentation, NDT protocols) are transferable to EW production.
- Integrated cladding solutions: For complex components requiring both welded and explosion-welded cladding (e.g., a steel producer needing clad plate for ladle linings and welded overlay for casting rolls), the company can offer a unified technical solution.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
- WPS/PQR database expansion: Each successful CC roll overlay project generates qualified welding procedure records that can be reused for similar applications, accelerating future project execution.
- Operator certification: The process requires certified welders (per GB/T 985.1 or ASME Section IX), building a certified workforce that can be deployed across all overlay projects.
- ISO 3834 compliance: The systematic approach to procedure qualification, NDT, and documentation supports ISO 3834 certification for welding quality management systems.
- Industry-specific certifications: Successful CC roll overlay projects contribute to certifications required by steel industry customers (e.g., POSCO, Baowu, Shagang, etc.).
8.2 Product Delivery
- Shorter lead times: The "new process" implies improved productivity, enabling faster roll rebuild cycles and shorter delivery times.
- Higher first-pass yield: Process improvements reduce rework and rejection rates, improving on-time delivery performance.
- Scalable capacity: Standardized procedures and trained operators enable capacity expansion without proportional increases in technical staff.
- Consistent quality: Automated and semi-automated welding reduces variability, ensuring consistent product quality across multiple rebuilds.
8.3 Customer Value
- Cost savings: Roll rebuild costs are 30–50% of new roll costs, providing significant savings to steel producers.
- Reduced downtime: Faster rebuild cycles and extended roll life reduce unplanned downtime at continuous casting lines.
- Improved product quality: Superior overlay surface quality translates to better cast steel surface quality, reducing finishing costs and improving downstream product value.
- Technical partnership: The company's deep process knowledge positions it as a technical partner rather than a simple service provider, enabling collaborative development of new overlay solutions tailored to specific steel grades and casting configurations.
- Risk mitigation: Rigorous qualification, NDT, and documentation provide customers with confidence in overlay integrity and service life, reducing warranty claims and operational risk.
9. Conclusion and Recommendations
The "New Weld Overlay Process for Continuous Casting Rolls" represents a significant technical advancement that strengthens Cladding Technology Shanxi Co., Ltd.'s position in the steel industry after-market segment. The systematic documentation of process learnings (学习心得) demonstrates a commitment to knowledge management and continuous improvement that is essential for maintaining competitive advantage in technically demanding weld overlay applications.
Key recommendations for further development include:
- Formal WPS qualification: Ensure the new process is fully qualified under GB/T 985.1, EN ISO 15614-7, or ASME Section IX, with comprehensive PQR testing including hardness, dilution, bond strength, and NDT.
- Automation integration: Invest in robotic MIG welding with CMT or pulsed GMAW for high-volume CC roll rebuild applications to improve consistency, productivity, and reduce operator dependency.
- Multi-layer alloy optimization: Conduct systematic studies on 3-layer and 4-layer overlay designs to optimize the hardness gradient, dilution profile, and thermal fatigue resistance for specific steel grades (e.g., stainless steel, HSLA, tool steel).
- In-service performance tracking: Establish a database correlating overlay process parameters with in-service roll life to enable data-driven process optimization and predictive maintenance recommendations for customers.
- Cross-route technology transfer: Actively leverage metallurgical and NDT knowledge from CC roll overlay to strengthen HEB and EW capabilities, creating integrated cladding solutions that address the full range of customer needs.
- Customer-facing technical publications: Develop white papers and technical case studies from the new process learnings to enhance the company's technical credibility and support business development activities.
By continuing to invest in process innovation, qualification development, and knowledge management, Cladding Technology Shanxi Co., Ltd. can establish itself as a leading provider of advanced weld overlay solutions for the continuous casting industry, delivering measurable value to customers through extended equipment life, improved product quality, and reduced operational costs.