Submerged Arc Weld Overlay Flux Development for Continuous Casting Roll Surfacing
1. Definition and Technical Principles
Submerged arc weld overlay (SAWO) is a thick-deposit, high-deposition-rate cladding process in which a submerged arc welding (SAW) consumable system—comprising a solid or flux-cored wire electrode and a sintered flux blanket—deposits a controlled alloy layer onto a base substrate. When applied to continuous casting rolls (CCRs), the objective is to restore worn roll surfaces or to apply a hardfacing alloy layer that resists the combined thermal, mechanical, and chemical degradation encountered during hot-metal contact in steelmaking and aluminum casting operations.
The sintered flux in this process serves multiple critical functions simultaneously: it melts to form a protective slag layer that shields the molten weld pool from atmospheric contamination; it acts as a chemical buffer that controls the oxygen and nitrogen pickup in the weld metal; it provides arc stability through controlled conductivity; it modifies the weld bead profile and penetration characteristics; and it influences the microstructure and hardness of the deposited overlay through dilution control and cooling rate management. The development of a purpose-designed sintered flux for cast roll SAWO is therefore not merely a consumable selection exercise but a materials engineering challenge that directly determines overlay performance in service.
Continuous casting rolls are typically manufactured from high-chromium white cast iron (e.g., ASTM A396 Type 1) or martensitic cast iron with chromium, molybdenum, and vanadium additions. These substrates present unique metallurgical challenges for weld overlay: high carbon and alloy content in the base metal promotes excessive dilution into the weld metal; the high hardness of the base (typically 500–650 HB) can cause cracking during welding due to thermal stresses; and the inherent brittleness of cast iron requires careful heat input management. A properly formulated sintered flux addresses these challenges by controlling dilution, moderating thermal gradients, and promoting a ductile, crack-resistant weld microstructure.
2. Category and Business Positioning
This technology entry falls under the company's weld overlay route, specifically within the submerged arc weld overlay (SAWO) sub-category. While the company's core welding technologies center on TIG (GTAW) and MIG (GMAW) weld overlay processes, the development of SAWO fluxes for cast roll applications represents a strategic expansion into the high-deposition-rate overlay segment. This positioning is significant because:
- Market demand: The global continuous casting industry requires regular roll refurbishment—typically every 3 to 12 months depending on steel grade and casting speed. SAWO offers deposition rates of 30–80 kg/h, far exceeding the 5–15 kg/h typical of TIG/MIG overlay, making it economically essential for thick overlay builds (6–12 mm).
- Technical complementarity: SAWO excels at building thick, uniform overlay layers on large cylindrical geometries, while TIG/MIG overlay provides superior control for thin transition layers and complex geometries. A combined approach—TIG transition layer followed by SAWO build-up—leverages the strengths of both processes.
- Qualification depth: Developing proprietary flux formulations positions the company as a full-solution provider rather than a process-only contractor, enhancing qualification credentials with steelmakers and foundries.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The development of a dedicated sintered flux for cast roll SAWO aims to achieve the following performance targets:
- Controlled dilution: Limit base metal dilution to the weld metal to ≤30% to preserve the overlay alloy's corrosion and wear resistance properties.
- Hardness uniformity: Achieve overlay hardness in the range of 55–65 HRC with ≤5 HRC variation across the deposited layer, ensuring consistent roll surface performance.
- Crack resistance: Eliminate transverse and longitudinal cracking in both the weld metal and heat-affected zone (HAZ) during and after welding.
- Low porosity: Achieve porosity levels below the acceptance threshold of GB/T 3323 or ISO 5817 Level B.
- Good machinability: Ensure the as-welded overlay can be ground and machined to the final roll surface finish (Ra ≤ 1.6 μm) without excessive tool wear.
3.2 Business Value
The proprietary flux development delivers measurable value across the company's business model:
- Cost reduction: By controlling dilution and deposition efficiency, the flux reduces the volume of expensive overlay alloy wire required per roll refurbishment, typically by 15–25%.
- Quality assurance: A purpose-designed flux eliminates the variability associated with generic commercial fluxes, resulting in more consistent overlay quality and fewer rework events.
- IP protection: Proprietary flux formulations constitute intellectual property that differentiates the company's offerings from competitors relying on off-the-shelf consumables.
- Process qualification: The flux development program inherently generates the welding procedure specifications (WPS) and procedure qualification records (PQR) required for customer audits and standard compliance.
4. Key Process and Implementation Points
4.1 Flux Composition Design
The sintered flux formulation for cast roll SAWO typically comprises the following component groups, each serving a specific metallurgical function:
| Flux Component Group | Typical Composition (wt%) | Primary Function |
|---|---|---|
| Manganese silicates (MnSiO₃) | 15–25 | Slag viscosity control, Mn alloying |
| Calcium fluoride (CaF₂) | 25–35 | Arc stability, desulfurization, slag fluidity |
| Silica (SiO₂) | 10–18 | Slag structure, Si alloying |
| Lime (CaO) | 8–15 | Alkalinity control, desulfurization |
| Alumina (Al₂O₃) | 5–10 | Slag viscosity, arc shielding |
| Iron oxide (Fe₂O₃) | 3–8 | Slag conductivity, Fe alloying |
| Carbon powder | 2–5 | Deoxidization, hardness enhancement |
| Alloying additions (Cr, Mo, V, Ni) | 5–12 | Overlay hardness and wear resistance |
The alkalinity ratio (AR = (CaO + CaF₂ + MgO) / (SiO₂ + Al₂O₃)) is a critical design parameter, typically maintained in the range of 1.5–2.5 for cast roll applications. A higher AR promotes better desulfurization and reduces the risk of hot cracking, while a lower AR improves slag fluidity and arc stability. The optimal AR is determined through systematic trial welding on representative cast iron substrates.
4.2 Flux Manufacturing Process
The sintered flux production follows a controlled sequence:
- Raw material preparation: Mineral and alloying raw materials are crushed, screened to particle size distribution of 0.5–2.0 mm, and chemically analyzed per GB/T 223 series standards.
- Batch blending: Raw materials are weighed according to the formulated recipe and blended in a rotary mixer for a minimum of 30 minutes to ensure homogeneity.
- Sintering: The blended powder is sintered at 900–1100°C in a controlled atmosphere furnace. The sintering temperature and dwell time (typically 2–4 hours) are critical to achieving the target slag melting range and particle integrity.
- Cooling and crushing: The sintered cake is cooled, crushed, and screened to the final particle size distribution (typically 0.5–1.5 mm for cast roll SAWO).
- Quality inspection: Each batch is tested for chemical composition, moisture content (≤0.5%), particle size distribution, slag melting range, and basicity.
4.3 Welding Process Parameters
The following parameters represent the typical SAWO conditions for continuous casting roll overlay, optimized with the developed flux:
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding current (I) | 300–500 A (DC, electrode positive) | Higher current for thicker deposits; DCEN may be used for deeper penetration |
| Welding voltage (V) | 28–36 V | Depends on wire diameter and flux characteristics |
| Wire feed speed (WFS) | 6–12 m/min | Adjusted for wire diameter (1.6–2.4 mm) |
| Travel speed (TS) | 200–400 mm/min | Lower speed for thicker single-pass deposits |
| Wire diameter | 1.6–2.4 mm | 2.4 mm preferred for high deposition rate |
| Flux coverage rate | ≥10 kg/h | Adequate flux supply ensures complete slag coverage |
| Preheat temperature | 250–400°C | Depends on base metal carbon equivalent and wall thickness |
| Interpass temperature | ≤300°C | Monitored with infrared pyrometer or thermocouple |
| Post-weld cooling rate | ≤200°C/h | Controlled furnace cool or insulated slow cool to prevent cracking |
| Overlay thickness per pass | 3–6 mm | Multiple passes for total overlay of 8–15 mm |
4.4 Multi-Pass Overlay Strategy
For continuous casting rolls requiring substantial overlay build-up (typically 8–15 mm total), a multi-pass strategy is employed:
- Transition layer (Pass 1): A dilution-control layer is deposited using a low-alloy wire (e.g., ER50-6 or a proprietary transition alloy) with the developed flux. This layer acts as a metallurgical buffer between the high-carbon cast iron base and the subsequent hardfacing layers, reducing the risk of cracking and controlling dilution into the final overlay.
- Build-up layers (Passes 2–N-1): Successive passes of the hardfacing alloy wire (e.g., high-chromium, high-carbon, or cobalt-based) are deposited using the developed flux. Each pass is overlapped by 50–70% of the previous pass width to ensure complete coverage and uniform composition.
- Final surface layer (Pass N): A final pass with a composition optimized for surface hardness and wear resistance is applied. This layer may use a slightly different flux formulation or wire composition to achieve the target surface properties.
5. Applicable Standards and Acceptance Criteria
5.1 Flux Qualification Standards
- GB/T 5293: Submerged arc welding fluxes—classification and designation (Chinese national standard for flux classification).
- GB/T 12468: Submerged arc welding fluxes for carbon and low-alloy steel—requirements and test methods.
- ISO 11134-1: Welding consumables—Submerged arc welding fluxes—Part 1: Classification and designation.
- ASTM A395: Standard Specification for Submerged Arc Welding Flux for Carbon and Low-Alloy Steel.
- GB/T 223.1–223.69: Chemical analysis methods for iron, steel, and alloys (series of standards for flux raw material and product analysis).
5.2 Weld Procedure Qualification Standards
- GB/T 985.1: Methods of welding procedure qualification for steels—Part 1: Qualification requirements for fusion-welding procedures.
- GB/T 19866: Welding procedure specification and qualification—General requirements.
- ISO 15614-1: Qualification procedures for welding of metallic materials—Part 1: Qualification procedures for fusion welding.
- ASME Section IX: Qualification Rules for Welding, Brazing, and Faying Procedures (when required by customer specification).
- NB/T 47014: Qualification rules for welding procedure specification for pressure vessels (applicable when rolls are used in pressure-containing applications).
5.3 Acceptance Criteria for Cast Roll Overlay
| Acceptance Parameter | Criteria | Test Method |
|---|---|---|
| Overlay hardness | 55–65 HRC (or per customer specification) | HBW or HRC per GB/T 231.1 / GB/T 230.1 |
| Dilution | ≤30% base metal dilution | Spectroscopic analysis per GB/T 223.66 |
| Porosity | No porosity > 1 mm; no clustered porosity | Visual + radiographic (GB/T 3323 Level II) |
| Cracking | No cracks in weld metal or HAZ | PT (GB/T 18851) + RT (GB/T 3323) |
| Undercut | ≤0.5 mm depth, ≤10% of weld length | Visual + profile gauge |
| Overlay thickness uniformity | ±0.5 mm variation across roll face | Ultrasonic thickness measurement (GB/T 7995) |
| Surface finish (post-grinding) | Ra ≤ 1.6 μm | Surface profilometer per GB/T 1031 |
| Impact toughness (transition layer) | ≥27 J at -20°C (if required) | Charpy V-notch per GB/T 229 |
6. Common Risks and Controls
6.1 Metallurgical Risks
- Cracking (hot and cold): The high carbon and alloy content of cast iron substrates creates a high susceptibility to cracking. Controls: Adequate preheating (250–400°C), controlled interpass temperature, low hydrogen flux formulation, and a ductile transition layer to accommodate thermal stresses.
- Excessive dilution: If dilution exceeds the design limit, the overlay alloy's wear and corrosion resistance properties degrade. Controls: Transition layer strategy, optimized flux chemistry to limit base metal pickup, and spectroscopic verification after each pass.
- Soft spots in overlay: Incomplete melting of the previous pass or flux contamination can create localized soft areas. Controls: Proper interpass cleaning, consistent flux application, and systematic hardness mapping after overlay.
6.2 Process Risks
- Flux moisture absorption: Sintered flux is hygroscopic, and moisture pickup leads to hydrogen-induced porosity and cracking. Controls: Flux storage in conditioned environment (RH ≤ 60%), pre-weld flux drying at 250–350°C for 2 hours, and first-flux-out/last-flux-in inventory management.
- Inconsistent slag coverage: Inadequate flux supply or excessive travel speed can expose the weld pool to atmospheric contamination. Controls: Automated flux feeder with flow rate monitoring, travel speed verification, and visual inspection of slag coverage.
- Geometric distortion: The high heat input of SAWO can cause roll barrel distortion, affecting dimensional accuracy. Controls: Symmetrical welding sequence (alternating sides), controlled heat input, and post-weld straightening if necessary.
6.3 Quality Control Measures
- First-pass verification: Chemical analysis and hardness testing of the first production pass to confirm flux performance before proceeding with full overlay.
- In-process monitoring: Real-time monitoring of welding parameters (current, voltage, WFS, TS) with automated data logging and deviation alarms.
- Post-overlay NDT: Mandatory PT and/or RT inspection of the complete overlay per the company's NDT quality plan, with documented acceptance records.
- Flux batch traceability: Each flux batch is assigned a unique identification number and linked to the weld procedure qualification records, enabling full traceability from raw material to finished overlay.
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay Integration
The developed SAWO flux is most effectively deployed in a hybrid process sequence with the company's TIG/MIG overlay capabilities:
- TIG transition layer + SAWO build-up: A TIG-welded transition layer (0.5–1.5 mm) provides superior dilution control on the initial pass, followed by SAWO passes with the developed flux for rapid build-up to the target thickness. This approach leverages TIG's precision for the critical first pass and SAWO's efficiency for subsequent passes.
- MIG overlay for repair + SAWO for refurbishment: Localized wear or damage on a casting roll can be repaired with MIG weld overlay, while full roll refurbishment employs the SAWO process with the developed flux for maximum deposition efficiency.
- Edge and end-face preparation: TIG welding is used to prepare the roll end faces and shoulder areas where SAWO equipment cannot reach, ensuring complete overlay coverage across the entire roll surface.
7.2 Hydraulic Explosive Bonding Complementarity
While the SAWO flux development primarily serves the weld overlay route, it contributes to the company's overall qualification portfolio in the following ways:
- Post-bonding repair and refurbishment: Rolls that have been refurbished via hydraulic explosive bonding may require localized weld repair or additional overlay in specific wear zones. The developed flux enables high-quality SAWO repair on previously bonded surfaces, extending roll service life.
- Material compatibility data: The metallurgical knowledge gained from flux development—particularly regarding dilution control on high-alloy substrates—directly informs the selection of bondable material combinations for hydraulic explosive bonding applications.
7.3 Explosion Welding Synergy
- Clad roll refurbishment cycle: For casting rolls that have reached end-of-life after multiple SAWO refurbishment cycles, explosion welding can be used to apply a new clad layer onto the roll barrel. The SAWO flux technology supports the transition layer application between the explosion-welded clad and the subsequent hardfacing overlay.
- Process qualification cross-reference: The WPS and PQR generated during SAWO flux development can be cross-referenced with explosion welding qualification records to demonstrate the company's comprehensive capability in multi-process cladding solutions for casting roll applications.
8. Qualification Building and Customer Value
8.1 Qualification Building
The flux development program generates a comprehensive set of qualification documentation that strengthens the company's market position:
- Flux type test reports: Independent laboratory testing of flux composition, slag properties, and welding performance per GB/T 12468 and ISO 11134-1.
- Welding procedure qualification records (PQR): Full PQR documentation including mechanical testing (tensile, hardness, impact), metallurgical examination (macro and micro), and NDT results, compliant with GB/T 985.1 and ISO 15614-1.
- Welding procedure specifications (WPS): Production-ready WPS documents specifying all essential variables, consumable details, and process parameters for each cast roll overlay application.
- Flux approval certificates: Documentation confirming flux suitability for specific base metals and overlay alloys, traceable to the company's internal quality management system (ISO 9001:2015 compliant).
8.2 Customer Value Delivery
The development of proprietary SAWO flux for cast roll overlay delivers quantifiable value to the company's customers in the steelmaking and aluminum casting industries:
- Extended roll service life: Optimized overlay properties extend roll life by 30–50% compared to generic flux solutions, reducing roll changeover frequency and associated production downtime.
- Reduced refurbishment cost: Higher deposition efficiency and lower dilution reduce material consumption per refurbishment cycle, typically achieving 15–25% cost savings.
- Consistent quality: Proprietary flux eliminates batch-to-batch variability, ensuring consistent overlay performance and reducing the risk of premature roll failure.
- Technical support: The company provides comprehensive technical support including flux selection guidance, WPS development, on-site process monitoring, and post-overlay quality verification.
- Integrated solutions: Customers benefit from a single-source solution combining flux supply, process qualification, and overlay execution, simplifying procurement and quality management.
9. Conclusion
The development of sintered flux for submerged arc weld overlay of continuous casting rolls represents a strategically significant capability within the company's technical portfolio. It bridges the gap between the company's established TIG/MIG weld overlay expertise and the high-deposition-rate requirements of heavy-duty roll refurbishment. The resulting proprietary flux, validated through rigorous qualification testing and process verification, enables the company to deliver superior overlay quality, reduced refurbishment costs, and extended roll service life to customers in the steelmaking and non-ferrous casting industries. The technical knowledge and qualification documentation generated through this program also reinforce the company's credibility and competitive position across all three cladding technology routes—weld overlay, hydraulic explosive bonding, and explosion welding—establishing a comprehensive, multi-process capability for metallurgical cladding solutions.