Weld Overlay Clad Backup Rollers for Continuous Casting — Development and Application at Pangang Steel
1. Definition and Technical Background
Continuous casting backup rollers are critical components in the strand solidification and shaping zone of a continuous caster. During operation, these rollers are subjected to extreme thermal cycling, mechanical contact stress from the molten steel shell, and abrasive erosion from oxide inclusions. Conventional solid alloy rollers suffer from limited service life due to rapid surface degradation, necessitating frequent replacement and downtime.
The Weld Overlay Clad Backup Roller technology addresses this challenge by applying a wear- and corrosion-resistant alloy overlay layer onto a cost-effective steel substrate roller body. The composite structure leverages the toughness and economic base material for structural integrity while providing a hardened, wear-resistant surface through the overlay cladding. This approach was developed and validated through a collaborative project with Pangang Steel (Panzhihua Iron and Steel Group Co., Ltd.), resulting in the successful deployment of clad rollers in production continuous casting lines.
2. Category and Business Positioning
This technology falls squarely within the TIG/MIG Weld Overlay route of the company's three core cladding technologies. It represents a high-value industrial application where weld overlay engineering directly translates into measurable customer savings through extended roller service life, reduced replacement frequency, and decreased caster downtime.
Within the company's business portfolio, this entry positions the organization as a qualified supplier of metallurgically sound, field-proven overlay products for the continuous casting segment of the steel industry — a market segment demanding stringent qualification, traceability, and performance guarantee.
3. Technical Purpose and Value
- Extended Service Life: The overlay layer provides superior resistance to thermal fatigue cracking, abrasive wear from oxide scale, and galling at the roller-strand contact interface, typically achieving 3–8 times the service life of unclad equivalent rollers.
- Reduced Total Cost of Ownership: Although the unit cost of a clad roller exceeds that of a plain carbon steel roller, the extended interval between replacements yields significant cumulative savings in material procurement, labor, and caster downtime.
- Improved Strand Quality: A harder, more dimensionally stable roller surface produces a more uniform shell thickness, reducing surface defects on the cast product and lowering downstream finishing costs.
- Process Flexibility: Unlike fully forged alloy rollers requiring specialized heat treatment, clad rollers allow selection of the optimal overlay composition independently of the substrate, enabling tailored solutions for different casting grades and conditions.
4. Key Process and Implementation Points
4.1 Substrate Roller Preparation
The base roller body is typically fabricated from medium-carbon steel (e.g., 45# steel, Q345B, or equivalent per GB/T 699 or GB/T 1591). Surface preparation is critical to ensuring metallurgical bond strength between the substrate and overlay:
- Machining of the cylindrical surface to achieve Ra ≤ 1.6 μm finish
- Removal of scale, oil, and contamination through grinding and degreasing
- Preheating to 200–300°C to reduce hydrogen pickup and minimize cracking susceptibility
- Dimensional verification of roller diameter, parallelism, and runout within specified tolerances
4.2 Overlay Welding Process Parameters
The overlay is typically applied using either Gas Tungsten Arc Welding (GTAW/TIG) for precision single-pass or multi-pass builds, or Gas Metal Arc Welding (GMAW/MIG) for higher deposition rates on larger diameter rollers. The following table summarizes representative parameters:
| Parameter | TIG (GTAW) Overlay | MIG (GMAW) Overlay |
|---|---|---|
| Shielding Gas | Argon (99.99%) or Ar/CO₂ (80/20) | Ar/CO₂ (80/20) or Ar/He (75/25) |
| Welding Current | 180–280 A | 220–350 A |
| Travel Speed | 60–120 mm/min | 150–300 mm/min |
| Wire Diameter | 2.4–3.2 mm (consumable tungsten) | 1.2–1.6 mm (solid wire) |
| Interpass Temperature | ≤ 250°C | ≤ 250°C |
| Preheat Temperature | 200–300°C | 200–300°C |
| Typical Layer Thickness | 0.5–1.5 mm per pass | 1.0–2.5 mm per pass |
| Total Overlay Build-Up | 3–6 mm (multi-pass) | 4–8 mm (multi-pass) |
4.3 Overlay Material Selection
The selection of overlay consumable is dictated by the specific service conditions of the continuous casting application:
| Overlay Material Type | Typical Composition | Key Properties | Application Zone |
|---|---|---|---|
| High-Cr Alloy (AISI 410/420 equivalent) | 11–16% Cr, 0.4–0.8% C | HRC 40–50, excellent thermal fatigue resistance | Backup roll surface |
| Stainless (AISI 310/309 equivalent) | 20–25% Cr, 2–3% Ni | HRC 25–35, oxidation resistance | Transition layer / high-temp zone |
| Hardfacing (Stellite 6/Co-Cr-W) | 55–65% Co, 25–30% Cr, 5–10% W | HRC 40–45, excellent abrasive resistance | High-wear contact zone |
| Maraging Steel (18Ni equivalent) | 18% Ni, 8% Co, 5% Mo | HRC 45–55 after aging, superior toughness | Heavy-duty backup rolls |
4.4 Post-Weld Treatment
- Controlled cooling (air or furnace cool) to minimize residual stress
- Stress-relief annealing at 550–650°C for 2–4 hours where required
- Machining of the overlay surface to final dimensional tolerances (typically H7/h6 fit for roller journals)
- Surface grinding to achieve Ra ≤ 0.4 μm for critical contact surfaces
- Final hardness verification (Rockwell C) across the overlay cross-section
4.5 Bond Strength and Integrity Verification
The metallurgical bond between substrate and overlay is the single most critical quality attribute. Verification methods include:
- Microscopic examination of the weld interface (no porosity, lack of fusion, or cracking)
- Shear test or bend test on coupon samples per ASTM A563 or equivalent qualification procedure
- Magnetic particle inspection (MT) of the entire overlay surface per ASTM E1444 or GB/T 2690
- Ultrasonic testing (UT) of the overlay-substrate interface per ASTM E2391 or GB/T 11345
5. Applicable Standards and Acceptance Criteria
| Standard / Code | Scope of Application |
|---|---|
| GB/T 13916 | Technical conditions for weld overlay steel materials |
| GB/T 2690 | Magnetic particle testing of welds |
| GB/T 11345 | Ultrasonic testing of welds |
| ASTM A563 | Standard specification for weld overlay cladding of steel |
| ASTM A240 | Standard specification for stainless steel plate/sheet (substrate reference) |
| ASTM E1444 | Standard practice for magnetic particle testing |
| ASTM E2391 | Standard practice for UT of weld overlay cladding |
| ASME Section IX | Welding qualification procedures (WPS/PQR qualification) |
| ISO 9001:2015 | Quality management system requirements |
| NACE MR0175 / ISO 15156 | Susceptibility of metals to sulfide stress cracking (where applicable) |
Acceptance Criteria Summary
- Overlay hardness: ≥ specified minimum (typically HRC 35–50 depending on material)
- Overlay thickness: ≥ 3.0 mm nominal, with minimum local thickness ≥ 2.5 mm
- Bond strength: No separation at the interface under specified test conditions
- NDT: Zero Level I indications per ASTM E1444 / GB/T 2690
- Dimensional tolerance: Roller diameter ±0.05 mm, runout ≤ 0.02 mm TIR
- Surface roughness: Ra ≤ 0.4 μm on the final machined overlay surface
6. Common Risks and Controls
| Risk | Cause | Control Measure |
|---|---|---|
| Hot cracking in overlay | High sulfur/phosphorus in consumable; inadequate preheat | Use low-S, low-P wire; maintain preheat ≥ 200°C; control interpass temp | Lack of fusion at interface | Inadequate heat input; surface contamination | Ensure proper surface cleaning; optimize current/travel speed; verify with UT | Residual stress cracking | Rapid cooling; mismatch in thermal expansion | Apply stress-relief heat treatment; use compatible transition layer | Hardness non-uniformity | Inconsistent wire feed; heat input variation | Automated welding systems with parameter monitoring; map hardness across surface |
| Roller dimensional distortion | Excessive total heat input; asymmetric welding sequence | Use symmetric multi-pass welding sequence; post-weld machining to true dimensions |
| Porosity in overlay | Shielding gas breakdown; moisture in flux/wire | Ensure gas flow rate ≥ 15 L/min; use dry consumables; maintain wind screens |
7. Application Across Company Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route for This Application)
The Pangang continuous casting clad roller project is executed entirely through the TIG/MIG weld overlay route. This route is selected because:
- The roller geometry (large-diameter cylinder) is well-suited to automated orbital or circumferential welding
- Weld overlay provides excellent metallurgical bonding without the thermal shock associated with explosive cladding methods
- Material flexibility allows selection of overlay composition tailored to specific casting conditions (slab, billet, or bloom caster)
- Post-weld machining capability ensures precise final dimensions
7.2 Hydraulic Explosive Bonding (HEB) — Complementary Role
While HEB is not the primary route for roller fabrication, it finds application in the manufacture of clad steel plates used for roller journal housings and structural support components. HEB provides a diffusion-bonded interface with zero interfacial voids, suitable for high-pressure containment components in the caster hydraulic system.
7.3 Explosion Welding (EW) — Complementary Role
Explosion welding is applicable for producing large-format clad plates (e.g., Ni-based or Co-based alloy on steel) that may serve as raw material for machining roller end caps or specialized roller segments requiring extreme corrosion resistance. The EW route offers superior interface cleanliness compared to weld overlay for flat-plate applications.
8. Qualification Building and Customer Value
8.1 Qualification and Certification Impact
The successful development and field validation of clad backup rollers at Pangang Steel establishes the following qualification credentials:
- WPS/PQR Qualification: Valid welding procedure specifications qualified per ASME Section IX and applicable GB standards, covering the full range of overlay materials and substrate combinations used
- Customer Approval: Field-proven performance data from a major steel producer (Pangang Steel) serves as a powerful reference for customer qualification in the continuous casting market
- NDT Capability: Demonstrated proficiency in MT and UT inspection of overlay welds at production scale
- Traceability System: Full material traceability from consumable certification through to final product release, meeting ISO 9001:2015 requirements
8.2 Quantifiable Customer Value
| Value Metric | Unclad Roller | Clad Overlay Roller | Improvement |
|---|---|---|---|
| Typical Service Life | 1–3 months | 6–24 months | 3–8× extension |
| Replacement Frequency | 4–12 per year | 1–2 per year | 75–90% reduction |
| Caster Downtime (replacement) | 8–16 hours per change | 2–4 hours per change | ~60% reduction |
| Strand Surface Quality | Higher defect rate | Reduced surface marks | Improved yield |
| Annual Cost per Roller Position | Baseline (100%) | 55–70% | 30–45% savings |
9. Lessons Learned and Continuous Improvement
The Pangang project yielded several critical technical insights that have been incorporated into the company's standard operating procedures:
- Transition Layer Importance: When overlaying high-alloy materials (e.g., 13Cr or Co-based) directly onto low-carbon steel substrates, a 309L or 309 stainless transition layer (1–2 passes) significantly reduces dilution-related cracking and ensures a compatible metallurgical gradient at the interface.
- Welding Sequence Optimization: A circumferential multi-pass approach with 120° overlap between passes produces more uniform hardness distribution than a single-direction spiral build-up, reducing the risk of localized soft spots.
- Field Monitoring: Installing thermocouples on the roller body during service enables tracking of thermal cycling amplitude, which correlates directly with overlay fatigue life and informs replacement scheduling.
- Consumable Certification: Maintaining mill test certificates (MTC) for all overlay wire consumables, with lot-specific traceability, is essential for customer acceptance and regulatory compliance.
- Post-Service Analysis: Retrieving worn clad rollers for metallographic examination provides valuable feedback on overlay degradation mechanisms, enabling iterative improvement of material selection and process parameters for subsequent orders.
10. Conclusion
The development and application of weld overlay clad backup rollers for Pangang Steel continuous casting lines represents a mature, field-validated application of the company's TIG/MIG weld overlay technology. The project demonstrates end-to-end capability from WPS qualification and consumable selection through to manufacturing, NDT verification, and field performance validation. This entry strengthens the company's qualification portfolio for the steel industry continuous casting segment and provides a replicable technical framework for extending clad roller solutions to other steel producers and continuous casting configurations globally.