Weld Overlay Technology for Hot Strip Mill Back-up Roll Restoration
1. Definition and Technical Principles
Weld overlay on back-up rolls (支承辊) of hot strip rolling mills (热连轧机) is a specialized surface engineering process that applies hardfacing or wear-resistant alloy layers onto the working surface and bearing shoulders of large-diameter cylindrical rolls. Back-up rolls in hot continuous strip mills (HSM) operate under extreme conditions: temperatures exceeding 800°C at the roll surface, sustained contact pressure of 200–400 MPa, and continuous rolling of carbon steel, stainless steel, or alloy strip at speeds up to 15 m/s. The primary failure modes include surface spalling, cracking, indentation, and bearing shoulder wear that leads to bearing seizure.
The fundamental principle of weld overlay restoration relies on depositing a metallurgically compatible, thermally stable, and wear-resistant alloy layer through arc melting. The deposited overlay creates a graded microstructure at the substrate-overlay interface, combining the toughness of the original cast steel roll body (typically 40CrNiMoA or equivalent) with the hardness and thermal fatigue resistance of the overlay alloy. Common overlay alloys include Cr-Ni based austenitic systems (e.g., AISI 309/310), Cr-Mo-V martensitic systems, and high-alloy Ni-based systems selected based on the severity of service conditions.
2. Category and Business Positioning
This capability falls squarely within the company's TIG/MIG Weld Overlay technology route, specifically in the sub-category of heavy industrial equipment restoration and surface hardening. Within the company's business portfolio, this application serves as a high-value qualification project that demonstrates:
- Large-diameter workpiece handling capability — Back-up rolls typically range from 800 mm to 1,200 mm in diameter with lengths exceeding 1,800 mm, requiring specialized rotational fixtures and multi-wire feeding systems.
- Thick-section weld overlay expertise — Overlay thicknesses of 3–8 mm are common, often requiring multiple passes with interpass temperature control.
- Critical equipment restoration credibility — Successful delivery positions the company as a trusted partner for steel mill OEMs and service providers.
3. Technical Purpose and Value
The primary technical objectives of weld overlay on HSM back-up rolls are:
- Surface hardening — Increasing surface hardness from the base material's 200–250 HB to 350–500 HB through overlay alloy selection and post-weld heat treatment.
- Thermal fatigue resistance — Mitigating the cyclic thermal stress (ΔT of 400–600°C per rolling cycle) that causes surface cracking and spalling.
- Bearing shoulder restoration — Rebuilding worn bearing seat dimensions to specification tolerance (typically ±0.02 mm) to ensure proper bearing fit and load distribution.
- Service life extension — Extending roll service life by 2–4 times compared to unoverlaid replacement rolls, delivering significant cost savings to the customer.
- Dimensional recovery — Restoring worn roll geometry to original specifications, eliminating the need for full roll replacement.
The economic value is substantial: a single HSM back-up roll replacement costs $50,000–$120,000, while overlay restoration costs $8,000–$25,000. For mills operating 250+ days per year, this translates to annual savings of $200,000–$500,000 per mill train.
4. Key Process and Implementation Points
4.1 Substrate Preparation
Proper substrate preparation is the single most critical factor in achieving sound weld overlay on back-up rolls. The following sequence must be followed:
- Complete cleaning — Removal of all scale, oxide, lubricant, and contamination through grinding (Grit 60–80) or shot blasting. The prepared surface must be free of defects visible at 5× magnification.
- Edge preparation — For overlay at bearing shoulders or end faces, a 45° chamfer with 2 mm leg length is machined to ensure adequate fusion and avoid undercut at the overlay boundary.
- Preheating — The entire roll must be preheated uniformly to 250–350°C using induction heating or gas torches. Temperature uniformity within ±25°C across the working surface is mandatory to prevent differential thermal distortion.
- Defect inspection — Pre-existing cracks, inclusions, or segregation in the roll body must be identified via ultrasonic testing (UT) or magnetic particle inspection (MT) before overlay application. Any crack longer than 2 mm requires repair by gouging and welding prior to overlay.
4.2 Weld Overlay Process Parameters
| Parameter | Specification | Rationale |
|---|---|---|
| Welding Process | SAW (Submerged Arc Welding) primary; MIG (GMAW) for finishing | SAW provides high deposition rate (8–15 kg/h) and deep penetration; MIG offers precision for thin final passes |
| Overlay Alloy (Typical) | AISI 309L / AISI 310 / Cr17Ni12Mo2 | Austenitic composition prevents thermal cracking; Mo addition enhances wear resistance |
| Wire Diameter | 2.4 mm – 3.2 mm (SAW); 1.2 mm – 1.6 mm (MIG) | Large diameter for high deposition; small diameter for final surface finish |
| Current (SAW) | 400 – 600 A | Ensures adequate heat input for fusion with preheated substrate |
| Voltage (SAW) | 28 – 34 V | Controls arc stability and bead profile |
| Travel Speed | 300 – 500 mm/min | Balances deposition rate with adequate cooling to avoid grain coarsening |
| Interpass Temperature | 250 – 350°C (maintained throughout) | Prevents cold cracking in martensitic substrates; maintains ductility for subsequent passes |
| Number of Passes | 3 – 6 passes (typical) | Multiple passes ensure complete fusion and uniform microstructure |
| Shielding Gas (MIG finishing) | 99.99% Ar or Ar/2% O₂ | Pure argon for austenitic alloys; trace O₂ for improved wetting |
| Flux (SAW) | Low-hydrogen basic flux (e.g., HJ431) | Controls hydrogen pickup; ensures low diffusion hydrogen content (<10 mL/100g) |
| Post-Weld Heat Treatment | 600°C × 2h (stress relief) or 1050°C × 1h + air cool (if specified) | Relieves residual stresses; optionally re-austenitizes for uniform microstructure |
4.3 Multi-Pass Overlay Strategy
The overlay is applied in a controlled multi-pass sequence:
- First pass (binding pass): Applied at reduced current (80% of nominal) to achieve full fusion with the substrate while limiting dilution to 15–25%. This pass creates the metallurgical bond.
- Intermediate passes: Applied at full parameters with full overlap (75% minimum) to build thickness. Each pass must be ground flush before the next to ensure uniform heat distribution.
- Final pass: Applied using MIG for precise bead control. The final pass determines surface quality and must achieve Ra ≤ 1.6 μm.
- Post-overlay machining: The overlay is machined to final dimensions with a minimum remaining thickness of 1.5 mm to ensure the hardened layer is not removed during subsequent roll regrinding in service.
4.4 Equipment and Fixturing
Back-up roll overlay requires specialized equipment:
- Rotational welding fixture: A turntable or roll holder capable of rotating the roll at 0.5–2 RPM during SAW application, with adjustable speed for varying roll diameters.
- Multi-wire SAW system: For high-productivity applications, a 3-wire or 5-wire SAW system deposits 3–5 beads simultaneously, reducing total welding time by 60–80%.
- Induction preheating system: A medium-frequency (1,000–5,000 Hz) induction heater capable of uniformly preheating rolls up to 1,200 mm diameter.
- Insulated ceramic backing: For end-face overlay, ceramic backing rings maintain heat input and prevent sagging of molten metal.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Applicability |
|---|---|
| GB/T 12469-2009 | Welding procedure qualification — General requirements for PQR/WPS development |
| GB/T 985.1-2008 | Weld joint preparation — Groove dimensions for overlay preparation |
| GB/T 3375-2011 | Welding terminology — Standard nomenclature for overlay welding |
| GB/T 19542-2004 | Steel — Hardfacing weld overlay — Classification and requirements |
| ASTM A396/A396M | Standard specification for cast steel rolls (substrate reference) |
| ASTM A276 | Stainless steel wire for overlay (309L, 310L wire qualification) |
| ASME Section IX | Welding procedure and performance qualification if pressure-retaining components are involved |
| ISO 13919-1 | Welding — Weld procedure qualification — General requirements |
| ISO 14555 | Welding — Welding procedure qualification for weld overlay |
| NACE MR0175/ISO 15156 | Applicable if overlay alloys must resist sulfide stress cracking in sour service environments |
5.2 Acceptance Criteria
- Visual inspection (VT): No undercut, porosity, cracks, or incomplete fusion visible on the overlay surface. Surface roughness Ra ≤ 1.6 μm after machining.
- Magnetic particle inspection (MT): 100% coverage of overlay surface. No indication of cracks, linear indications > 1 mm, or clusters of round indications > 3 mm in any 100 mm length.
- Ultrasonic testing (UT): Full coverage of overlay thickness. No lack of fusion or volumetric defects at the overlay-substrate interface. Sensitivity: 20 dB above reference reflector.
- Hardness verification: Vickers hardness (HV10) measured at 3 mm intervals along the overlay surface. Acceptance: 350–550 HV for Cr-Ni austenitic overlay; 400–600 HV for Cr-Mo-V martensitic overlay.
- Dilution control: Chemical analysis of the first 0.5 mm of overlay must show dilution ≤ 25% for austenitic overlays on ferritic substrates. Dilution is calculated from Cr and Ni content at the interface.
- Dimensional tolerance: Final overlay thickness within ±0.1 mm of specified thickness. Roll diameter tolerance ±0.05 mm. Runout ≤ 0.02 mm TIR.
- Impact testing (if required): Charpy V-notch impact at 25°C: minimum 27 J for austenitic overlay; minimum 47 J for martensitic overlay at -20°C.
6. Common Risks and Controls
| Risk | Cause | Control Measure |
|---|---|---|
| Hot cracking in overlay | Low melting point eutectics at grain boundaries; high sulfur/phosphorus in filler metal | Use ultra-low sulfur filler metals (S < 0.01%); maintain interpass temperature; avoid excessive heat input |
| Lack of fusion at interface | Inadequate preheating; insufficient current; surface contamination | Mandatory preheating to 250°C minimum; visual verification of fusion before each pass; strict cleaning protocol |
| Hydrogen-induced cracking | Diffusion hydrogen from flux or electrode coating; rapid cooling of martensitic substrate | Use low-hydrogen flux; bake flux at 300°C for 2h; maintain interpass temperature; post-weld bake at 200°C for 2h |
| Excessive dilution | Too high heat input; first pass too deep; wrong alloy selection | Reduce first-pass current by 20%; use overlay alloy with sufficient alloying content to compensate for dilution; perform dilution analysis after first pass |
| Residual stress cracking | High residual stress from thick multi-pass overlay; thermal gradient | Post-weld stress relief at 600°C × 2h; use alternating weld direction; control interpass temperature tightly |
| Roll distortion | Asymmetric heat input; inadequate rotational speed | Multi-wire simultaneous deposition; consistent rotation speed; symmetric weld sequence; post-weld straightness verification |
| Overlay spalling in service | Poor metallurgical bond; thermal expansion mismatch; insufficient overlay thickness | Verify interface quality by UT; select overlay with thermal expansion coefficient within 5% of substrate; ensure minimum 3 mm remaining thickness after machining |
| Inconsistent hardness | Variation in cooling rate; improper heat treatment | Controlled cooling rate (≤ 5°C/min for martensitic); verify hardness at multiple locations; post-weld heat treatment per WPS |
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Application)
This is the primary and most mature technology route for back-up roll overlay. The company's MIG-based overlay capability enables:
- Single-wire precision overlay for thin overlays (1–3 mm) on high-precision bearing shoulders where dimensional accuracy is critical.
- Multi-wire SAW overlay for full-circumference surface hardening (3–8 mm) on the roll working surface.
- Transition layer deposition when overlaying dissimilar materials (e.g., Ni-based on carbon steel substrate) using a graded approach: first pass with 309L transition, subsequent passes with 310 or Ni-base hardfacing.
- Repair welding of existing cracks or damage in the roll body before overlay application.
The company's qualification for this application involves developing and qualifying a WPS/PQR package per GB/T 12469 and ISO 13919-1, demonstrating weldability, mechanical properties, and service performance under simulated rolling conditions.
7.2 Hydraulic Explosive Bonding (Complementary Application)
While hydraulic explosive bonding is not directly applied to roll restoration, it serves a complementary role in the company's product ecosystem:
- Clad roll shell fabrication: Production of bimetallic roll shells where a wear-resistant surface layer (e.g., high-carbon steel or ceramic-composite) is bonded to a tough substrate (e.g., 40CrNiMoA) using hydraulic explosive bonding. These pre-clad shells can then receive a final weld overlay pass for dimensional finishing.
- Hybrid clad-overlay products: For applications requiring both a thick wear-resistant base layer (achieved by explosive bonding) and a thin precision overlay (achieved by TIG/MIG), the company offers a two-stage process combining both technologies.
- Technical synergy: Understanding of metallurgical bonding mechanisms from explosive bonding informs overlay alloy selection and interface engineering for weld overlay applications.
7.3 Explosion Welding (Strategic Extension)
Explosion welding contributes to the back-up roll application in the following ways:
- Large-scale clad plate production: For mills requiring replacement roll bodies rather than surface restoration, explosion welding produces full clad plates (e.g., 20 mm overlay on 100 mm substrate) that are then machined into roll blanks.
- Alloy selection validation: The company's explosion welding capability provides a platform for testing novel overlay alloy combinations (e.g., tungsten carbide composite on steel) that can then be adapted for weld overlay applications where explosive bonding is impractical.
- Customer education and qualification: Demonstrating explosion welding capability positions the company as a comprehensive surface engineering partner, increasing credibility for weld overlay contracts on critical mill equipment.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
This application is a cornerstone qualification for the company's heavy industrial equipment restoration business. Successful execution demonstrates:
- WPS qualification for thick-section overlay: The multi-pass overlay on thick substrates (>100 mm) validates the company's welding procedure for heavy industrial applications.
- NDT capability verification: Full UT and MT inspection of large-diameter cylindrical surfaces validates the company's NDT infrastructure and personnel qualifications.
- Heat treatment expertise: Post-weld stress relief of large mass components demonstrates capability in managing thermal gradients in heavy sections.
- Industry certification: Successful delivery to steel mill customers supports qualification for API 577 (welding of steel equipment) and ISO 3834 (quality requirements for fusion welding) certification.
8.2 Customer Value
- Reduced downtime: Overlay restoration can be performed off-site while a replacement roll is in service, minimizing mill downtime compared to full roll replacement and reconditioning.
- Cost savings: 70–85% cost reduction compared to new roll procurement, with equivalent or superior service life.
- Performance improvement: Properly selected overlay alloys can improve roll surface life by 2–4× compared to the original cast material, reducing changeover frequency and improving mill productivity.
- Technical partnership: The company provides metallurgical analysis, alloy selection consultation, and in-service monitoring support, positioning itself as a strategic partner rather than a transactional supplier.
8.3 Product Delivery Excellence
Delivery of back-up roll overlay services requires adherence to a rigorous quality management system:
- Pre-delivery: Customer consultation on service conditions, alloy selection, and specification development. NDE of incoming roll to identify pre-existing defects.
- During production: Real-time monitoring of welding parameters, interpass temperature, and rotational speed. In-process UT after each major pass. Hardness verification at defined intervals.
- Post-production: Final NDE (100% MT + UT), dimensional verification (CMM or laser scanner), hardness mapping, and comprehensive documentation package including WPS, PQR, NDE reports, hardness maps, and dimensional certificates.
- Post-delivery: In-service monitoring support, performance tracking, and periodic condition assessment to optimize overlay selection for future restoration cycles.
9. Conclusion
The application of weld overlay technology on hot strip mill back-up rolls represents a high-value, technically demanding capability that positions Cladding Technology Shanxi Co., Ltd. as a leading provider of surface engineering solutions for critical metallurgical equipment. By combining deep metallurgical expertise, advanced welding technology, comprehensive NDT capability, and rigorous quality management, the company delivers solutions that extend equipment life, reduce operational costs, and minimize production downtime for steel mill customers. This qualification serves as a platform for expanding into related heavy industrial applications including mill housing repair, gearbox surface hardening, and conveyor roll restoration.