Weld Overlay Repair of Hot Rolled Coil Coiler Pick-Up Rollers
1. Definition and Technical Principles
Hot rolled coil coiler pick-up rollers (also known as pinch rolls or grip rollers) are critical mechanical components in the finishing section of hot strip rolling mills. These rollers are responsible for gripping, transferring, and guiding the hot steel strip—typically at temperatures ranging from 750°C to 900°C—into the coiling head to form the final hot-rolled coil product. During continuous operation, pick-up rollers are subjected to an extreme combination of thermal cycling, mechanical impact loading, abrasive wear from the rough oxide scale on the strip surface, and corrosive attack from mill scale and water mist cooling systems.
Weld overlay repair of pick-up rollers is a specialized surface engineering technology that involves the removal of the damaged or worn surface layer through machining or grinding, followed by the deposition of one or more layers of wear-resistant and heat-resistant alloy weld metal through arc welding processes. The fundamental principle relies on building up a functional surface layer with superior mechanical properties—such as high hardness, thermal shock resistance, and abrasion resistance—while maintaining the structural integrity of the roller substrate.
The repair process is governed by metallurgical compatibility between the substrate (typically a forged carbon steel or low-alloy steel such as 45# steel, 50CrMo, or 42CrMo) and the overlay weld metal. The thermal gradient created during welding must be carefully controlled to prevent cracking, excessive distortion, and loss of dimensional accuracy in the cylindrical roller geometry.
2. Category and Business Positioning
This technology entry falls squarely within the TIG/MIG Weld Overlay technology route of Cladding Technology Shanxi Co., Ltd. It represents a high-value-added service in the industrial equipment repair and maintenance segment, specifically targeting the steelmaking and metal rolling industry. The business positioning is as follows:
- Industry Segment: Hot strip rolling mill equipment maintenance and repair services
- Service Type: Critical component restoration—extending service life of high-cost rollers without full replacement
- Value Proposition: Reducing unplanned downtime in hot rolling mills by 60-80% compared to roller replacement cycles, while achieving 40-60% cost savings versus procuring new forged rollers
- Technical Complexity Level: Medium-high—requires expertise in high-temperature alloy welding, thermal management of large-diameter cylindrical components, and post-repair dimensional verification
This entry, framed as a "learning reflection" (学习心得), indicates a knowledge transfer and qualification-building activity within the company's technical team. It represents the codification of practical experience into repeatable, standardized procedures—a critical step in building the company's WPS (Welding Procedure Specification) library and technician certification program.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The weld overlay repair of coiler pick-up rollers addresses four fundamental degradation mechanisms:
- Surface Abrasion: The rough mill scale on hot strip (Fe₂O₃, Fe₃O₄, FeO) abrades the roller surface at high sliding speeds (up to 20 m/s), creating grooves and reducing grip surface area
- Thermal Fatigue Cracking: Repeated heating and cooling cycles generate thermal stresses that cause surface cracking and spalling, particularly in the contact zone
- Indentation and Plastic Deformation: High contact pressure between the roller and the soft hot strip causes localized yielding and permanent indentation
- Oxidative Corrosion: Exposure to water mist cooling and oxidizing atmosphere accelerates surface degradation
3.2 Economic and Operational Value
| Parameter | New Roller | Weld Overlay Repair | Savings |
|---|---|---|---|
| Unit Cost (approx.) | RMB 80,000–150,000 | RMB 15,000–35,000 | 70–85% |
| Lead Time | 8–16 weeks | 3–7 days | 90%+ |
| Service Life Extension | Baseline | 80–95% of new roller life | — |
| Mill Downtime Avoided | — | 1–2 weeks per repair event | RMB 500,000–2,000,000 |
3.3 Contribution to Qualification Building
The documented learning experience from this repair project contributes directly to the company's qualification infrastructure in three ways:
- WPS Development: Each successful repair generates validated welding parameters that feed into the company's standardized WPS library for hot-work repair applications
- Personnel Certification: Technicians who participate in these repairs accumulate documented hours toward AWS D10.12 or ISO 9606-1 qualification renewal
- Customer Trust: Documented case studies with quantifiable results serve as evidence in tender submissions for steel mill maintenance contracts
4. Key Process and Implementation Points
4.1 Substrate Characterization and Assessment
Before initiating any repair, a comprehensive assessment of the pick-up roller is mandatory. This includes:
- Material Identification: Spectrometric analysis (OES) to confirm base metal composition. Common substrates include 45# carbon steel, 50CrMo, 42CrMo, or custom forge grades with carbon equivalent (CE) of 0.35–0.55
- Hardness Mapping: Surface and subsurface hardness profiling (HV30) to determine the depth of damage and identify any existing hardening layers or previous weld overlays
- Crack Detection: Magnetic particle testing (MT) or liquid penetrant testing (PT) per ASTM E1444 or ASTM E709 to identify subsurface cracks that must be removed or addressed
- Dimensional Survey: Laser scanning or coordinate measurement to establish out-of-roundness, taper, and step height deviations from the original drawing
4.2 Surface Preparation
Surface preparation is the single most critical factor in overlay weld quality. The following sequence is recommended:
- Machining/Grinding: Remove all damaged material to a minimum depth of 2 mm below the deepest defect. For severe thermal fatigue cracking, removal depth may need to be 5–8 mm. Surface roughness should be achieved to Ra ≤ 12.5 μm (Rz ≤ 63 μm)
- Crack Removal: Any detected cracks must be fully removed by grinding to a V-groove with a 60° included angle, followed by a second MT inspection to confirm crack-free condition
- Cleaning: Remove all oil, scale, and contaminants using acetone or a dedicated degreaser. The welding zone must be free of moisture and hydrocarbons
- Preheating: Apply uniform preheat to the entire roller body using induction heating or gas flame. The preheat temperature must be controlled and verified with thermocouples at multiple locations
4.3 Weld Overlay Process Parameters
The following table summarizes recommended welding parameters for pick-up roller overlay repair:
| Parameter | Transition Layer (Pass 1) | Wear Layer (Pass 2–4) |
|---|---|---|
| Welding Process | TIG (GTAW) or Pulse MIG (GMAW) | Submerged Arc (SAW) or Pulse MIG (GMAW) |
| Weld Wire / Electrode | ER80S-D2 (AWS) / SAlMn13 (GB) | ER60S-D3 (AWS) / SAlSiMnCu (GB) |
| Wire Diameter | Φ1.6–2.0 mm | Φ2.4–3.2 mm (SAW) / Φ1.6 mm (MIG) |
| Shielding Gas | Ar + 2% O₂ or Ar + 5% CO₂ | Ar + 2% O₂ (MIG) / Flux (SAW) |
| Current | 120–180 A | 350–500 A (SAW) / 180–260 A (MIG) |
| Voltage | 18–22 V | 28–35 V (SAW) / 20–26 V (MIG) |
| Travel Speed | 4–8 cm/min | 12–20 cm/min (SAW) / 8–14 cm/min (MIG) |
| Preheat Temperature | 200–350°C (for CE > 0.40 substrates) | |
| Interpass Temperature | ≤ 350°C (monitored with IR thermometer) | |
| Weld Bead Width | 10–15 mm | 15–25 mm |
| Weld Bead Height | 2–3 mm | 2–4 mm per pass |
| Overlay Thickness | 3–5 mm total | 5–12 mm total |
| Post-Weld Heat Treatment | PWHT at 550–650°C for 2–4 hours (for CE > 0.45) | |
4.4 Multi-Layer Overlay Strategy
The overlay structure follows a three-zone metallurgical design:
- Zone 1 — Dilution Control Layer (Pass 1): A low-carbon, high-ductility wire (e.g., ER70S-6 or ER80S-D2) is used to minimize dilution effects from the substrate. This layer acts as a metallurgical buffer, reducing the carbon and alloy concentration gradient between the base metal and the wear layer. Target dilution rate: ≤ 30%
- Zone 2 — Transition/Bonding Layer (Pass 2): A medium-alloy wire (e.g., ER80S-D2) provides adequate strength and thermal fatigue resistance. This layer ensures sound metallurgical bonding between the substrate and the wear layer
- Zone 3 — Functional Wear Layer (Pass 3–N): A high-carbon, high-alloy wire (e.g., ER60S-D3, ER60NiCrMo, or specialized hardfacing wire per GB/T 12470) provides the primary wear resistance. Target surface hardness: HV 400–600 for abrasion resistance against mill scale
4.5 Welding Technique for Cylindrical Geometry
Welding on a large-diameter cylindrical roller (typically Φ400–800 mm) presents unique challenges. The following techniques must be applied:
- Directional Welding Sequence: Weld in a spiral or helical pattern to distribute heat uniformly around the circumference. Avoid continuous longitudinal welds that create asymmetric thermal gradients and ovality distortion
- Heat Input Management: Limit heat input to ≤ 25 kJ/cm for the transition layer and ≤ 35 kJ/cm for the wear layer. Use pulse welding where possible to reduce peak temperatures
- Rotational Welding: Rotate the roller at a constant speed (0.5–2 rpm) while the welding torch remains stationary, or use a multi-torch arrangement for simultaneous circumferential coverage
- Thermal Monitoring: Place thermocouples at the weld zone, mid-thickness, and opposite side of the roller. Interpass temperature must be logged and verified before each subsequent pass
4.6 Post-Weld Heat Treatment
For substrates with carbon equivalent CE ≥ 0.40, post-weld heat treatment (PWHT) is mandatory to relieve residual stresses and prevent delayed hydrogen cracking. The PWHT procedure should follow:
- Heating Rate: ≤ 200°C/hour to the holding temperature
- Holding Temperature: 550–650°C (below the Ac₁ transformation temperature of the substrate)
- Holding Duration: 1 hour per 25 mm of the thickest section, minimum 2 hours
- Cooling Rate: Controlled cooling to ≤ 100°C at a rate of ≤ 100°C/hour, then air cool
- Equipment: Induction heating with embedded thermocouples, or a dedicated stress-relief furnace for smaller rollers
4.7 Final Machining and Dimensional Verification
After PWHT, the overlay surface is machined to the final dimensional specification:
- Machining Allowance: 3–5 mm left on the overlay surface for final grinding
- Final Surface Finish: Ra ≤ 6.3 μm for the grip surface; Ra ≤ 3.2 μm for bearing journals
- Dimensional Tolerances: Out-of-roundness ≤ 0.1 mm; taper ≤ 0.05 mm/m; step height ≤ 0.05 mm
- Hardness Verification: Surface hardness (HV30) measured at 10 equally spaced points around the circumference; target range HV 400–600 for the wear layer
- Balance Verification: Dynamic balancing to G6.3 grade per ISO 21940-11 (for operating speeds > 150 rpm)
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard Number | Title / Scope | Application |
|---|---|---|
| GB/T 12470 | Welding consumables for hardfacing | Weld wire specification for wear overlay layers |
| GB/T 985.1 | Groove dimensions for butt welds | Crack repair groove geometry |
| GB/T 3323 | Non-destructive testing—Radiographic testing of welds | RT inspection of overlay welds (if applicable) |
| GB/T 24717 | Non-destructive testing—Magnetic particle testing | MT inspection for crack detection |
| GB/T 18851 | Non-destructive testing—Liquid penetrant testing | PT inspection of surface cracks |
| GB/T 11345 | Non-destructive testing—Ultrasonic testing of welds | UT inspection of overlay weld bonds |
| ASTM A743 | Castings, iron castings for pressure parts | Reference for cast iron/steel roller materials |
| ASTM E1444 | Non-destructive testing—Magnetic particle examination | MT acceptance criteria |
| ASTM E709 | Non-destructive testing—Liquid penetrant examination | PT acceptance criteria |
| ASTM E23 | Impact testing of metallic materials | Impact toughness verification of overlay welds |
| AWS D10.12 | Specification for Welding Operator Qualification and Certification | Welder qualification requirements |
| ISO 9606-1 | Qualification testing of welders—Welding by fusion | International welder certification |
| ISO 13919 | Welding—Welding position classification | Weld position classification for overlay |
| ISO 15614 | Welding—Approval of welding procedures | WPS/PQR qualification framework |
| NACE MR0175 | Sour service materials (if applicable) | Material selection for H₂S-resistant overlay (if required) |
| ASME BPVC Section IX | Welding, Brazing, and Fusing Qualifications | WPS qualification and welder performance qualification |
5.2 Acceptance Criteria Summary
- Visual Inspection (VT): No undercuts > 0.5 mm, no spatter, no porosity visible on the surface. Weld profile uniformity within ±0.5 mm of the designed bead profile
- Penetrant Testing (PT): Per ASTM E709, no linear indications > 2 mm in length. All indications must be evaluated and documented
- Magnetic Particle Testing (MT): Per ASTM E1444, no indications of cracks, laps, or inclusions. All indications must be evaluated per the applicable acceptance level (typically Level 2)
- Hardness: Surface hardness HV 400–600 (wear layer); transition layer hardness HV 250–400; substrate hardness unchanged within ±50 HV from original specification
- Impact Toughness: Charpy V-notch impact energy ≥ 27 J at 20°C for the transition layer weld metal (per ASTM E23)
- Dimensional: Out-of-roundness ≤ 0.1 mm; surface roughness Ra ≤ 6.3 μm on grip surface
- Balance: Dynamic balance to G6.3 per ISO 21940-11
6. Common Risks and Controls
| Risk | Cause | Mitigation Control |
|---|---|---|
| Hydrogen-induced cracking (cold cracking) | High CE substrate, inadequate preheat, excessive hydrogen in weld metal | Preheat to 250–350°C; use low-hydrogen consumables (H ≤ 5 mL/100g); control interpass temperature ≤ 350°C; apply PWHT within 2 hours of final weld |
| Thermal fatigue cracking of overlay | High hardness (> HV 600) reduces thermal shock resistance; excessive dilution creates brittle microstructure | Limit surface hardness to HV ≤ 600; ensure ≥ 30% dilution control in transition layer; use Ni-Cr alloy wires for improved thermal fatigue resistance |
| Roller distortion and ovality | Asymmetric heat input; excessive heat per pass; inadequate rotational speed | Use spiral/helical welding pattern; limit heat input ≤ 35 kJ/cm; rotate at 0.5–2 rpm; monitor with dial indicators during welding |
| Delamination of overlay from substrate | Poor surface preparation; oxide inclusions; inadequate wetting | Grind to bare metal with Ra ≤ 12.5 μm; clean with acetone; use transition layer with good wetting characteristics; perform UT bond testing |
| Excessive wear after repair | Inappropriate wire selection; insufficient overlay thickness; poor surface finish | Select wire based on wear mechanism (abrasive vs. adhesive vs. impact); ensure ≥ 5 mm overlay thickness; achieve final Ra ≤ 6.3 μm |
| Bearing journal damage | Heat input affecting the bearing seat area; inadequate shielding of non-weld zones | Shield bearing journals with ceramic blanket or refractory tape; monitor temperature at bearing seats (must remain < 150°C); re-machine bearing seats to tolerance after repair |
| Residual stress-induced failure | Incomplete stress relief; excessive welding sequence | Mandatory PWHT for CE ≥ 0.40 substrates; verify residual stress reduction by X-ray diffraction or strain gauge method |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay (Primary Route for This Application)
The pick-up roller repair application is the core domain of the company's TIG/MIG weld overlay technology route. This route is uniquely suited for the following reasons:
- Geometric Flexibility: TIG and MIG processes can be applied to large-diameter cylindrical surfaces in all positions (horizontal, vertical, overhead) without specialized equipment
- Material Versatility: The route accommodates a wide range of substrate compositions (carbon steel, low-alloy steel, cast steel) and overlay consumables (manganese, chromium, nickel-based hardfacing alloys)
- On-Site Applicability: Portable TIG/MIG equipment enables field repair at the steel mill, eliminating the need to ship heavy rollers to a fabrication facility
- Multi-Pass Capability: The process naturally supports the multi-layer overlay strategy (transition layer + wear layer) required for metallurgical compatibility
The documented learning experience from this project directly contributes to the company's TIG/MIG overlay WPS library. Each successful repair validates specific combinations of substrate, wire, parameters, and technique that can be replicated for similar applications across different steel mill customers.
7.2 Hydraulic Explosive Bonding (Secondary/Complementary Route)
While hydraulic explosive bonding is not directly applicable to roller repair, the technology principles transfer in the following ways:
- Knowledge Transfer: Understanding of interfacial metallurgy, shock wave propagation, and dynamic deformation from hydraulic bonding expertise informs the selection of overlay alloy systems with optimal bonding characteristics
- Material Selection: The company's expertise in compatible metal pairings (developed through hydraulic bonding work) supports informed decisions on transition layer alloy selection for roller overlay
- NDT Synergy: The ultrasonic and radiographic inspection techniques developed for bonded joint qualification are directly applicable to overlay weld bond verification
7.3 Explosion Welding (Tertiary/Strategic Route)
The explosion welding route contributes to the roller repair application indirectly but significantly:
- High-Velocity Deformation Insights: Understanding of material behavior under extreme strain rates (developed through explosion welding) provides insight into the microstructural evolution in rapidly solidified overlay welds
- Advanced Consumable Development: The company's metallurgical research capabilities, built through explosion welding projects, support the development of proprietary overlay consumables optimized for specific wear mechanisms
- Qualification Framework: The rigorous qualification and certification protocols established for explosion welding (WPS, PQR, NDE protocols) set the quality benchmark for all overlay welding services, including roller repair
8. Integration into the Company's Quality Management System
8.1 WPS and PQR Framework
Each pick-up roller repair project should be governed by a qualified Welding Procedure Specification (WPS) supported by a Procedure Qualification Record (PQR). The qualification framework should follow:
- ISO 15614-1: For qualification of welding procedures for ferrous metals
- ASME BPVC Section IX: For qualification per the ASME framework (if required by the customer)
- AWS D1.1: For structural steel welding qualification requirements
The WPS should specify: welding process, consumable type and size, polarity, current range, voltage range, travel speed, gas composition and flow rate, preheat temperature, interpass temperature, and post-weld heat treatment parameters.
8.2 Documentation and Traceability
A complete repair documentation package should include:
- As-found inspection report (visual, MT/PT, dimensional survey, hardness map)
- Repair plan with approved WPS reference
- Welding log (parameters, times, welder ID, thermocouple readings)
- PWHT record (heating curve, holding time, cooling curve)
- Post-repair NDT report (PT/MT results with acceptance criteria)
- Final dimensional and hardness verification report
- Welder qualification certificate (valid within the qualification window)
8.3 Customer-Specific Requirements
Steel mill customers (Baosteel, HBIS, Shagang, Angang, etc.) typically impose additional requirements beyond base standards:
- Internal Specifications: Many mills have proprietary repair specifications that supersede general standards. These must be obtained and incorporated into the repair plan
- Witness Inspection: Customer quality representatives may require presence during critical steps (surface preparation, welding, NDT, machining)
- Service Life Guarantee: The company may be required to guarantee a minimum service life (e.g., ≥ 6 months or ≥ 50,000 tons of coil processed) before the next repair
- Spare Parts Integration: Repaired rollers must be dimensionally interchangeable with new rollers in the mill's roller inventory
9. Continuous Improvement and Knowledge Management
The "learning reflection" (学习心得) format of this entry highlights the company's commitment to knowledge management. Key practices for continuous improvement include:
- Post-Service Failure Analysis: When a repaired roller fails in service, conduct a root cause analysis (RCA) using fractography, microstructural examination, and chemical analysis of the failed zone. Feed findings back into WPS revisions
- Consumable Performance Tracking: Maintain a database correlating specific consumable batches, welding parameters, and service life outcomes to identify optimal combinations
- Technician Training Programs: Use documented case studies as training material for new technicians. Each repair project should include a structured debrief session
- Standard Development: Accumulate sufficient project data to develop company-specific technical standards that can be proposed to customers as best-practice guidelines
- Cross-Route Learning: Encourage knowledge sharing between the TIG/MIG overlay team and the hydraulic bonding/explosion welding teams to leverage metallurgical insights across all technology routes
10. Conclusion
The weld overlay repair of hot rolled coil coiler pick-up rollers represents a technically demanding and commercially significant application within Cladding Technology Shanxi Co., Ltd.'s service portfolio. It requires a deep understanding of metallurgy, welding science, thermal management, and quality assurance—precisely the competencies that the company has developed across its three core technology routes.
This documented learning experience serves as both a technical reference and a qualification-building artifact. It codifies practical knowledge into repeatable procedures, supports WPS development, demonstrates technical capability to customers, and contributes to the professional development of the welding engineering team. As the company continues to expand its presence in the steel mill maintenance market, the systematic accumulation and application of such knowledge will be a critical competitive advantage.
The integration of this repair capability with the company's broader expertise in hydraulic explosive bonding and explosion welding creates a unique value proposition: the ability to offer customers not only repair services but also advanced surface engineering solutions—custom overlay alloy development, bonded cladding for new roller fabrication, and comprehensive lifecycle management of critical rolling mill components.