CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Transfer Rollers on Hot-Strip Coilers
Overview and Background
Hot-rolled strip coilers are critical components in the finishing line of hot strip mills, responsible for controlled coiling of hot steel strip at temperatures typically ranging from 600°C to 950°C. The transfer rollers (also called guide rollers or transfer rolls) that handle the strip are subjected to extreme thermal cycling, mechanical impact, abrasive contact with scale, and occasional side-bending forces. Over time, the working surfaces of these rollers suffer from severe wear, surface cracking, and localized material loss, which can lead to strip surface defects, dimensional deviation, and unplanned shutdowns. Weld overlay repair of these rollers is therefore a well-established maintenance strategy in the hot rolling industry, offering significant cost savings compared to full roller replacement.
This study note examines the technical approach to weld overlay repair of coiler transfer rollers, drawing from published case studies and industry practice. The focus is on material selection, process parameters, heat treatment, and quality assurance procedures that ensure reliable long-term service.
Material Selection for Overlay Repair
The selection of overlay material is the most consequential decision in roller repair. The material must resist thermal fatigue, high-temperature oxidation, and abrasive wear from scale and scale fragments at operating temperatures above 700°C.
| Parameter | Typical Specification | Rationale |
|---|---|---|
| Base material | 42CrMo or 35CrMo (quenched and tempered) | High strength, good fatigue resistance |
| Overlay material | Fe-Cr-Al alloy (e.g., 20Cr25Ni5 or equivalent) or Ni-Cr-Mo alloy | Heat resistance, oxidation resistance |
| Overlay thickness | 3–8 mm (total) | Adequate wear life without excessive distortion |
| Hardness after repair | 28–35 HRC (overlay), 30–38 HRC (base) | Matching of mechanical properties |
| Service temperature | Up to 950°C | Hot strip temperature range |
The Fe-Cr-Al system is favored for its excellent oxidation resistance at elevated temperatures due to the formation of a stable Cr2O3 scale. Ni-Cr-Mo alloys offer superior thermal fatigue resistance but at a significantly higher material cost. In practice, a multi-layer approach is often employed: the first layer is a transition alloy (e.g., Ni-Fe or Ni-Cr) to prevent carbon depletion and chromium carbide precipitation at the base-overlay interface, followed by the functional overlay layer.
Welding Process and Parameters
Submerged arc welding (SAW) is the most commonly used process for roller overlay repair due to its high deposition rate, deep penetration, and ability to produce uniform multi-layer deposits. Gas tungsten arc welding (GTAW) is sometimes used for the first transition layer to achieve better control and cleanliness.
Typical SAW Overlay Parameters
| Parameter | Value |
|---|---|
| Welding current | 400–600 A |
| Arc voltage | 28–35 V |
| Travel speed | 200–350 mm/min |
| Wire diameter | 1.6–2.4 mm |
| Flux type | Basic (rutile-basic mixed) |
| Preheat temperature | 200–300°C |
| Interpass temperature | 200–350°C |
The preheat temperature is critical. Insufficient preheat leads to high cooling rates, which can cause hydrogen-induced cracking (HIC) in the heat-affected zone (HAZ) and the transition layer. Excessive preheat, on the other hand, reduces the hardness and strength of the base material near the surface. The interpass temperature must be maintained within a narrow window to prevent excessive grain growth and to ensure proper dilution control.
Multi-Layer Strategy
A typical repair sequence involves:
- Surface preparation: Grinding down the worn surface to a uniform geometry, followed by cleaning with acetone or similar solvent to remove oil and contaminants.
- Transition layer (1–2 passes): Using a Ni-Fe or Ni-Cr wire to prevent carbon diffusion from the base into the overlay, which would form brittle carbides and reduce the corrosion resistance of the overlay.
- Overlay layers (2–4 passes): Applying the functional alloy in multiple passes to build up the required thickness. Each pass should have a dilution ratio below 15% for the final layer.
- Post-weld heat treatment: Stress relief at 550–600°C for 2 hours, or full tempering if the base material requires it.
Common Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Hydrogen-induced cracking (HIC) | High hydrogen content, low preheat | Increase preheat to 250–300°C; use low-hydrogen flux; bake electrodes |
| Carbon depletion at interface | Carbon diffusion from base to overlay | Use Ni-Fe transition layer; limit base carbon content |
| Cracking in overlay | Thermal stresses, excessive cooling rate | Control interpass temperature; use compatible alloy system |
| Excessive dilution | High travel speed, low current | Optimize welding parameters; use smaller wire diameter |
| Surface porosity | Flux moisture, poor cleaning | Control flux drying; thorough surface cleaning |
Quality Assurance and Inspection
Post-repair inspection is mandatory to verify the integrity of the overlay. The following inspection methods are typically employed:
- Visual and dimensional inspection: Verify overlay thickness, surface profile, and absence of visible defects.
- Magnetic particle testing (MT): Detect surface and near-surface cracks in the overlay and HAZ.
- Ultrasonic testing (UT): Assess bond strength between overlay and base material. The overlay bond should be verified at intervals not exceeding 500 mm along the roller circumference.
- Hardness testing: Confirm that the overlay hardness meets specification and that the HAZ hardness does not exceed the allowable limit.
- Metallographic examination: Cross-sectional analysis to verify microstructure, carbide distribution, and absence of interfacial defects.
Engineering Practice Insights
From a practical standpoint, several lessons emerge from the literature and field experience:
- Roller geometry matters: The cylindrical shape of the roller creates a curved surface, which affects heat flow during welding. The cooling rate at the center of the roller surface is different from that at the edges, leading to non-uniform microstructures if parameters are not adjusted accordingly.
- Thermal distortion is manageable: With proper preheat and controlled interpass temperature, the dimensional change of a roller after multi-layer overlay is typically less than 0.2 mm in diameter, which is within the tolerance of most coiler applications.
- Service life improvement: A well-executed overlay repair can extend roller life by 3–5 times compared to the original bare roller, making it economically attractive even when considering the cost of inspection and repair.
- Repair frequency planning: Rollers should be inspected every 6–12 months of service, depending on the severity of the operating conditions. A wear rate of 0.1–0.3 mm per year is typical for properly overlaid rollers in hot strip coiler service.
Study Reflections
The weld overlay repair of coiler transfer rollers is a technically demanding task that requires careful balancing of material selection, process parameters, and heat treatment. The key insight is that the overlay is not merely a wear-resistant skin but a functional interface between the base material and the harsh operating environment. The transition layer, often overlooked in cost-conscious operations, is essential for preventing carbon depletion and ensuring long-term performance. Furthermore, the quality of surface preparation before welding cannot be overstated; any residual scale, oil, or oxide on the roller surface will act as a defect nucleation site and compromise the bond strength.
In summary, successful overlay repair of hot-rolled strip coiler transfer rollers depends on a systematic approach that integrates material science, welding process knowledge, and rigorous quality control. Engineers should treat each repair as a critical operation that demands careful planning, precise execution, and thorough verification.