CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Technology for Hot-Rolled Support Rolls
Literature Overview
This study focuses on the overlay welding technology applied to hot-rolled support rolls (HRSR) in steel rolling mills, where severe thermal fatigue, abrasion, and contact fatigue are the primary failure modes. Support rolls, typically made of 45 steel or 50MnV, operate under extreme conditions with surface temperatures reaching 800–1000 °C and contact pressures exceeding 100 MPa. The overlay layer must provide thermal barrier protection, abrasion resistance, and dimensional stability while maintaining the structural integrity of the roll body.
The study evaluates various overlay processes including SAW, GTAW, and plasma arc welding, and compares the performance of different filler metal systems including Cr-Mo alloy steels, austenitic stainless steels, and nickel-based alloys.
Core Technical Findings
Failure Modes of Support Rolls
| Failure Mode | Mechanism | Typical Location | Service Life Impact |
|---|---|---|---|
| Thermal fatigue | Cyclic heating and cooling | Surface and near-surface | 30–50% reduction |
| Abrasive wear | Contact with strip scale and oxide | Cylindrical surface | 20–40% reduction |
| Contact fatigue | Hertzian contact stress | Subsurface (0.5–1.5 mm) | 10–20% reduction |
| Chatter marks | Vibration during rolling | Full cylindrical surface | Surface quality degradation |
| Scaling | Oxidation at high temperature | Surface layer | Dimensional loss |
The overlay layer addresses thermal fatigue and abrasive wear but must be carefully designed to avoid exacerbating contact fatigue, which is governed by subsurface stress conditions.
Overlay Layer Design
| Filler Metal | Composition (wt%) | Hardness (HV) | Thermal Conductivity (W/m·K) | Service Temperature |
|---|---|---|---|---|
| Cr-Mo alloy steel (4140) | Cr 0.8–1.1, Mo 0.20–0.30 | 280–320 | 28–32 | 600 °C |
| Austenitic SS (310) | Cr 24–26, Ni 19–22 | 180–220 | 20–24 | 1000 °C |
| Ni-base alloy (625) | Ni bal., Cr 20–23, Mo 8–10 | 250–300 | 11–13 | 1100 °C |
| High-Cr cast iron | Cr 26–30, C 2.5–3.5 | 600–750 | 15–20 | 800 °C |
| Cu-base alloy | Cu bal., Ni 5–8 | 120–150 | 40–50 | 600 °C |
The selection of the overlay material depends on the specific operating conditions. For hot band mills, austenitic stainless steel or Ni-base alloys are preferred due to their superior thermal fatigue resistance. For cold rolling backup rolls, Cr-Mo alloy steel overlays provide adequate protection at lower cost.
Process Parameters and Technique
Recommended Welding Parameters for Support Roll Overlay
| Parameter | GTAW | SAW | PTA |
|---|---|---|---|
| Current (A) | 150–250 | 400–600 | 200–400 |
| Voltage (V) | 18–22 | 28–34 | 30–38 |
| Travel speed (mm/min) | 100–200 | 200–400 | 150–300 |
| Heat input (kJ/mm) | 3–8 | 10–25 | 5–12 |
| Preheat (°C) | 200–300 | 300–400 | 250–350 |
| Layer thickness (mm) | 1.0–2.0 | 3.0–6.0 | 1.0–2.0 |
| Interpass temp (°C) | < 350 | < 400 | < 350 |
The key challenge in support roll overlay is maintaining dimensional accuracy. The cylindrical geometry requires a multi-axis welding setup, typically using a robotic GTAW or PTA system with a rotating roll chuck. The overlay thickness must be uniform within ±0.1 mm to ensure proper roll gap control during rolling.
Multi-Layer Overlay Sequence
- Surface preparation: Grind the roll surface to remove scale and oxide, achieving a surface roughness of Ra ≤ 6.3 μm.
- Bonding layer: Apply a single pass of compatible filler (e.g., 309L) at low heat input to ensure complete melting and bonding.
- Build-up layers: Apply 2–4 passes of the selected overlay material, maintaining consistent travel speed and torch angle.
- Surface finishing: Grind and polish the overlay surface to achieve Ra ≤ 1.6 μm for hot rolling applications.
- Post-weld treatment: Stress-relief annealing at 550–650 °C for 2 hours to reduce residual stresses without softening the overlay.
Defect Analysis and Quality Control
| Defect | Detection Method | Acceptance Criteria | Countermeasure |
|---|---|---|---|
| Undercut | Visual, MT | Depth < 0.5 mm | Reduce current, increase travel speed |
| Porosity | UT, RT | Single pore < 1 mm | Improve shielding, clean wire |
| Cracking | MT, PT | No surface cracks | Preheat, reduce interpass temperature |
| Excessive dilution | Hardness mapping | Dilution < 15% | Use multiple thin layers |
| Dimensional deviation | CMM, laser scan | ±0.1 mm | Robotic welding, in-process monitoring |
Non-destructive testing per JB/T 4730 or EN ISO 17640 is mandatory for support roll overlays. Magnetic particle testing (MT) covers 100% of the overlay surface, while ultrasonic testing (UT) is applied to the bonding interface to verify full bond integrity.
Engineering Practice and Case Study
In a typical hot strip mill application, support rolls with a diameter of 800 mm and length of 1200 mm are overlaid with a 3 mm thick austenitic stainless steel layer. The overlay is applied using a robotic GTAW system with ER310 filler wire, operating at 200 A, 20 V, and 150 mm/min. The total welding time per roll is approximately 8–10 hours, including surface preparation and post-weld grinding.
A critical observation from field experience is that the overlay layer can develop thermal fatigue cracks after 200–500 hours of service, depending on the rolling schedule. The cracks typically initiate at the overlay surface and propagate inward, reaching the bonding interface after extended service. To mitigate this, the overlay thickness should be sufficient to ensure that the cracks do not reach the base metal within the expected service interval. A minimum overlay thickness of 3 mm is recommended for hot band mills, while 2 mm is acceptable for warm rolling applications.
Study Insights and Implications
The overlay technology for hot-rolled support rolls is a prime example of how welding engineering must be integrated with mechanical design and operational requirements. The overlay layer is not merely a protective coating but a structural component that must withstand cyclic thermal and mechanical loading. The key insight from this study is that the thermal fatigue life of the overlay is governed by the temperature gradient through the layer thickness, which is directly related to the thermal conductivity of the overlay material. Lower thermal conductivity materials (e.g., Ni-base alloys) create steeper temperature gradients and may paradoxically reduce thermal fatigue life despite their higher service temperature capability. Engineers must therefore balance thermal conductivity, hardness, and thermal fatigue resistance when selecting the overlay material. The future of support roll overlay lies in functionally graded deposits that transition from a high-conductivity layer at the surface to a high-strength layer at the interface, optimizing both thermal and mechanical performance.