CLADDING TECHNOLOGY SHANXI CO., LTDResearch on Roll Surfacing for Industrial Rolling Mills
Overview and Technical Background
Rolls in industrial rolling mills are subjected to extreme conditions: high contact stresses, severe plastic deformation of the workpiece, high temperatures, and abrasive wear. The roll surface is the critical component that determines the surface quality of the rolled product, the dimensional accuracy, and the service life of the roll. Traditional roll materials (cast steel, alloy steel) have limited wear resistance, and rolls must be replaced or reground frequently, leading to significant downtime and cost.
Roll surfacing (cladding) involves depositing a wear-resistant layer on the roll surface to extend service life and improve surface quality. This study note examines the metallurgical challenges, process optimization, and engineering outcomes of roll surfacing for industrial rolling mills.
Metallurgical Challenges of Roll Surfacing
The metallurgical challenges of roll surfacing are unique and demanding. The roll surface must withstand:
- High contact stresses (up to 3 GPa in hot rolling)
- Thermal cycling (from room temperature to 1200 °C in hot rolling)
- Abrasive wear from oxide scale and iron particles
- Adhesive wear from the rolled workpiece
- Impact loading from scale breakers and roll chills
The following table summarizes the key performance requirements for roll surfacing alloys.
| Property | Requirement | Typical Value |
|---|---|---|
| Hardness | > 600 HV | 600–800 HV |
| Thermal stability | Retain > 90% hardness at 800 °C | 550–700 HV at 800 °C |
| Thermal fatigue resistance | > 1000 cycles at 800 °C | No cracking after 1000 cycles |
| Wear resistance | > 5× base material | 5–15× base material |
| Bond strength | > 200 MPa (peel test) | 200–350 MPa |
| Surface finish (as-deposited) | Ra < 6 μm | 3–6 μm |
The primary metallurgical challenge is achieving a high-hardness surface layer that is thermally stable and resistant to thermal fatigue cracking. The high hardness is typically achieved through the formation of hard carbides (Cr7C3, Cr3C2, or Mo2C), but these carbides can coarsen and dissolve at elevated temperatures, leading to hardness loss and cracking.
Alloy Selection and Process Parameters
The following table compares the performance of different surfacing alloys for hot rolling mill rolls.
| Alloy System | Hardness (HV) | Hardness at 800 °C (HV) | Thermal Fatigue Life (cycles) | Wear Rate (mg/N·m) | Cost Index |
|---|---|---|---|---|---|
| Cr12MoV (H13) | 550 | 350 | 200 | 8.5 | 1.0 |
| Co-Cr-C (Stellite 6) | 400 | 380 | 500 | 4.2 | 3.5 |
| Fe-Cr-C (cast iron) | 650 | 450 | 300 | 3.8 | 1.5 |
| Ni-Cr-C (carbide) | 500 | 480 | 600 | 3.5 | 4.0 |
| Co-W-C (high-speed) | 550 | 500 | 700 | 2.8 | 5.0 |
| Fe-Ni-Cr-C (composite) | 600 | 520 | 650 | 3.0 | 3.0 |
The Co-W-C (high-speed steel) alloy offers the best thermal fatigue resistance and wear resistance, but at the highest cost. The Fe-Ni-Cr-C composite alloy provides a good balance of performance and cost, with a wear rate of 3.0 mg/N·m and a cost index of 3.0.
The welding process selection for roll surfacing is constrained by the roll geometry, the required deposit thickness, and the need to minimize distortion. The following table compares the suitability of different processes.
| Process | Deposition Rate | Distortion | Surface Finish | Suitability |
|---|---|---|---|---|
| Manual GMAW | 0.5–2 kg/h | Moderate | 3–6 μm | Small rolls |
| SAW (wire + flux) | 3–8 kg/h | Low | 6–12 μm | Large rolls |
| GMAW (automated) | 2–5 kg/h | Low | 2–4 μm | Medium rolls |
| PTA (powder) | 1–4 kg/h | Very low | 1–3 μm | Precision rolls |
| Laser cladding | 0.5–3 kg/h | Very low | 0.5–2 μm | High-precision rolls |
For large-diameter rolls (above 500 mm), automated GMAW or SAW is preferred due to the high deposition rate and low cost. For small-diameter precision rolls, PTA or laser cladding is preferred due to the fine surface finish and low distortion.
The process parameters for automated GMAW surfacing of a Co-Cr-C alloy on a 600 mm diameter hot rolling mill roll are summarized below.
| Parameter | Value |
|---|---|
| Wire diameter | 1.2 mm |
| Current | 250–300 A |
| Voltage | 28–32 V |
| Shielding gas | 98% Ar + 2% CO2 |
| Travel speed | 400–500 mm/min |
| Wire feed speed | 5–7 m/min |
| Interpass temperature | < 200 °C |
| Number of passes | 3–4 |
| Final deposit thickness | 3.0–5.0 mm |
Defect Analysis and Countermeasures
The following table presents the common defects encountered during roll surfacing and the recommended countermeasures.
| Defect | Root Cause | Countermeasure |
|---|---|---|
| Cracking at weld/base interface | Thermal expansion mismatch; high carbon in base | Preheat to 200–300 °C; use low-carbon transition layer; control interpass temperature |
| Thermal fatigue cracking | Coarse carbides; high residual stress | Refine carbides with lower heat input; apply post-weld stress relief |
| Excessive dilution | High heat input; thin deposit | Use lower current; increase wire feed speed; use multiple thin passes |
| Surface irregularities | Torch positioning error; wire stick-out variation | Stabilize torch; maintain constant stick-out; use push-pull feed |
| Porosity | Inadequate shielding; contaminated wire | Ensure 100% gas coverage; use dry, clean wire; pre-gas purge |
| Insufficient bond strength | Poor surface preparation; low heat input | Clean surface thoroughly; increase heat input; use transition layer |
Engineering Practice and Field Experience
In a practical application, the surface of a 600 mm diameter hot rolling mill roll (base material: 4Cr5MoSiV) was surfaced with a Fe-Ni-Cr-C composite alloy using automated GMAW. The roll was preheated to 250 °C, and the surfacing deposit was 4.0 mm thick. The process parameters were: current 280 A, voltage 30 V, travel speed 450 mm/min, shielding gas 98% Ar + 2% CO2.
The as-deposited surface was machined to Ra 1.6 μm. Metallographic examination revealed a fine, uniformly distributed carbide structure with carbide sizes of 3–8 μm. Hardness testing showed a gradient from 620 HV at the surface to 450 HV at the interface, with no cracks or porosity detected.
After 12 months of continuous operation in a hot strip mill (rolling 1200 °C steel slabs), the roll showed no measurable wear on the surfacing layer. The original roll had a service life of only 4 months before requiring regrinding or replacement. This represents a 3-fold improvement in service life, with significant savings in maintenance costs and downtime.
Study Insights and Reflections
The research on roll surfacing reveals that the key to success is the integration of alloy design, process optimization, and quality control. The most critical factor is the carbide morphology and distribution, which determines the wear resistance and thermal fatigue resistance of the surfacing layer.
The field experience demonstrates that a well-designed surfacing alloy can extend roll service life by 2–3 times, with significant economic benefits. However, the success of the surfacing process depends on rigorous quality control, including pre-weld cleaning, process parameter monitoring, and post-weld inspection.
The most significant challenge is achieving a balance between hardness and toughness. High hardness is required for wear resistance, but excessive hardness can lead to thermal fatigue cracking. The solution is to use a composite alloy with a gradient in hardness and carbide distribution, providing a hard surface layer with a tougher transition zone.
Future work should focus on developing multilayer surfacing systems that combine different alloy systems to optimize the surface properties, and on developing in-situ monitoring techniques that can assess the condition of the surfacing deposit during service.
In conclusion, roll surfacing is a mature technology that offers significant benefits in terms of service life extension and production cost reduction. The key to success lies in a thorough understanding of the wear mechanisms, careful alloy design, and rigorous process control.