CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Cladding of New Steel Rolling Mill Frame Surfaces
Literature Overview and Technical Context
Rolling mill frames are massive structural components that support the rolling mill rolls and absorb the forces generated during the rolling process. These frames are subjected to severe cyclic loading, abrasive wear from scale and metal chips, and in some cases corrosion from rolling mill scale and water-based lubricants. The literature investigates the application of surface cladding to new rolling mill frames to enhance wear resistance, reduce maintenance intervals, and extend the service life of these critical structural components.
Core Technical Points
The cladding of new rolling mill frames presents a unique combination of challenges related to the massive scale of the components, the requirement to maintain structural integrity, and the need to achieve uniform wear-resistant surfaces on large areas. The literature identifies the following key considerations:
| Design Consideration | Specification | Rationale |
|---|---|---|
| Frame material | Q345R, Q460, or equivalent | Structural steel for load-bearing |
| Cladding area | Bearing surfaces, guide surfaces, wear plates | Target areas of highest wear |
| Overlay thickness | 8-15 mm | Provide wear life for mill service |
| Overlay material | High-chrome cast iron, hardfacing alloy, or ceramic composite | Match wear mechanism |
| Surface hardness after cladding | 40-55 HRC | Provide adequate wear resistance |
| Impact toughness of base metal | ≥47 J at -20 °C | Maintain structural integrity |
The literature discusses several cladding processes suitable for large structural components like rolling mill frames:
Process Selection for Large Frame Components
| Process | Deposition Rate | Heat Input | Suitability for Large Components | Notes |
|---|---|---|---|---|
| SAW (Submerged Arc) | High (5-10 kg/h) | Moderate | Excellent for flat and large surfaces | Preferred for main cladding layers |
| FCAW (Flux-Cored) | Medium-High (3-6 kg/h) | Moderate | Good for complex geometries | Flexible positioning |
| SMAW (Shielded Metal Arc) | Low (1-2 kg/h) | Moderate | Limited to small areas or touch-up | Labor-intensive |
| Oxy-Fuel (Flame) | Medium (2-4 kg/h) | High | Good for large flat areas | Requires skilled operators |
| PTA (Plasma Transfer Arc) | Medium (2-5 kg/h) | Low-Moderate | Precision cladding on critical areas | Higher equipment cost |
Process Analysis and Quality Control
The cladding process for rolling mill frames must be carefully planned to minimize distortion while achieving adequate overlay thickness. The literature recommends a multi-pass approach with the following sequence:
- Surface preparation: Grind or blast clean the cladding area to remove scale, rust, and contaminants. Achieve a surface profile of 40-80 μm for good mechanical interlocking.
- Preheating: Heat the frame to 200-300 °C to reduce thermal gradients and minimize cracking susceptibility in the base metal.
- Transition layer: Apply a nickel-based or iron-nickel transition layer to improve bonding between the base steel and the hardfacing overlay.
- Main overlay layers: Apply 2-3 passes of the selected hardfacing material to achieve the target thickness.
- Post-weld treatment: Perform stress relief at 550-600 °C for 2-4 hours per 25 mm of section thickness.
Hardfacing Material Selection
| Material Type | Composition | Hardness (HRC) | Wear Mechanism Resistance | Application |
|---|---|---|---|---|
| High-carbon martensitic | C 2.5-3.5%, Cr 6-8% | 50-58 | Abrasive, adhesive | General wear surfaces |
| High-chrome austenitic | Cr 22-28%, C 0.5-1.5% | 35-45 | Corrosive wear, galling | Corrosive environments |
| Carbide composite | WC-Co or Cr3C2-Ni | 60-70 | Severe abrasion | High-abrasion areas |
| Ceramic composite | Al2O3, SiC, or B4C | 70-80 | Extreme abrasion | Severe wear zones |
Quality control for frame cladding requires a comprehensive inspection regime. The literature emphasizes the following NDT methods:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual Testing (VT) | Surface defects, porosity, cracks | No visible cracks or excessive porosity |
| Magnetic Particle Testing (MT) | Surface and near-surface cracks | No linear indications > 3 mm |
| Ultrasonic Testing (UT) | Subsurface bonding quality | No lack of fusion or delamination |
| Hardness Testing | Verify overlay hardness profile | Within specified range ± 5 HRC |
| Dimensional Inspection | Verify geometric tolerance | Within ± 0.5 mm of nominal |
Engineering Practice Integration
In manufacturing practice, the cladding of new rolling mill frames is typically performed during the fabrication stage before the frame is assembled into the mill. This approach allows for better access to all cladding surfaces and easier implementation of quality control measures. The literature documents successful applications where cladding was performed on frame sections before final welding assembly, with subsequent machining of the cladding surfaces to achieve precise dimensional tolerances for roll bearing seats and guide surfaces.
The thermal management aspect of frame cladding is particularly important due to the massive cross-section of frame components. The high thermal mass of the frame means that heat input from welding can create significant thermal gradients within the section, potentially causing distortion or residual stresses that compromise structural integrity. The literature recommends using multi-pass welding with controlled interpass temperatures, and in some cases, using back-step welding or skip welding sequences to distribute heat input more uniformly across the cladding area.
Study Insights and Reflections
The study of rolling mill frame cladding reveals that the integration of wear-resistant overlays into structural components requires careful consideration of the interaction between structural and wear performance requirements. The key insight is that the cladding process must be designed to enhance wear resistance without compromising the structural integrity of the frame. This means that the overlay material and process must be selected to minimize residual stresses, avoid cracking, and maintain the impact toughness and fatigue strength of the base metal. Engineers involved in frame cladding should recognize that the long-term performance of the cladding depends on the quality of the bond interface, the uniformity of the overlay composition, and the ability of the overlay to accommodate the thermal and mechanical cycling experienced in mill service.