CLADDING TECHNOLOGY SHANXI CO., LTDStrip Cladding Technology for Rolls of Double-Roll Reversible Rolling Mills
Literature Overview
The study by Gan Meijun from Ningbo Institute of Metallurgy, published in Shanghai Metals in 2001, addresses a critical industrial challenge: the restoration and enhancement of work rolls in double-roll reversible rolling mills through strip cladding technology. Double-roll reversible mills are workhorses of the steel industry, where the rolls endure extreme thermal cycling, mechanical wear, and oxidation under the demanding conditions of hot rolling. The original roll surface degrades rapidly, leading to reduced product quality, shortened service intervals, and significant downtime costs. This literature presents a systematic approach to applying wear-resistant overlay layers using the strip cladding method, which offers superior metallurgical bonding and uniform deposition compared to conventional arc welding techniques.
Core Technical Points
Material Selection and Metallurgical Considerations
The selection of cladding materials for reversible mill rolls requires careful consideration of the service environment. The overlay layer must resist high-temperature oxidation, thermal fatigue cracking, and abrasive wear simultaneously. Typically, high-carbon, high-chromium martensitic stainless steels or austenitic stainless steels are selected as the cladding material. The carbon content is generally controlled between 0.5% and 1.2%, while chromium content ranges from 12% to 26%, depending on whether the primary requirement is oxidation resistance or wear resistance. The dilution ratio between the cladding material and the base roll steel is a critical parameter that directly affects the final properties of the overlay.
| Parameter | Typical Value | Purpose |
|---|---|---|
| Carbon content (cladding) | 0.5-1.2 wt% | Hardenability and wear resistance |
| Chromium content (cladding) | 12-26 wt% | Oxidation and corrosion resistance |
| Dilution ratio | 10-30% | Balance hardness and toughness |
| Overlay thickness | 3-8 mm | Wear life and thermal stress accommodation |
| Hardness (HRC) | 45-58 | Wear resistance |
Electroslag Welding Process Parameters
The strip cladding process employs electroslag welding (ESW) with a consumable strip electrode, which provides deep penetration and uniform heat input distribution. The process parameters are carefully optimized to ensure complete bonding between the overlay and the base roll steel while minimizing thermal distortion and residual stresses.
| Process Parameter | Range | Notes |
|---|---|---|
| Welding current | 400-600 A | Depends on strip thickness |
| Welding voltage | 28-38 V | Maintains stable slag pool |
| Travel speed | 100-200 mm/min | Controls dilution and cooling rate |
| Strip thickness | 2-4 mm | Affects penetration profile |
| Preheat temperature | 200-350 C | Reduces cracking risk |
| Interpass temperature | 250-400 C | Controls microstructure |
| Flux composition | CaF2-Al2O3-SiO2 system | Stabilizes slag pool |
The flux composition plays a vital role in the electroslag welding process. A CaF2-Al2O3-SiO2 based flux system is typically employed to ensure stable slag pool formation, adequate deoxidation, and proper wetting of the strip electrode. The viscosity of the slag pool must be carefully controlled to maintain a uniform weld profile and prevent undercut or excessive reinforcement.
Process Implementation and Quality Control
Pre-Welding Preparation
The surface preparation of the roll is a critical step that directly influences the quality of the cladding layer. The original roll surface must be machined to remove any damaged or oxidized layers, and the surface roughness should be controlled to ensure uniform contact with the cladding material. Surface cleaning with grinding or shot blasting removes contaminants that could lead to porosity or lack of fusion defects.
Post-Weld Heat Treatment
After the cladding process is completed, the roll must undergo a controlled heat treatment to relieve residual stresses and optimize the microstructure. The typical heat treatment cycle involves:
- Stress relief annealing at 550-650 C for 2-4 hours, followed by furnace cooling to room temperature
- Optional tempering treatment at 400-500 C to adjust the hardness of the overlay layer
- Controlled cooling rate to prevent thermal cracking during the cooling stage
The cooling rate during post-weld heat treatment is particularly important. Too rapid cooling can lead to the formation of brittle martensite in the overlay layer, increasing the risk of thermal fatigue cracking during service. Conversely, too slow cooling may result in excessive grain growth and reduced hardness.
Non-Destructive Testing Requirements
The quality of the cladding layer is verified through a combination of non-destructive testing methods:
| NDT Method | Purpose | Acceptance Criteria |
|---|---|---|
| Magnetic particle testing (MT) | Surface cracks | No linear indications |
| Ultrasonic testing (UT) | Lack of fusion, internal defects | Per GB/T 11345 |
| Hardness testing | Uniformity and adequacy | Within specified range |
| Penetrant testing (PT) | Surface defects | No linear indications |
| Metallographic examination | Bond strength, microstructure | No cracks at interface |
Engineering Challenges and Countermeasures
The application of strip cladding to reversible mill rolls presents several engineering challenges that must be addressed:
- Thermal distortion: The asymmetric heat input can cause the roll to warp, which affects the dimensional accuracy of the rolled product. Countermeasures include symmetric welding sequences, fixture design, and controlled cooling.
- Residual stress: High residual stresses in the overlay layer can lead to delamination during service. Stress relief heat treatment and proper welding sequence planning are essential.
- Interface bonding: Incomplete metallurgical bonding between the overlay and base material can result in spalling. Ensuring adequate heat input and proper surface preparation is critical.
- Cracking susceptibility: The high-carbon, high-chromium overlay material is susceptible to cold cracking. Preheating and controlled interpass temperature are necessary.
Integration with Engineering Practice
In practical applications at steel mills, the strip cladding process is typically performed on dedicated roll refurbishment lines. The rolls are machined to the required dimensions, the cladding is applied in multiple passes to achieve the target thickness, and the final surface is ground to the precise geometry required for the rolling operation. The entire process, from strip cladding to final machining, typically requires 48-72 hours per roll set, depending on the number of passes required and the heat treatment cycle.
The economic benefits of strip cladding are substantial. Compared to full roll replacement, the cladding approach reduces material costs by approximately 60-70% and reduces downtime by 40-50%. Furthermore, the ability to select the optimal cladding material for specific service conditions allows for performance optimization that is not achievable with standard roll materials.
Key Questions and Reflections
The literature raises several important questions that deserve further investigation. First, the long-term service life of strip-cladded rolls under aggressive thermal cycling conditions is not fully characterized in this study. Accelerated thermal fatigue testing and field service data would provide valuable validation of the predicted performance. Second, the influence of the number of cladding passes on the microstructure and properties of the overlay layer warrants systematic study, as each additional pass introduces additional thermal cycles that can modify the microstructure. Third, the interaction between the cladding material and the base roll steel during the welding process, particularly regarding elemental diffusion and the formation of intermetallic compounds at the interface, requires further metallographic investigation.
Study Insights and Implications
This study demonstrates that strip cladding is a mature and reliable technology for the refurbishment of reversible mill rolls. The key to successful application lies in the careful selection of cladding materials, optimization of welding parameters, and rigorous quality control. The electroslag welding process provides the deep penetration and uniform heat distribution required for thick overlay deposits, while the strip electrode ensures consistent composition and properties throughout the overlay layer.
The practical implications of this research extend beyond roll refurbishment. The methodology of material selection, process optimization, and quality control can be applied to other cladding applications in the steel industry, including guide rolls, backup rolls, and other wear-critical components. The systematic approach to balancing wear resistance, thermal fatigue resistance, and toughness provides a framework that can be adapted to different service conditions.
In conclusion, this literature provides a solid foundation for understanding the application of strip cladding technology to reversible mill rolls. The combination of appropriate material selection, optimized process parameters, and rigorous quality control ensures reliable performance in demanding industrial environments. Future research should focus on extending service life predictions through accelerated testing and field validation, and on developing new cladding materials that offer even better performance under extreme thermal and mechanical loading conditions.