Zhuojin Pipe Fitting Co., LtdCurrent Status of Continuous Casting Roll Hardfacing Technology
Literature Overview
The review article by Yang Zhengyong from Baosteel Equipment Maintenance Company, published in Welding (2006, No. 10, pp. 57-59), provides a comprehensive overview of the hardfacing technology used for continuous casting rolls in steel enterprises. Continuous casting is the backbone of modern steelmaking, and the rolls used in the casting process are subjected to extreme thermal cycling, mechanical loading, and chemical attack from molten steel and mold flux. Hardfacing is the primary method for extending roll service life and maintaining casting quality.
Role of Continuous Casting Rolls in Steel Production
Continuous casting rolls serve several critical functions in the steelmaking process:
- Support and shaping: The rolls support the weight of the solidifying steel shell and guide it through the mold and secondary cooling zones.
- Heat extraction: The rolls extract heat from the molten steel to promote controlled solidification.
- Surface quality: The roll surface condition directly affects the surface quality of the cast product.
The operating environment for casting rolls is extremely harsh:
| Parameter | Typical Value | Challenge |
|---|---|---|
| Steel shell temperature | 1300-1500°C | Extreme thermal load |
| Thermal cycling rate | Rapid heating and cooling | Thermal fatigue |
| Mechanical load | 20-50 MPa contact pressure | Wear and deformation |
| Mold flux contact | Continuous chemical exposure | Corrosion and erosion |
| Cooling water | 15-30°C | Thermal shock |
Hardfacing Technology Requirements
The hardfacing deposits on casting rolls must meet several demanding requirements:
- Thermal fatigue resistance: The overlay must withstand repeated thermal cycling without cracking or spalling.
- Wear resistance: The surface must resist abrasive and adhesive wear from the moving steel shell.
- Thermal conductivity: The overlay should facilitate heat extraction from the steel shell.
- Weldability: The overlay must bond strongly to the roll substrate without cracking.
- Surface quality: The overlay surface must be smooth enough to produce high-quality cast products.
Hardfacing Processes and Materials
The review discusses several hardfacing processes used for casting rolls:
Submerged Arc Welding (SAW)
SAW is widely used for applying thick, uniform overlay layers to casting rolls. The process offers high deposition rates and good penetration. Common SAW overlay compositions include:
- High-silicon cast irons: Excellent thermal fatigue resistance but lower wear resistance.
- Nickel-based alloys: Good combination of thermal fatigue resistance and wear resistance.
- Stainless steels: Moderate performance with good weldability.
Plasma Transfer Arc Welding (PTAW)
PTAW offers precise control over the overlay composition and microstructure through the use of external powder feeding. This allows for tailored overlay designs with optimized properties for specific service conditions.
Electron Beam Welding (EBW)
EBW provides very high energy density and minimal dilution with the base metal. It is particularly suitable for applying thin, high-quality overlay layers.
Laser Cladding
Laser cladding offers excellent control over the overlay microstructure and minimal thermal distortion. It is increasingly used for repair and surface treatment of casting rolls.
Typical Overlay Compositions and Performance
The review highlights several commonly used overlay compositions for casting rolls:
| Overlay Type | Typical Composition | Hardness | Thermal Fatigue Life | Application |
|---|---|---|---|---|
| High-Si Cast Iron | 15-20% Si, 3-4% C | 500-600 HV | Excellent | Mold rolls |
| Ni-Base Alloy | Ni-Cr-Si | 300-400 HV | Good | Secondary cooling rolls |
| Stainless Steel | Fe-Cr-Ni | 250-350 HV | Moderate | General purpose |
| Cr-Cu Alloy | Cr-Cu-Ni | 200-300 HV | Good | Hot face applications |
Engineering Practice Considerations
From a practical standpoint, several factors must be considered when applying hardfacing to casting rolls:
- Pre-heating: Adequate pre-heating (typically 200-300°C) is essential to prevent cracking in the overlay and the base metal.
- Interpass temperature control: Maintaining interpass temperatures within specified limits prevents excessive grain growth and cracking.
- Post-weld heat treatment: Stress relief annealing after hardfacing is often required to reduce residual stresses.
- Surface finishing: The overlay surface must be ground or turned to achieve the required surface roughness for quality casting.
Study Insights and Reflections
This review article, while published in 2006, remains relevant as a foundational reference for understanding the hardfacing technology landscape for continuous casting rolls. The evolution of hardfacing technology for casting rolls has continued to advance, with newer processes such as cold spray and advanced laser cladding gaining prominence. However, the fundamental principles discussed in this review—balancing thermal fatigue resistance, wear resistance, and weldability—remain the core considerations in overlay design.
One notable observation is the trade-off between thermal fatigue resistance and wear resistance. High-silicon cast irons excel in thermal fatigue resistance but are relatively soft and susceptible to wear. Nickel-based alloys offer a better balance but at higher cost. The selection of the optimal overlay composition depends on the specific service conditions, including the type of steel being cast, the casting speed, and the cooling intensity.
The article also implicitly highlights the importance of process control in hardfacing applications. Even with the best overlay material, improper welding parameters can lead to defects such as cracking, porosity, and poor bonding, which compromise the service life of the roll.