Current Status of Continuous Casting Roll Weld Overlay Technology

Literature Overview

The technical review by Yang Zhengyong from Baosteel Equipment Maintenance Company, published in 2006, provides a comprehensive overview of the state of the art in continuous casting roll weld overlay technology at that time. Continuous casting rolls are critical components in steelmaking that directly contact the molten steel and solidifying shell, subjecting them to extreme thermal, mechanical, and chemical loading conditions. The overlay of wear-resistant and thermally conductive materials on roll surfaces is essential for extending roll service life and maintaining casting quality.

This review is particularly valuable because it comes from an industrial practitioner at one of China's largest steel producers, providing direct insight into the practical challenges and solutions encountered in continuous casting roll overlay applications. The perspective is grounded in real production experience rather than purely academic investigation.

Technical Background

Continuous casting rolls experience a complex combination of service conditions that make them one of the most demanding wear applications in metallurgy:

• Thermal loading — Surface temperatures reach 1000–1400 °C during casting, with rapid cooling during contact with water spray
• Mechanical loading — Contact pressure between the roll and solidifying shell can exceed 50 MPa, with cyclic loading during each casting cycle
• Chemical loading — Contact with molten steel and slag leads to oxidation, carburization, and slag penetration
• Thermal shock — Rapid temperature changes during casting and cooling can induce thermal fatigue cracking

Overlay Material Selection

The selection of overlay material depends on the specific roll position and service conditions. Mold rolls, which form the initial solidification shell, require high thermal conductivity and resistance to thermal fatigue. Stripping rolls, which separate the solidified shell from the mold, require high wear resistance and low friction. Intermediate rolls experience a combination of wear and thermal fatigue.

Overlay Process Technologies

Submerged Arc Welding (SAW) Overlay

SAW overlay remains the most widely used process for continuous casting roll overlay due to its high deposition rate, good penetration, and relatively low cost. The process is particularly suitable for overlaying thick layers of wear-resistant materials such as high-speed steel or hardfacing alloys. However, SAW produces a relatively coarse surface finish that requires subsequent machining to achieve the required surface quality for casting applications.

Key process parameters for SAW overlay of casting rolls include:

• Wire diameter: 1.6–3.2 mm
• Welding current: 600–1000 A
• Welding voltage: 28–38 V
• Travel speed: 200–400 mm/min
• Flux type: Low-hydrogen, self-shielded, or specialized hardfacing flux

Gas Metal Arc Welding (GMAW) Overlay

GMAW overlay offers greater flexibility than SAW, particularly for repairing damaged rolls or applying overlay to specific areas. The process produces a finer microstructure and better surface quality than SAW, reducing the amount of subsequent machining required. However, the deposition rate is lower than SAW, making it less economical for full roll overlay applications.

Laser Cladding

By 2006, laser cladding was emerging as a promising technology for continuous casting roll overlay. The process offers several advantages:

• High dilution control (typically 5–15% base metal dilution)
• Fine microstructure with minimal heat-affected zone
• Excellent surface quality requiring minimal machining
• Ability to apply thin, precise overlay layers

The limitations of laser cladding at that time included high equipment cost, limited deposition rate for large area coverage, and challenges with substrate preparation and thermal management.

Electroslag Welding (ESW) Overlay

ESW overlay is particularly suitable for applying thick overlay layers to cylindrical components such as casting rolls. The process produces deep penetration and uniform deposit composition, making it ideal for overlaying thick layers of thermally conductive materials. However, ESW is limited to vertical or near-vertical positions and requires specialized equipment.

Microstructural Considerations

The microstructure of the overlay deposit is critical to roll performance. Key microstructural features include:

Heat-Affected Zone

The HAZ of the overlay weld is often the weakest link in roll performance. Excessive heat input can lead to grain growth, carbide dissolution, and softening in the substrate. Conversely, excessive cooling rates can produce hard, brittle phases in the HAZ that are susceptible to cracking.

Overlay Microstructure

For high-speed steel overlays, the microstructure typically consists of martensite with dispersed carbides. The carbide distribution and morphology directly influence wear resistance and thermal fatigue behavior. Fine, uniformly dispersed carbides provide superior wear resistance compared to coarse, clustered carbides.

For austenitic stainless steel overlays, the retained austenite fraction is critical. Higher retained austenite content improves thermal fatigue resistance through strain-induced transformation toughening but reduces hardness and wear resistance.

Interface Quality

The bond strength between the overlay and substrate is essential for roll integrity. Common interface defects include:

• Lack of fusion due to inadequate cleaning or insufficient heat input
• Slag inclusions trapped at the interface
• Cracking due to thermal stresses or hydrogen embrittlement
• Excessive dilution leading to composition gradient

Quality Control and Inspection

Quality control for continuous casting roll overlay involves multiple inspection methods:

The inspection requirements for casting rolls are stringent because overlay defects can lead to roll failure during casting, resulting in production shutdown and potential safety hazards. The inspection program should be tailored to the criticality of the roll application and the consequences of failure.

Engineering Practice and Case Studies

Baosteel, as one of China's largest steel producers, has extensive experience with casting roll overlay technology. The company has developed specialized overlay procedures and inspection protocols for its various continuous casting lines. Key lessons from industrial practice include:

1. Surface preparation is critical — Inadequate cleaning of the roll surface before overlay leads to lack of fusion and reduced bond strength. The surface should be cleaned to white metal using grinding or shot blasting.
2. Heat input control is essential — Excessive heat input leads to substrate damage and reduced roll life. Process parameters should be optimized to minimize heat input while maintaining adequate penetration.
3. Post-overlay machining is necessary — Even with advanced overlay processes, surface machining is required to achieve the required surface finish for casting applications. The machining allowance should be planned to avoid removing too much or too little overlay material.
4. Roll storage and handling matter — After overlay and machining, rolls should be stored in a controlled environment to prevent corrosion and damage. Improper handling can introduce surface defects that reduce roll life.

Study Insights and Industry Development

This review captures the state of casting roll overlay technology at a particular point in time and provides insight into the challenges and opportunities facing the industry. Several trends are evident:

The industry is moving toward higher-performance overlay materials that can extend roll life and improve casting quality. High-speed steel and ceramic-reinforced overlays are replacing conventional cast iron overlays for mold roll applications, while advanced nickel-based and cobalt-based alloys are being developed for special service applications.

Process technology is evolving toward more precise and flexible methods. Laser cladding and plasma transferred arc (PTA) cladding are gaining acceptance for high-value rolls where the precision and quality of the overlay justify the higher process cost. Hybrid approaches combining multiple overlay processes for different roll sections are also being explored.

Quality control is becoming more sophisticated, with the adoption of advanced non-destructive testing methods such as phased array ultrasonic testing and thermography. In-line monitoring of overlay quality during production is an emerging area that promises to improve consistency and reduce scrap rates.

The economic considerations driving overlay technology development are significant. The cost of roll overlay is a small fraction of the cost of a new roll, and the extended roll life achieved through overlay can significantly reduce production costs. However, the quality and consistency of the overlay must be maintained to avoid costly production interruptions due to roll failure.

This review provides a valuable snapshot of casting roll overlay technology and highlights the continuous evolution of the field. The practical perspective offered by an industrial practitioner ensures that the technical content is directly relevant to production environments, making it a useful reference for engineers involved in casting roll maintenance and improvement programs.