Cladding Material and Process Development for Long-Life Continuous Casting Rolls

Literature Overview

Continuous casting rolls are among the most critical components in steelmaking operations, subjected to extreme thermal cycling, chemical attack from molten steel, and mechanical abrasion from the solidifying shell. The development of appropriate cladding materials and processes is therefore essential for extending roll service life, reducing unplanned downtime, and improving overall production economics. This literature review examines the selection of cladding materials, the evaluation of different cladding processes, and the engineering considerations for achieving maximum roll longevity.

Cladding Material Selection

The cladding material for continuous casting rolls must satisfy several demanding requirements simultaneously: resistance to thermal shock, resistance to chemical attack by molten steel, adequate thermal conductivity to facilitate heat extraction, and sufficient hardness to resist abrasion from the steel shell. Based on these requirements, several material systems have been evaluated and implemented in industry.

The selection of material depends heavily on the specific casting application. For slab casting, where the roll diameter is large and the thermal gradient is moderate, high-speed steel cladding offers a good balance of hardness and thermal fatigue resistance. For strip casting, where the roll diameter is small and the thermal cycling frequency is high, materials with superior thermal shock resistance such as H13 or nickel-based alloys are preferred.

Cladding Process Evaluation

Several cladding processes have been evaluated for continuous casting rolls, each with distinct advantages and limitations:

Plasma Transfer Arc (PTA) Cladding

PTA offers precise control over dilution and layer composition, making it suitable for applying high-alloy overlays on carbon steel roll cores. The process is well-suited for automated production and can produce smooth, defect-free surfaces that require minimal post-grinding. Typical parameters include a current of 200-280 A, a travel speed of 150-300 mm/min, and a powder feed rate of 0.8-1.5 kg/h. The dilution ratio can be controlled to 10-15%, ensuring that the overlay retains its designed microstructure and properties.

Electroslag Welding (ESW) Cladding

ESW is commonly used for applying thick cladding layers on large-diameter rolls. The process produces a deep, uniform melt pool with excellent bonding strength. However, the high heat input associated with ESW can lead to significant distortion and residual stress, requiring stress-relief heat treatment after cladding. The process is best suited for batch production of large rolls where throughput is a priority.

High-Frequency Induction Heating Cladding

This process is particularly effective for applying localized cladding to the working surface of rolls. The rapid heating and cooling cycle produces a fine-grained microstructure with high hardness. The process is well-suited for repair applications where only a portion of the roll surface requires cladding. However, the limited depth of penetration restricts the maximum cladding thickness to approximately 3-5 mm per pass.

Laser Cladding

Laser cladding offers the highest precision and lowest dilution among all cladding processes. The rapid solidification produces fine microstructures with excellent mechanical properties. However, the process is limited by deposition rate and equipment cost, making it more suitable for high-value, small-diameter rolls or for applying thin, wear-critical layers over a base cladding.

Process Development and Optimization

The development of a robust cladding process for continuous casting rolls involves a systematic approach that addresses material selection, process parameter optimization, quality control, and performance validation. The following framework outlines the key steps:

1. Material-substrate compatibility assessment: Evaluate the thermal expansion coefficient mismatch between the cladding material and the roll core material. A mismatch exceeding 1.5 x 10^-6 /K may lead to interfacial cracking during thermal cycling.
2. Process parameter optimization: Conduct a Design of Experiments (DOE) study to identify the optimal combination of current, travel speed, powder feed rate, and interpass temperature. The objective is to minimize dilution while ensuring complete bonding and defect-free deposition.
3. Heat treatment strategy: Determine whether post-cladding heat treatment is required. For high-alloy cladding materials such as Stellite 6 or high-speed steel, a tempering treatment at 550-650 degrees Celsius for 2-4 hours is typically necessary to relieve residual stress and stabilize the microstructure.
4. Surface finishing: After cladding, the roll surface must be ground to achieve the required dimensional accuracy and surface finish. The grinding allowance should be planned during the cladding process design to ensure that the functional cladding layer is not removed during finishing.
5. Quality inspection: Implement a comprehensive inspection protocol including ultrasonic testing (UT) for subsurface defects, magnetic particle testing (MT) for surface cracks, and hardness profiling across the cladding thickness.

Engineering Practice Cases

In a steel mill producing hot-rolled strip, the continuous casting rolls were originally clad with a Cr12MoV alloy using ESW. The service life was approximately 2.5 months before the rolls required replacement due to surface cracking and abrasive wear. After switching to a two-layer cladding strategy consisting of a Ni-based bonding layer (0.5 mm) followed by a high-speed steel working layer (2.5 mm) applied by PTA, the service life was extended to 5.5 months. The Ni-based bonding layer accommodated the thermal expansion mismatch between the carbon steel core and the high-speed steel working layer, significantly reducing interfacial cracking.

Another case involved the cladding of thin-strip casting rolls with a diameter of 350 mm. The small roll diameter and high thermal cycling frequency posed significant challenges. A laser cladding process was adopted to apply a 1.5 mm thick Ni-based alloy layer with a dilution ratio of less than 5%. The resulting rolls achieved a service life of 4 months, a 60% improvement over the previous unclad rolls.

Key Defects and Countermeasures

Study Insights and Implications

The literature reinforces the principle that roll cladding is not a standalone surface treatment but an integral part of the roll design philosophy. The cladding material, process, and heat treatment must be selected in concert with the roll core material, the casting conditions, and the expected service environment. A systematic approach that considers the entire lifecycle of the roll, from fabrication through service to repair, leads to superior performance and economic outcomes.

The concept of a multi-layer cladding strategy, where different materials are applied in sequence to address different performance requirements, is particularly valuable. The bonding layer ensures metallurgical compatibility, while the working layer provides the required wear and thermal resistance. This approach, while more complex to implement, offers a significantly higher return on investment through extended service life and reduced downtime.

Conclusion

The development of effective cladding solutions for continuous casting rolls requires a multidisciplinary approach that integrates materials science, welding engineering, and process technology. The selection of cladding material and process must be driven by the specific demands of the casting application, and the process must be rigorously qualified through systematic experimentation and field validation. The engineering community should continue to explore advanced cladding technologies such as laser cladding and hybrid processes to further extend roll life and improve the economics of steelmaking operations.