Development of Cladding Materials and Processes for Long-Life Continuous Casting Rolls

Literature Overview

This study, conducted jointly by Northeastern University and Baosteel Research Institute under National Natural Science Foundation grant 50674022 and Baosteel major research project ZA9815, was published in the Journal of Northeastern University (Natural Science) in 2008. The research addresses the critical engineering challenge of extending the service life of continuous casting copper rolls, which are subjected to extreme thermal cycling, molten steel erosion, and mechanical loading during the steelmaking process. The authors investigated the development of specialized cladding materials and overlay welding processes aimed at achieving significantly improved wear and thermal fatigue resistance compared to conventional roll designs.

Core Technical Content

The fundamental problem addressed in this work is that continuous casting rolls experience a combination of thermal shock, thermal fatigue, and abrasive wear from the solidifying steel shell. Traditional monolithic copper rolls suffer from limited service life, typically requiring replacement every 150 to 300 heats depending on the steel grade being cast. The researchers proposed a multi-layer cladding approach where a functionally graded overlay system is deposited onto the roll substrate to create a composite structure with optimized mechanical and thermal properties.

The cladding material system developed in this study incorporated a base layer of Ni-based alloy for bonding compatibility with the copper substrate, followed by an intermediate transition layer to manage thermal expansion mismatch, and a surface wear layer containing hard carbide-forming elements such as chromium and tungsten. The thermal fatigue resistance was evaluated through thermal cycling tests simulating actual casting conditions, where the roll surface temperature was cycled between room temperature and approximately 1200°C to represent the thermal gradient experienced during solidification of the steel shell.

Process Development and Key Findings

The overlay welding process selected for the cladding operation was submerged arc welding (SAW), chosen for its high deposition rate, excellent penetration, and suitability for thick multi-layer deposits on cylindrical geometries. The process parameters were carefully optimized to minimize dilution between the cladding layers while ensuring full metallurgical bonding. A critical finding of this research was that the thermal expansion coefficient mismatch between the copper substrate (approximately 17×10⁻⁶/K) and the Ni-based cladding layers (approximately 13×10⁻⁶/K) must be managed through intermediate transition layers with gradually varying composition.

The researchers employed a stepwise thermal expansion matching strategy where each layer's composition was designed to have a thermal expansion coefficient intermediate between the adjacent layers. This approach effectively reduced the residual stresses developed during cooling and during subsequent thermal cycling in service. The interlayer bond strength was verified through shear testing and metallographic examination, confirming full metallurgical bonding without interfacial defects such as cracks, pores, or unmelted particles.

The thermal fatigue resistance improvement was attributed to two mechanisms: first, the high hardness of the surface layer (500-650 HV) provided excellent resistance to abrasive wear from the solidifying steel shell; second, the functionally graded structure reduced thermal stresses at the substrate-cladding interface by accommodating differential thermal expansion through the gradual compositional transition. The results demonstrated that the clad rolls could withstand 3 to 5 times more thermal cycles before surface cracking initiated compared to conventional copper rolls.

Engineering Practice Implications

From an engineering application perspective, this research has direct relevance to steel plant operators seeking to reduce roll replacement frequency and associated production downtime. The key implementation considerations include: preheating the copper roll to 200-300°C before cladding to reduce thermal stresses; controlling the interpass temperature between 150-250°C to prevent excessive softening of previously deposited layers; and performing post-weld stress relief annealing at 600°C for 2 hours to reduce residual stresses below the fatigue limit.

Quality control during production cladding operations should include ultrasonic testing (UT) of the bond interface, hardness profiling across the full cladding thickness, and thermal cycling qualification testing on coupon samples before full-scale roll cladding. The economic benefit is substantial: extending roll life from 200 to 800 heats reduces the cost per ton of steel produced by approximately 30-40% in terms of roll-related expenses alone.

Study Insights and Reflections

This research represents a mature application of functionally graded materials (FGM) concepts to an industrial welding problem. The systematic approach of matching thermal expansion coefficients through layer-by-layer composition design is a methodology that can be applied to other bimetallic composite components in the steel and energy industries. The success of this project demonstrates that careful materials design combined with appropriate welding process control can dramatically extend component life without requiring changes to the base material or the operating environment. The practical significance is underscored by the fact that Baosteel, one of China's largest steel producers, funded this work as a major research project, indicating clear industrial demand for improved roll technology.