High-Temperature Damage Mechanism of Ni–Co Coatings on the Narrow Face Copper Plates of Slab Mold with High-Speed Continuous Casting
摘要
The service life of the narrow face copper plates in high-speed slab continuous casting molds is limited by the thermal fatigue damage of Ni–Co coatings. This study systematically investigates the high-temperature damage mechanism of the coating by integrating industrial failure analysis, 3D thermo-mechanical coupled numerical simulation, and high-temperature experimental characterization. Results show that the meniscus region (100 to 140 mm below the mold top) of the narrow face is a high-risk zone for crack initiation, exhibiting peak temperature of 386.5 °C and von Mises stress of 397.7 MPa. This stress exceeds the yield strength of the coating at the corresponding temperature (357 MPa), causing plastic strain accumulation. Microstructural analysis reveals that elevated temperatures transform the initial bimodal nanocrystalline structure (average grain size approximately 235 nm) into micron-sized grains (approximately 1.45 μm), while geometrically necessary dislocation (GND) density decreases substantially from 10.04 × 1012 to 2.87 × 1012 m−2. These microstructural evolutions reduce grain boundary strengthening from 373 to 150 MPa and twin boundary strengthening from 117 to 62 MPa, fundamentally explaining the high-temperature softening and yield strength degradation. This study clarifies that coating cracking is essentially a thermal fatigue process driven by both thermo-mechanical overload and microstructural instability, providing a theoretical basis for developing high-performance crack-resistant Ni–Co coatings to extend the service life of high-speed continuous casting molds.