<p>Composite rollers serve as critical components in modern slab continuous casting systems, particularly the No. 0 segment rollers positioned at the mold exit, which endure severe service conditions and intermittent thermal shocks. The service stability of these composite rollers directly determines the continuous operation capability of casting lines and per-ton steel cost control, making reliability enhancement a critical research topic in the field of metallurgical equipment. In this study, 414N/42CrMo heterogeneous composite rollers with gradient properties were fabricated by additively manufacture. The multi-scale characterization techniques were conducted on the composite rollers after online service for 2.5 months in a continuous casting line, including SEM, EBSD, and X-ray computed tomography for 3D defect mapping. The elemental interdiffusion kinetics, microstructure, and defect distribution were systematically elucidated. The directional migration of Cr/Mo elements enables the formation of a gradient interfacial architecture: heterogeneous interfacial thickness with alternating peak-valley morphology driven by differential elemental diffusion behaviors, progressive transitions in grain boundary configurations from low-angle to high-angle, and gradient variations in grain size distribution across adjacent microstructural zones. X-ray computed tomography reveals that the defect distribution across the entire interfacial region exhibits spatial heterogeneity, characterized by the presence of distinct defect clusters from the 414N cladding layer near the interface, accompanied by localized aggregations of irregular defects (with an equivalent diameter greater than 50 µm). Therefore, a remanufacturing strategy is proposed to eliminate defect-rich interfacial layers and reconstruct gradient-compatible interfaces for performance restoration, which offers practical solutions for extending service life through precision remanufacturing.</p>