Atmospheric Plasma Sprayed Fe-Based Coating and Argon Arc Remelting Layer Microstructure and Mechanical Properties

Literature Overview

This study by Dong Tianshun, Zheng Xiaodong, Li Guolu, Wang Haidou, Zhou Xiukai, and Li Yalong, published in Materials Reports (Volume 33, Issue 4, 2019, pp. 679-683), investigates the microstructural evolution and mechanical property improvement of Fe-based coatings produced by atmospheric plasma spraying (APS) when subjected to post-spray argon arc remelting treatment. The research was conducted at Hebei University of Technology and the Key Laboratory of Equipment Remanufacturing Technology at the Academy of Armored Force Engineering, supported by the National Natural Science Foundation of China. The work addresses a critical gap in surface engineering technology where thermally sprayed coatings, despite their excellent depositability and versatility, often suffer from inherent porosity, unmelted particles, and weak mechanical interlocking with the substrate.

Core Technical Findings

Microstructural Transformation

The APS Fe-based coating exhibits a typical layered structure characteristic of plasma spraying, with porosity around 4 percent, unmelted particles, and inclusions distributed throughout the deposit. The coating microstructure is composed of microcrystalline regions, nanocrystalline regions, and transition zones, with relatively poor crystallinity and disordered atomic arrangements. The interface between the as-sprayed coating and the substrate shows clear gaps, indicating that bonding is purely mechanical in nature rather than metallurgical.

After argon arc remelting, the layered structure, porosity, unmelted particles, and inclusions are essentially eliminated. The porosity drops dramatically from 4 percent to 0.4 percent, resulting in a dense and homogeneous remelted layer. The remelted layer microstructure consists of single crystal regions and (Fe,Cr)₂₃C₆ phases. Notably, no microcracks are observed at the interface between precipitated phases and the matrix. The crystallinity improves significantly, with more regular atomic arrangements. At the interface between the remelted layer and the substrate, a "white bright band" forms, indicating metallurgical bonding has been achieved.

Mechanical Property Improvement

The remelted layer demonstrates substantially improved hardness and elastic modulus, while the surface roughness is reduced by over 43 percent. The metallurgical bonding at the substrate interface provides far superior adhesion compared to the mechanical interlocking of the as-sprayed coating.

Process Analysis and Technical Insights

The argon arc remelting process effectively consolidates the porous APS deposit by partially or fully melting the sprayed layer, allowing capillary forces and surface tension to eliminate voids. The formation of the "white bright band" at the remelted layer-substrate interface suggests localized melting and resolidification of the substrate surface, which promotes true metallurgical bonding. The appearance of (Fe,Cr)₂₃C₆ carbide phases indicates that carbon from the coating material has reacted with chromium during the remelting process, which contributes to the observed increase in microhardness.

From a metallurgical perspective, the transition from a poorly crystallized, disordered microstructure to a well-crystallized, ordered single crystal and carbide phase structure represents a fundamental improvement in the material's intrinsic properties. The elimination of microcracks at precipitate-matrix interfaces is particularly significant for fatigue and creep resistance in service conditions.

Engineering Practice Implications

For pipeline and pressure vessel applications where corrosion-resistant or wear-resistant surface treatments are required, the combination of APS deposition followed by argon arc remelting offers a practical pathway to achieve dense, well-bonded coatings with enhanced mechanical properties. This hybrid approach is particularly relevant for repairing or refurbishing pipeline components where full component replacement is not economical. The technique could be adapted for overlay welding applications on carbon steel and alloy steel pipelines operating in corrosive environments, provided that thermal distortion and residual stress are properly managed through appropriate heat input control and possibly post-weld stress relief.

Key Reflections

The study demonstrates that post-spray remelting is not merely a surface finishing operation but a fundamental metallurgical treatment that transforms the coating from a mechanically bonded, porous deposit into a metallurgically bonded, dense layer with superior mechanical properties. The 90 percent reduction in porosity alone would significantly improve the coating's resistance to corrosion penetration and fatigue failure. However, engineers must consider that the remelting process introduces additional thermal input to the substrate, which may cause distortion or alter the substrate's heat-affected zone properties, particularly for thin-walled components or materials sensitive to thermal cycles.

Reference Value and Outlook

This research provides a clear technical foundation for hybrid surface engineering approaches in pipeline and equipment refurbishment. Future work should focus on optimizing remelting parameters for different substrate materials and coating thicknesses, evaluating long-term service performance under cyclic loading and corrosion conditions, and extending the methodology to multilayer coatings and functionally graded designs. The principles demonstrated here are directly transferable to overlay welding and cladding operations in the pipeline industry, where achieving dense, crack-free, metallurgically bonded deposits is essential for long-term structural integrity and corrosion resistance.