Microstructure Analysis of Plasma Overlay Welded Ni-Coated SiCp Reinforced Cobalt-Based Overlay

Literature Overview

This paper published in Special Casting and Nonferrous Alloys in 2016 by Pan Chenggang, Xiao Qin, Yang Huqun, Ma Wenchao, Chang Qingming from Wuhan University of Science and Technology and Wang Huachang from Wuhan University of Technology investigates the microstructure of cobalt-based overlay layers reinforced with Ni-coated SiCp (silicon carbide particles) produced by plasma overlay welding. Funded by the National Natural Science Foundation of China (Project 51375353) and the Provincial-Ministry Co-built Key Laboratory of Refractory Materials and Metallurgy (Project 2014QN03), this study addresses a critical challenge in ceramic-reinforced overlay welding: reducing SiCp burn-off and improving particle wetting during the welding process. The work appears on pages 788-791 of Volume 36, Issue 8.

Core Technical Content

Ceramic particle-reinforced metal matrix composites offer exceptional wear resistance due to the high hardness of ceramic particles. However, incorporating ceramic particles into overlay weld deposits presents significant challenges:

• Particle burn-off: SiC reacts with carbon and oxygen during the high-temperature welding process, forming CO gas and reducing the effective ceramic content.
• Poor wetting: SiC particles exhibit poor wetting by molten metals, leading to particle debonding and reduced composite integrity.
• Particle agglomeration: Without proper mixing, particles tend to cluster, creating weak regions in the composite.
• Thermal shock cracking: Thermal expansion mismatch between ceramic particles and metal matrix can cause cracking during cooling.

Ni-Coating Solution

The researchers address these challenges through Ni-coating of SiCp particles. The nickel coating serves multiple functions:

1. Reaction barrier: The Ni layer prevents direct contact between SiC and the molten metal, reducing chemical reactions that cause particle degradation.
2. Wetting improvement: Ni has excellent wetting characteristics with both SiC and cobalt-based alloys, serving as a bridge between the ceramic particle and the metal matrix.
3. Bond strength enhancement: The Ni coating creates a metallurgical bond between the SiC particle and the cobalt matrix, improving particle-matrix adhesion.
4. Thermal stress accommodation: The ductile Ni layer can accommodate thermal expansion mismatch between SiC and the cobalt matrix, reducing cracking tendency.

Microstructural Characterization

The plasma overlay welded composite deposit exhibits a layered microstructure:

The microhardness exhibits a gradient distribution from the surface (approximately 800 HV) toward the substrate, reflecting the decreasing SiCp content and increasing matrix dominance in the depth direction.

Phase Identification

X-ray diffraction analysis identifies the following phases in the overlay layer:

• Cr silicides: Formed by reaction between Cr from the cobalt alloy and Si from SiCp particles
• Cr23C6 carbide: Primary chromium carbide phase providing hardness
• Cr7C3 carbide: Secondary chromium carbide phase
• Co solid solution: Matrix phase providing toughness and ductility
• SiCp particles: Residual silicon carbide particles, primarily in the top layer

The presence of Cr silicides indicates some degree of SiCp reaction, but the Ni-coating effectively limits this reaction compared to uncoated SiCp systems.

Plasma Overlay Welding Process Optimization

The modified plasma spray welding torch developed by the researchers addresses specific challenges of ceramic particle delivery:

The plasma overlay welding process parameters are critical for achieving optimal composite properties:

• Plasma power: Controls melting temperature and dilution level
• Travel speed: Affects cooling rate and microstructure refinement
• Powder feed rate: Determines deposition rate and particle content
• Arc length: Influences heat input and powder melting efficiency
• Shielding gas: Protects molten pool from atmospheric contamination

Engineering Applications

The Ni-coated SiCp reinforced cobalt-based overlay offers significant advantages for steel pipe and pipe fitting applications:

• Abrasive service: The high hardness (800 HV) provides excellent resistance to abrasive wear in applications such as sand handling pipes, slurry transport systems, and mining equipment.
• High-temperature wear: Cobalt-based alloys maintain hardness at elevated temperatures, making them suitable for hot work applications including furnace components and hot gas ducts.
• Corrosive-abrasive environments: The combination of cobalt alloy corrosion resistance and SiCp wear resistance addresses dual degradation mechanisms common in chemical processing and environmental protection applications.

The gradient hardness profile provides an additional benefit by transitioning smoothly from the hard surface layer to the tougher substrate, reducing the risk of delamination under cyclic loading.

Key Reflections and Study Insights

This study demonstrates a sophisticated approach to overcoming the fundamental challenges of ceramic particle incorporation in overlay welding. The Ni-coating strategy represents a materials engineering solution that addresses multiple failure mechanisms simultaneously through a single modification.

The gradient microstructure observed in the deposit reflects the natural solidification behavior of the plasma overlay welding process, where particle concentration decreases with depth due to preferential particle settling and limited particle transport. Understanding this gradient behavior is essential for predicting service performance and optimizing the overlay design.

For engineers in the steel pipe industry, this research highlights the potential of particle-reinforced overlay welding as a technology for addressing severe wear conditions. The combination of high surface hardness, thermal stability, and corrosion resistance offered by Ni-coated SiCp reinforced cobalt-based overlays represents a significant advancement in surface engineering capabilities.

The modified plasma torch design developed by the researchers addresses practical implementation challenges, demonstrating that process innovation combined with materials engineering can overcome historical limitations in ceramic-metal composite welding. This integrated approach to problem solving provides a model for future development of advanced overlay welding technologies.