Microstructure and Properties of Plasma-Clad Nickel-Based Coating on Shaker Hammer Head

Literature Overview

This 2014 study published in the Chinese Journal of Mechanical Engineering by Zhang Guodong, Li Li, Liu Nian, Cao Hongmei, and Mao Yan investigates the microstructure and performance of nickel-based overlay coatings deposited on Q235 steel shaker hammer heads via plasma arc cladding. The research is funded by the Hubei Provincial Natural Science Foundation (2009CDB300), the Central Universities Basic Research Business Fee Special Fund (2012208020201), and the Ministry of Education Doctoral Academic Newcomer Award (5052012208001). Shaker hammer heads are critical components in blast furnace ironmaking operations where they are subjected to severe abrasive, thermal, and corrosive degradation from molten slag and refractory materials.

Core Technical Content and Coating Design

Three different composite nickel-based powder compositions were evaluated: 50%WC, 40%WC, and 30%WC+TiC. The base nickel alloy powder serves as the matrix binder, while the hard carbide particles (WC and TiC) provide the primary wear and corrosion resistance. The plasma arc cladding process was selected for its ability to produce coatings with relatively low dilution rates and good metallurgical bonding to carbon steel substrates.

The selection of these three compositions reflects a systematic approach to understanding the relationship between hard phase content, type, and the resulting coating performance. The inclusion of TiC in Variant C is particularly interesting because titanium carbide possesses higher hardness (2300 HV) and better thermal stability than tungsten carbide (1500 HV), while also providing enhanced resistance to chemical attack.

Microstructural Analysis and Bond Quality

All three coating variants exhibited a microstructure consisting of gamma-Ni solid solution matrix with dispersed hard carbide phases. The WC particles in Variants A and B retained their original morphology to varying degrees, depending on the thermal exposure during cladding. In Variant C, the presence of both WC and TiC particles created a more complex microstructure with multiple hard phase types distributed throughout the nickel matrix.

The (Ti,V)C phase observed in Variant C represents a ternary carbide formed by the interaction of TiC particles with vanadium present in the base nickel alloy powder. This phase transformation is significant because TiC particles can dissolve partially during the high-temperature plasma arc process and reprecipitate as (Ti,V)C, which has different hardness and stability characteristics than pure TiC.

The metallurgical bonding at the coating-substrate interface was confirmed to be good for all three variants, with low dilution rates. The interface examination revealed a diffusion zone of limited thickness, indicating that the dilution effect was well-controlled. This is attributed to the relatively low heat input and high cooling rate characteristic of plasma arc cladding, which limits the depth of substrate melting and the extent of alloy mixing.

Performance Evaluation: Wear, Corrosion, and Thermal Resistance

The wear testing results demonstrated that all three nickel-based coatings exhibited significantly improved wear resistance compared to the uncoated Q235 steel substrate. However, the performance ranking was clear: Variant C (30%WC+TiC) exhibited the highest wear resistance, followed by Variant B (40%WC), and then Variant A (50%WC). This counterintuitive result, where the highest WC content did not produce the best wear resistance, can be explained by the balance between hard phase volume fraction and matrix continuity.

At 50%WC content, the high volume fraction of tungsten carbide particles creates a discontinuous matrix structure with potential inter-particle cracking pathways. The WC particles, while hard, are inherently brittle and can fracture under impact loading, creating wear debris that accelerates further wear. At 40%WC, the matrix continuity is improved while maintaining sufficient hard phase content. The optimal performance of Variant C (30%WC+TiC) is attributed to the synergistic effect of the dual carbide system, where the harder and more thermally stable TiC particles complement the WC particles in resisting both abrasive and thermal degradation.

The electrochemical corrosion testing showed that all three coatings exhibited significantly improved corrosion resistance compared to the base Q235 steel. The nickel-based matrix provides inherent corrosion resistance through the formation of a stable passive film, while the dispersed carbide particles contribute additional resistance by reducing the number of active dissolution sites. Variant C again demonstrated the best corrosion performance, which is attributed to the thermodynamic stability of the TiC phase and its resistance to chemical attack under high-temperature slag conditions.

Engineering Practice and Defect Prevention

The plasma arc cladding of nickel-based coatings on carbon steel hammer heads presents several practical challenges that must be addressed in industrial implementation:

1. Powder handling and storage: Nickel-based composite powders are hygroscopic and must be stored under controlled humidity conditions to prevent moisture absorption, which can lead to porosity in the deposited coating.
2. Preheating: Q235 steel substrates should be preheated to 150-250 degrees Celsius to reduce thermal gradients and minimize the risk of cracking at the coating-substrate interface.
3. Multi-pass deposition: For coating thicknesses exceeding 1 mm, multiple passes are required with interpass temperature control to prevent excessive heat accumulation and substrate dilution.
4. Surface preparation: The substrate surface must be thoroughly cleaned of scale, rust, and contaminants to ensure proper metallurgical bonding. Shot blasting or grinding to a clean metal surface is recommended.
5. Post-clad treatment: Stress relief annealing at 600-700 degrees Celsius for 1-2 hours is recommended to reduce residual stresses and improve coating durability.

Common defects and their prevention strategies include:

Study Insights and Implications

The most significant finding of this research is the demonstration that a dual carbide system (WC+TiC) outperforms a single carbide system (WC alone) in both wear and corrosion resistance, even when the total hard phase volume fraction is lower. This finding has profound implications for coating design philosophy: the quality and type of hard phase distribution are more important than simply maximizing the volume fraction of hard particles.

The optimal performance of the 30%WC+TiC variant suggests that coating engineers should focus on developing multi-carbide systems that combine particles with complementary properties rather than simply increasing the content of a single carbide type. This approach creates a more robust microstructure where different carbide phases contribute to different aspects of wear and corrosion resistance.

For blast furnace operations in the iron and steel industry, this technology offers a practical solution for extending the service life of shaker hammer heads, which are among the most rapidly consumed refractory-protected components in the blast furnace system. The improved performance of the nickel-based dual carbide coating could reduce replacement frequency by a factor of 2-3, resulting in significant cost savings and reduced unplanned maintenance downtime.

This study provides valuable guidance for the rational design of nickel-based overlay coatings for severe service applications and establishes a methodology for optimizing hard phase content and type through systematic experimental evaluation.