Microstructure and Performance of Nickel-Based Coatings on Shell-Breaking Hammer Heads by Plasma Cladding

Literature Overview

This study by Zhang Guodong, Li Li, Liu Nian, Cao Hongmei, and Mao Yan from Wuhan University, Shanghai Aircraft Manufacturing Company, and Shanghai Jiao Tong University investigates the microstructure and performance of nickel-based overlay coatings deposited on Q235 steel shell-breaking hammer heads via plasma arc cladding. The research was supported by the Hubei Provincial Natural Science Foundation (2009CDB300), Central University Basic Research Business Fee Special Fund (2012208020201), and Ministry of Education Doctoral Academic Newcomer Award (5052012208001), and was published in the Chinese Journal of Mechanical Engineering, Volume 50, Issue 20, pages 70-76, in 2014.

Experimental Design and Coating Compositions

The authors designed three different nickel-based composite powder compositions for plasma arc cladding: one containing 50% WC, another containing 40% WC, and a third containing 30% WC plus TiC. The Q235 carbon steel substrate represents a typical low-cost structural steel used for hammer head fabrication in foundry and metalworking applications. The inclusion of carbide reinforcement particles (WC and TiC) in the nickel-based powder is a well-established approach to enhancing wear and corrosion resistance through the dispersion of hard ceramic-like phases in a ductile metallic matrix.

The characterization methods employed include metallographic microscopy for microstructural observation, scanning electron microscopy for detailed morphology analysis, microhardness testing for mechanical property evaluation, and friction-wear testing for tribological assessment. Corrosion resistance was evaluated through electrochemical testing.

Microstructural Characteristics

All three nickel-based cladding layers exhibit a microstructure consisting of gamma-Ni solid solution matrix with dispersed hard compound phases in different morphologies, including WC and (Ti,V)C carbides. The gamma-Ni matrix provides ductility and corrosion resistance, while the dispersed carbide particles provide wear resistance through their high hardness. The interface between the cladding layer and the Q235 steel substrate shows good metallurgical bonding with low dilution rate, which is a critical requirement for the functional integrity of the coating.

The comparative study reveals that the coating containing 30% WC plus TiC exhibits superior wear resistance and thermal corrosion resistance compared to the coatings containing 50% WC or 40% WC alone. This result is metallurgically significant because TiC is an extremely hard carbide with a hardness of approximately 2800 HV, comparable to VC and harder than WC (approximately 2300 HV). The combination of WC and TiC provides a dual-hard-phase system where the two carbides may interact synergistically to improve overall coating performance.

Performance Comparison and Engineering Implications

The coating with 30% WC plus TiC demonstrates the best comprehensive performance among the three compositions tested. The addition of TiC to the WC-containing nickel-based powder appears to improve both wear resistance and thermal corrosion resistance simultaneously. This finding is important for shell-breaking hammer head applications where the hammer is subjected to both mechanical impact wear and thermal exposure from hot metal surfaces.

The good metallurgical bonding at the cladding-substrate interface with low dilution rate is a key quality indicator. In plasma arc cladding, dilution occurs when the molten substrate mixes with the deposited material, altering the intended composition of the cladding layer. Low dilution is essential for maintaining the desired properties of the coating, and the plasma arc process generally achieves dilution rates in the range of 10-30%, which is acceptable for most industrial applications.

Study Insights and Reflections

This research provides practical guidance for selecting nickel-based overlay coatings for hammer head applications in foundry and metalworking environments. The finding that a mixed carbide system (WC plus TiC) outperforms a single carbide system (WC alone) at a lower total carbide content is particularly noteworthy from a cost-effectiveness perspective. Engineers should consider that the optimal carbide content and type depend on the specific service conditions, including the type of wear (abrasive, adhesive, erosive), temperature exposure, and corrosive environment. The plasma arc cladding process is well-suited for hammer head applications due to its ability to deposit relatively thick coatings with good metallurgical bonding on low-carbon steel substrates. The comprehensive performance of the 30% WC plus TiC coating suggests that this composition represents a promising candidate for industrial implementation, offering the potential for substantially extended hammer head service life.