Effect of Plasma Surfacing Current on Microstructure and Properties of Nickel-Based Alloy Cladding Layer

Literature Overview

The paper by Cui Wendong and colleagues, published in the journal Welding (2017, Vol. 12, pp. 36-40), investigates the influence of plasma arc surfacing current on the microstructure, phase composition, hardness, and wear resistance of nickel-based alloy cladding layers deposited onto Z2CN18-10 austenitic stainless steel substrate. The study was supported by the National Key R&D Program of China (2016YFB1100204/2013ZX06002-002) and Shenyang Science and Technology Bureau key R&D projects. This work is particularly relevant to nuclear pump manufacturing, where wear-resistant surface treatments are critical for extending component service life under aggressive fluid environments.

Core Technical Findings

The researchers systematically varied the plasma surfacing current and examined the resulting cladding layer characteristics using SEM, EDS, XRD, microhardness testing, and pin-on-disc tribological testing. The key findings are summarized below:

Interpretation of Microstructural Evolution

The microstructural evolution with increasing plasma current is a critical observation. At lower currents, the cooling rate is higher due to reduced heat input, producing fine, clustered rosette-like dendritic structures. As the current increases, the heat input rises, leading to slower cooling rates and the formation of coarser water-plant-like and elongated strip-shaped structures. This transition directly affects the mechanical properties:

• Low current regime: Fine grain structures provide good hardness but may not fully utilize the reinforcing effect of carbides due to incomplete melting and mixing.
• Optimal current (110 A): The balance between heat input and cooling rate produces a refined, well-distributed microstructure where carbides (Cr₂₃C₆, Cr₇C₃, CrB) are uniformly dispersed within the γ-Ni matrix, maximizing hardness and wear resistance.
• High current regime: Excessive heat input causes coarsening of the dendritic structure and potential dilution of the cladding layer by the substrate, reducing both hardness and wear performance.

Wear Mechanism Analysis

The tribological testing revealed a mixed wear mechanism operating in two stages:

1. Early stage: Adhesive wear and abrasive wear dominate, with material transfer occurring at asperity contacts and hard carbide particles (Cr₇C₃, CrB) acting as abrasives.
2. Late stage: Oxidative wear becomes significant as the wear temperature increases, forming an oxide layer that provides some protective effect but eventually spalls off.

The relative wear resistance of 13.8 at the optimal current indicates a dramatic improvement over the uncoated Z2CN18-10 substrate, which is essential for pump impellers and wear rings operating in slurry or erosive service.

Engineering Practice Implications

For nuclear pump manufacturers, this study provides actionable guidance:

• Process window definition: The optimal plasma surfacing current of 110 A for nickel-based alloy on Z2CN18-10 substrate should be incorporated into welding procedure specifications (WPS). Deviation beyond ±10 A may significantly degrade surface properties.
• Quality control checkpoints: In-process monitoring of arc current stability is essential. Fluctuations of more than 5 A during deposition can lead to microstructural non-uniformity across the cladding layer.
• Post-weld inspection: Hardness testing at multiple locations (surface, mid-thickness, interface) should verify that the cladding layer achieves minimum 800 HV, with the interface hardness gradient managed to prevent cracking.
• Dilution control: The dilution rate between substrate and cladding material directly affects the phase balance. Maintaining dilution below 20% is recommended to preserve the reinforcing carbide distribution.

Key Questions and Reflections

One question that arises from this study is the long-term stability of the cladding layer under cyclic thermal and mechanical loading, as typical of pump service conditions. The study focuses on room-temperature tribological performance, but nuclear pumps may experience temperature cycling and cavitation erosion that could alter the wear mechanism. Additionally, the role of residual stress from plasma surfacing on the fatigue life of the cladding-substrate system warrants further investigation.

The study also highlights the importance of matching the cladding alloy chemistry to the specific wear environment. For nuclear pump applications involving radioactive slurry or high-chloride environments, the corrosion-wear synergy may differ significantly from the laboratory pin-on-disc test conditions.

Study Insights and Implications

This research demonstrates the strong coupling between plasma surfacing process parameters and the resulting surface engineering properties. The 110 A optimal current represents a process window where the thermal cycle produces an ideal microstructure for wear resistance. For engineering practice, this translates into the need for precise current control systems and real-time monitoring during production. The finding that the phase composition remains constant while the morphology changes with current is particularly instructive—it confirms that microstructural refinement, not alloy chemistry modification, is the primary lever for property optimization within a given alloy system.

Reference Value and Outlook

The work by Cui et al. provides a solid foundation for process parameter optimization in nuclear-grade wear-resistant cladding applications. Future studies should extend to multi-pass cladding scenarios, evaluate performance under actual pump operating conditions, and investigate the synergistic effects of residual stress and microstructure on the overall durability of the cladded components.