Effect of Plasma Cladding Current on Microstructure and Properties of Nickel-Based Alloy Cladding Layer on Austenitic Stainless Steel

Literature Overview

This study, published in the journal Welding (2017, Issue 12, pp. 36-40) by Cui Wendong, Wang Shuang, Zhang Song, Tan Junzhe, Guan Meng, and Liu Kai, investigates the influence of plasma cladding current on the microstructure, phase composition, hardness, and wear resistance of nickel-based alloy cladding layers deposited on Z2CN18-10 austenitic stainless steel. The research was supported by the National Key R&D Program of China and the Shenyang Science and Technology Bureau Key R&D Program. The work originates from Shenyang Blower Works Nuclear Pump Co., Ltd. and the School of Materials Science and Engineering at Shenyang University of Technology, reflecting a strong industry-academia collaboration with clear engineering motivation.

Core Technical Content

The fundamental objective is to enhance the wear resistance of Z2CN18-10 austenitic stainless steel through plasma arc cladding with a nickel-based alloy. Plasma cladding is selected for its ability to produce dilution-controlled overlay deposits with good metallurgical bonding to the substrate. The researchers systematically varied the cladding current and examined the resulting microstructural evolution, phase formation, elemental distribution, microhardness profile, and tribological behavior using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), microhardness testing, and pin-on-disk friction and wear testing.

The phase composition of the nickel-based cladding layer was identified as a composite of gamma-nickel (γ-Ni) solid solution matrix with eutectic phases including FeNi₃, Cr₂₃C₆, Cr₇C₃, and CrB. This multi-phase architecture is critical because the hard carbide and boride phases dispersed within the ductile γ-Ni matrix provide the wear resistance enhancement while maintaining sufficient toughness to resist spalling during service.

Microstructural Evolution with Cladding Current

A key finding of this work is the systematic morphological transition of the cladding microstructure as the plasma cladding current increases. At lower currents, the microstructure exhibits a cluster-like or petal-shaped morphology, which is characteristic of relatively rapid solidification with limited thermal input. As the current increases, the thermal input rises, prolonging the time above the liquidus temperature and promoting grain growth and morphological instability. The microstructure progressively transforms from petal-shaped clusters to water-plant-like (dendritic) structures and eventually to elongated strip-like features at higher currents.

This morphological evolution directly correlates with the dilution behavior and grain size. Higher currents produce larger grains and greater substrate dilution, which can either enhance or degrade the final properties depending on the balance between the beneficial hard phases and the potential for excessive dilution that dilutes the alloying elements.

Hardness and Wear Performance

The microhardness results reveal a non-monotonic relationship with cladding current. The maximum average microhardness of 898 HV is achieved at a cladding current of 110 A. This represents a substantial improvement over the base Z2CN18-18-10 austenitic stainless steel, which typically exhibits hardness in the range of 180-220 HV. The relative wear resistance at 110 A reaches 13.8 times that of the substrate, indicating that the nickel-based cladding layer provides dramatic tribological enhancement.

The wear mechanism was identified as a mixed mode involving adhesive wear and abrasive wear in the early stages of the test, transitioning to oxidative wear in the later stages. This progression is typical of high-hardness overlay layers operating under sliding contact conditions, where the initial contact asperity interactions cause material transfer and micro-cutting, followed by oxide film formation that alters the subsequent wear behavior.

Key Process-Structure-Property Relationships

The following table summarizes the critical process-structure-property chain identified in this study:

Engineering Practice Implications

From an engineering perspective, this study provides actionable guidance for selecting plasma cladding parameters when applying nickel-based overlays to nuclear pump components or similar austenitic stainless steel equipment subjected to erosive or abrasive service. The 110 A current window represents a practical process parameter that balances productivity (higher current means faster deposition) with property optimization. However, engineers must also consider that plasma cladding requires careful control of arc stability, powder feed rate, and travel speed in conjunction with current to maintain consistent dilution and avoid defects such as porosity or cracking.

The identification of CrB as a hard phase in the microstructure is particularly noteworthy from a materials design standpoint. Chromium borides are extremely hard but inherently brittle, and their presence in a cladding layer must be weighed against the risk of microcracking during thermal cycling or mechanical loading. In nuclear pump applications where thermal shock resistance is important, the presence of these brittle phases should be evaluated through thermal cycling or fatigue testing in addition to the wear tests reported here.

Key Questions and Reflections

One question that arises from this study is whether the hardness maximum at 110 A corresponds to an optimal dilution level that maximizes the volume fraction of the hard eutectic phases without excessive substrate dilution. A complementary study varying powder composition or feed rate at fixed current could help decouple the effects of thermal input from those of alloy chemistry. Additionally, the wear testing appears to be conducted under dry sliding conditions, and the actual service environment in nuclear pump components may involve lubricated or corrosive sliding, which could alter the dominant wear mechanism and the relative importance of the different hard phases.

Another point of reflection concerns the scalability of plasma cladding for large-diameter pump housings or impellers. While the study demonstrates excellent wear properties in a controlled laboratory setting, translating these results to production-scale components requires addressing issues of geometric coverage, multi-pass deposition strategy, and residual stress management to prevent distortion or cracking in thick-walled components.

Study Insights and Implications

The most significant contribution of this paper is the establishment of a clear quantitative relationship between plasma cladding current and the resulting overlay properties for a specific nickel-based alloy system on austenitic stainless steel. The 110 A current yielding 898 HV and 13.8 times the substrate wear resistance provides a concrete design target for engineers specifying plasma cladding operations. The morphological evolution from petal to dendritic to strip-like structures with increasing current offers a visual diagnostic tool for process monitoring and quality control during production runs.

The study also underscores the importance of understanding the phase evolution in nickel-based overlay systems. The coexistence of γ-Ni solid solution with multiple hard intermetallic and carbide phases creates a microstructure that can be tailored through process parameter optimization. For engineers in the nuclear and pump industries, this work provides a foundation for developing standardized plasma cladding procedures that can be reliably applied to extend the service life of critical rotating equipment components. Future work should extend these findings to include cyclic loading, corrosion-wear interaction, and multi-pass deposition strategies to fully address the demands of nuclear-grade applications.