Effect of Plasma Surfacing Current on Microstructure and Wear Resistance of High-Vanadium Iron-Based Coatings

Literature Overview

This study, published in Iron and Steel Vanadium and Titanium (2018, Vol. 39, No. 6, pp. 81–87) by He Meng, Teng Yuancheng, Li Xin, and Lu Weiyuan from Southwest University of Science and Technology and Zhongwu Hongyu Technology Co., Ltd., investigates the influence of plasma arc surfacing current on the microstructural evolution and tribological performance of high-vanadium iron-based composite coatings. The work was supported by the Longshan Talent Research Support Program (18LZX412) and an industrial funding project (17Zh0284). The research is particularly relevant to engineers working on wear-resistant overlay applications in mining, cement, and material handling industries where high-vanadium carbide coatings are widely deployed.

Core Technical Findings

The authors employed a systematic experimental approach, varying the plasma surfacing current across a practical operating window and characterizing the resulting coatings through SEM, EDS, XRD, hardness testing, and pin-on-disc wear testing. The key findings are summarized below:

Phase Composition and Carbide Evolution

The coating microstructure is dominated by a martensitic matrix with dispersed vanadium carbide (VC) and M₇C₃-type eutectic carbides. The authors emphasize that the spherical VC particles and the (Fe,Cr,V)₇C₃ eutectic carbides form a wear-resistant skeleton within the martensitic matrix. This is a critical observation because it confirms that the wear resistance of high-vanadium coatings is not solely a function of overall hardness but depends heavily on the size, distribution, and continuity of the carbide network.

At lower currents (below 160 A), the thermal input is insufficient to achieve complete melting and homogenization of the deposited layer, leading to incomplete carbide dissolution and non-uniform microstructure. At higher currents (above 160 A), excessive thermal input causes carbide coarsening and potential dilution of the base material, which degrades the hardness and wear resistance.

Process Parameter Analysis

The plasma surfacing process offers precise thermal control compared to conventional arc surfacing methods. The current directly governs the heat input per unit length, which in turn determines:

1. Melting depth and dilution rate — Higher currents increase the dilution of base material into the overlay, reducing the effective vanadium concentration in the as-deposited layer.
2. Solidification rate — The cooling rate at the weld surface controls the martensite transformation temperature and the morphology of carbide precipitation.
3. Carbide dissolution and re-precipitation — VC carbides (Tm ≈ 2830°C) are extremely stable, but the M₇C₃ eutectic carbides are more susceptible to dissolution at elevated temperatures, leading to redistribution upon solidification.

The optimal current of 160 A represents a balance between sufficient thermal energy for complete melting and mixing, and limited thermal input to preserve carbide integrity and minimize dilution.

Engineering Practice Integration

In industrial applications, high-vanadium iron-based coatings are commonly applied to:

• Crusher hammers and jaws in mineral processing
• Cement mill liners and guide rollers
• Slurry pump impellers and wear rings
• Pipe fittings in abrasive slurry service

The study's findings have direct implications for production parameter selection. Engineers should note that the 160 A optimum is specific to the particular wire composition and wire feed rate used in this study. In practice, the optimal current must be determined through a parameter matrix approach, considering wire diameter, travel speed, and gas flow rate simultaneously.

A practical recommendation derived from this work is to perform a hardness gradient survey across the coating thickness. If the hardness drops below HRC 60 at any depth, it indicates excessive dilution or incomplete carbide formation, requiring process adjustment.

Key Questions and Reflections

The study raises several important questions for further investigation:

1. How does the interpass temperature affect the microstructure in multi-pass applications? The study focuses on single-pass or limited-pass conditions, but industrial components often require multiple passes.
2. What is the quantitative relationship between dilution rate and wear performance? A systematic dilution study would provide a more actionable process window.
3. How do the coatings perform under high-temperature wear conditions? Many industrial applications involve elevated temperatures that could alter the carbide stability and matrix properties.

The most significant insight from this paper is the confirmation that plasma surfacing current is not merely a process parameter but a microstructure-controlling variable. The ability to tune the carbide morphology and distribution through current selection provides a powerful tool for optimizing coating performance for specific wear mechanisms.

Study Insights and Implications

This research reinforces the principle that in hardfacing and surfacing applications, the microstructure is the bridge between process parameters and service performance. The identification of 160 A as the optimal current for this particular high-vanadium system provides a valuable starting point for process development, but engineers must always validate through their own parameter studies given the sensitivity of overlay microstructures to composition, base material, and geometric configuration.

The wear loss of 0.0427 g under standardized pin-on-disc testing, combined with HRC 63.3 hardness, positions this coating among the higher-performing iron-based hardfacing alloys. For engineers specifying coatings for severe abrasive wear, this data supports the selection of high-vanadium iron-based systems with plasma surfacing as the preferred process, provided the thermal input is carefully controlled within the identified optimal window.