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

Literature Overview

This paper by He Meng, Teng Yuancheng, Li Xin, and Lu Weiyuan, published in the journal Iron, Steel, Vanadium and Titanium in 2018, investigates the influence of plasma surfacing current on the phase composition, microstructure, hardness, and wear resistance of high-vanadium iron-based composite surfacing coatings. The research was conducted at Southwest University of Science and Technology and China Nuclear Hongyu Technology Co., Ltd., supported by the Longshan Talent Research Support Plan. The study employs a systematic approach using scanning electron microscopy (SEM), multi-function stereomicroscopy, energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), hardness testing, and wear testing to characterize coatings deposited at different plasma currents.

Core Technical Findings

The authors identified that the primary crystalline phases in the surfacing coating consist of martensite, VC, and M7C3-type eutectic carbides. The vanadium carbide (VC) particles and (Fe,Cr,V)7C3 eutectic carbides are dispersed within the martensitic matrix, forming a wear-resistant skeleton that governs the overall tribological performance of the coating. The study demonstrates that the surfacing current exerts a significant influence on both the microstructure and the comprehensive mechanical properties of the coating.

At a plasma surfacing current of 160 A, the alloy surfacing coating exhibited the optimal comprehensive mechanical properties, achieving a surface hardness of 63.3 HRC and a wear loss of only 0.0427 g. This finding is particularly significant for engineers selecting process parameters for high-wear applications such as mining equipment, cement grinding components, and wear plates in bulk material handling systems.

Key Process Parameters and Performance Data

Process Analysis and Metallurgical Discussion

The plasma surfacing process operates on the principle of arc melting with a controlled shielding atmosphere, allowing for precise thermal input management. The current directly governs the heat input rate, which in turn affects the cooling rate of the molten pool, the grain morphology, and the phase transformation behavior during solidification.

At lower currents, the thermal input is insufficient to achieve complete melting and homogenization of the alloying elements, potentially leading to unmelted particles and incomplete diffusion. At excessively high currents, the increased heat input promotes grain coarsening, excessive dilution from the substrate, and potentially the formation of brittle intermetallic phases or microcracking due to high residual stresses. The 160 A optimum represents a balance where sufficient thermal energy ensures complete melting and adequate alloying while maintaining a rapid enough cooling rate to preserve the fine dispersion of hard carbide phases within the martensitic matrix.

The formation of the martensitic matrix is critical to the coating's performance. The rapid solidification rates achievable in plasma surfacing suppress the formation of equilibrium phases and promote metastable martensite, which provides a hard and tough matrix capable of supporting the dispersed carbide particles. The VC particles, being among the hardest known carbides with a lattice parameter close to that of the iron matrix, form coherent or semi-coherent interfaces that enhance both hardness and wear resistance through dispersion strengthening mechanisms.

Engineering Practice Implications

For engineers working on wear-resistant surfacing applications, this study provides valuable guidance on current selection for plasma surfacing of high-vanadium alloys. The following practical considerations should be noted:

• The optimal current window is narrow, and deviations of more than 20 A from the 160 A optimum may result in significant degradation of coating properties.
• Preheating of the base material should be carefully controlled to prevent excessive heat accumulation, which could shift the effective thermal input beyond the optimal range.
• Multi-pass surfacing requires consideration of interpass temperature control to maintain the desired microstructure in subsequent passes.
• Substrate preparation, including machining to remove surface oxides and contaminants, is essential to ensure proper fusion and minimize defect formation.

The study also highlights the importance of understanding the relationship between process parameters and microstructural evolution. Engineers should not simply optimize for maximum hardness but rather seek the best balance between hardness, toughness, and wear resistance, as excessive hardness without adequate toughness can lead to spalling failure in impact-wear applications.

Study Insights and Reflections

This research exemplifies the systematic approach required in surfacing technology development. The correlation between current, microstructure, and wear performance is not linear, and the identification of an optimal window requires both experimental investigation and metallurgical understanding. The finding that 160 A produces the best results is specific to this particular alloy composition, electrode geometry, and shielding gas configuration, and should not be directly applied to other systems without appropriate qualification testing.

The dispersion of VC and M7C3 carbides within a martensitic matrix represents a classic case of composite reinforcement in surfacing alloys. The engineering significance lies in the ability to tailor the volume fraction and morphology of these carbides through process parameter control, thereby optimizing the coating for specific wear environments. For applications involving abrasive wear, such as those encountered in pipe fitting manufacturing and heavy-duty industrial equipment, this approach offers a reliable pathway to extending component service life.

The work by He Meng and colleagues contributes meaningfully to the body of knowledge on plasma surfacing of high-vanadium alloys and provides a practical reference for process development in related industrial applications.