Effect of Welding Current on Plasma Surfacing of Fe90 Self-Fluxing Alloy

Literature Overview

This paper, published in Journal of Thermal Engineering and Materials Processing (Volume 38, Issue 7, 2017, pp. 138-144) by Deng Dewei and colleagues from Dalian University of Technology and Shenyang Blower Works Group, investigates the effect of plasma welding current on the microstructure, hardness, wear resistance, and corrosion resistance of Fe90 self-fluxing alloy powder plasma surfacing deposits on 304L stainless steel substrates. The research is supported by multiple national and provincial funding programs, reflecting its significance in the context of China's major equipment manufacturing initiatives.

Process and Material Background

Plasma arc surfacing (PAS) offers several advantages over conventional arc surfacing methods:

Fe90 is a self-fluxing alloy powder system widely used in plasma surfacing for its excellent combination of wear resistance and corrosion resistance. The "self-fluxing" characteristic means the alloy contains elements (such as silicon and boron) that form a flux during melting, eliminating the need for a separate flux material and reducing porosity.

Microstructural Analysis

The surfacing deposit microstructure consists of the following phases:

The martensitic matrix provides the base hardness and toughness, while the carbide and boride phases provide additional hardening through dispersion strengthening and composite effect. The Fe3Si phase contributes to overall matrix strengthening.

Effect of Welding Current on Properties

The study demonstrates a clear trend where welding current directly influences the dilution ratio and thermal cycle, which in turn affects microstructure and properties:

The optimal welding current of 130 A represents a balance point where the dilution ratio is sufficiently low to maintain the alloy's inherent properties while still providing adequate wetting and bonding with the substrate. At lower currents, insufficient heat input leads to poor powder melting and incomplete bonding. At higher currents, excessive dilution degrades the alloy properties by incorporating too much substrate material into the deposit.

Mechanism of Property Enhancement

The hardness improvement of 3.5 to 5.2 times relative to the 304L substrate is achieved through multiple mechanisms:

1. Martensitic transformation — The rapid cooling rate in plasma surfacing promotes complete martensitic transformation, providing a hard and strong matrix.
2. Carbide precipitation — The (Cr,Fe)7C3 carbides form as a result of chromium and carbon enrichment during solidification, providing significant dispersion strengthening.
3. Boride network formation — CrB and CrFeB phases form a reinforcing network that impedes dislocation motion and crack propagation.
4. Microstructural refinement — The high cooling rate (typically 100-1000 K/s in plasma surfacing) produces a fine microstructure with small martensite lath packets and finely dispersed secondary phases.

Corrosion Resistance Considerations

An important finding is that the corrosion resistance of the Fe90 surfacing layer remains essentially unchanged relative to the 304L substrate. This is somewhat counterintuitive given the significant change in microstructure and composition. The explanation lies in the chromium content of Fe90 alloy, which is sufficient to maintain a protective passive film even in the martensitic matrix. The carbide and boride phases, while hard, do not significantly compromise the overall corrosion resistance because they are well-dispersed within the chromium-rich matrix rather than forming continuous networks.

Engineering Application Considerations

For practical implementation of Fe90 plasma surfacing, the following process parameters should be controlled:

• Welding current: 130 A (for the specific equipment and powder feed rate used in this study)
• Powder feed rate: Must be matched to current for optimal melting
• Travel speed: Affects heat input and dilution
• Gas flow rates: Shielding gas and carrier gas must be optimized
• Layer thickness: Multiple thin passes are preferred over single thick deposits

The ring-on-block wear test methodology used in this study provides a practical assessment of sliding wear resistance, which is directly relevant to applications such as pump impellers, valve components, and rotating equipment where sliding contact with wear particles occurs.

Study Insights

This research clearly demonstrates that welding current is a critical parameter in plasma surfacing of Fe90 alloy, with an optimal value that maximizes both hardness and wear resistance. The finding that 130 A provides the best performance is equipment-specific and should be validated for each production setup. The preservation of corrosion resistance while dramatically improving wear resistance makes Fe90 plasma surfacing an attractive solution for components that face both corrosive and abrasive environments simultaneously. For engineers working on upgrading existing 304L stainless steel components for enhanced wear performance without sacrificing corrosion resistance, this technology offers a proven solution with well-characterized process windows and predictable results.