Effect of Welding Current on Plasma Surfaced Fe90 Alloy Microstructure and Performance

Literature Overview and Research Context

This 2017 study by Deng Dewei and colleagues from Dalian University of Technology and Shenyang Blower Works Group Co., Ltd., published in the "Journal of Thermal Analysis and Calorimetry" (Volume 38, Issue 7, pages 138-144), investigates the influence of 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 was supported by multiple national and provincial funding programs, including the National 973 Program (Grant No. 2011CB013402) and Liaoning Provincial Natural Science Foundation (Grant No. 2014028002), reflecting its significance in the field of advanced surfacing technologies. The Fe90 alloy is a cobalt-free, iron-based self-fluxing alloy that has gained increasing attention as a cost-effective alternative to cobalt-based alloys for wear and corrosion protection.

Plasma Surfacing Process and Fe90 Alloy Characteristics

Plasma arc surfacing (also known as plasma transfer arc surfacing or plasma surfacing) is a thermal spray-adjacent welding process that uses a high-temperature plasma jet to melt and transfer alloy powder onto a substrate surface. The process offers several advantages over conventional arc welding methods for surfacing: precise control of dilution (typically 5-15%), high deposition efficiency, low heat input, and the ability to produce thin, uniform coatings. The plasma arc temperature, which is directly related to the welding current, is a critical process parameter that affects the melting behavior of the powder, the thermal cycle of the substrate, and ultimately the microstructure and properties of the deposit.

The Fe90 alloy is a self-fluxing powder alloy based on an iron-nickel-cobalt matrix (though in this study, the Fe90 variant is cobalt-free) containing carbide-forming elements such as chromium and carbon, along with boron and silicon as fluxing elements. The self-fluxing characteristic means that the alloy contains its own fluxing elements (typically boron and silicon), which form low-melting-point intermetallic compounds that melt first during the welding process, creating a fluid melt that wets the substrate and allows for good fusion without the need for external flux.

Experimental Parameters and Characterization Methods

The study employed optical microscopy (OM), X-ray diffraction (XRD), and transmission electron microscopy (TEM) for microstructural characterization. Mechanical and tribological properties were evaluated using microhardness testing and ring-on-block friction and wear testing. Corrosion resistance was assessed using saturated calomel electrode (SCE) potentiodynamic polarization measurements. The welding current was the primary variable, with multiple current levels tested to establish the current-property relationship.

Key Results on Microstructure and Properties

The microstructure of the Fe90 surfacing deposit consists of five distinct phases: martensite (the matrix phase), (Cr,Fe)7C3 (a chromium-rich complex carbide), CrFeB (a chromium-iron boride), CrB (chromium boride), and Fe3Si (iron silicide). This multi-phase microstructure is characteristic of self-fluxing alloys and provides a synergistic combination of hardness, wear resistance, and corrosion resistance through the interaction of hard ceramic-like phases (carbides and borides) with a tough metallic matrix.

The following table summarizes the key performance metrics across different welding currents:

The finding that corrosion resistance remains essentially unchanged across all current levels is notable. This suggests that the Fe90 alloy's corrosion performance is primarily determined by its inherent composition (particularly the chromium and nickel content) rather than by the microstructural variations induced by different welding currents. The martensitic matrix and chromium-rich carbides and borides provide sufficient passivation capacity to maintain corrosion resistance regardless of the specific microstructural state.

Current-Dependent Microstructural Evolution

The welding current directly controls the plasma arc temperature and energy input density. At lower currents, the thermal input is insufficient to fully melt and homogenize the powder particles, potentially leading to unmelted or partially melted particles within the deposit. At higher currents, the increased thermal input promotes more complete melting, finer grain refinement due to higher cooling rates (from deeper penetration and faster solidification), and potentially different phase transformations during cooling.

At the optimal current of 130 A, the deposit achieves the highest hardness and best wear resistance. This is likely because the thermal input at this current level is sufficient to fully melt and homogenize the Fe90 powder, producing a uniform distribution of hard phases (Cr7C3, CrFeB, CrB) within the martensitic matrix. The TEM analysis would have revealed the fine-scale dispersion of these hard phases, which are the primary contributors to the deposit's wear resistance through mechanisms such as ploughing resistance, micro-cutting resistance, and fatigue resistance.

Engineering Practice Implications for Plasma Surfacing

Plasma surfacing is increasingly being adopted in industries where precise control of coating composition and dilution is critical, such as in the repair and refurbishment of gas turbine components, chemical processing equipment, and marine applications. The study's findings on current optimization are directly applicable to process parameter selection in production environments.

For engineers implementing plasma surfacing of Fe90 alloy, several practical considerations emerge from this study. First, the optimal current of 130 A should serve as a starting point for process parameter optimization, but the specific optimum may vary depending on the powder feed rate, travel speed, nozzle-to-workpiece distance, and substrate material. Second, the 3.5 to 5.2 times hardness improvement over 304L stainless steel makes Fe90 surfacing an attractive option for protecting stainless steel components in wear-critical locations, such as pump impellers, valve seats, and mixer shafts. Third, the preservation of corrosion resistance is a significant advantage, as many hardfacing alloys sacrifice corrosion resistance for wear resistance, creating a trade-off that limits their applicability in corrosive environments.

The ring-on-block wear test configuration used in this study simulates sliding wear conditions, which are common in pump impellers, valve seats, and bearing surfaces. The 80-85% reduction in wear volume represents a substantial improvement in service life. For a pump impeller that originally lasted 500 hours before requiring replacement due to wear, Fe90 surfacing could potentially extend the service life to 3000-3500 hours, representing a significant reduction in maintenance downtime and replacement costs.

Process Parameter Interaction and Optimization Strategy

While this study focuses on the effect of welding current, plasma surfacing performance is governed by a complex interaction of multiple parameters. The following table outlines the key process parameters and their interactions:

The interaction between current and powder feed rate is particularly important. At low current with high feed rate, the powder may not fully melt, resulting in a porous deposit with poor mechanical properties. At high current with low feed rate, the excessive thermal input may cause excessive dilution with the base metal, reducing the beneficial effect of the Fe90 alloy composition. The optimal combination of current and feed rate must be determined through systematic experimentation for each specific application.

Study Insights and Independent Reflection

This study makes a compelling case for Fe90 self-fluxing alloy as a versatile surfacing material for stainless steel substrates. The combination of dramatic hardness improvement (3.5-5.2 times), substantial wear resistance enhancement (80-85% reduction in wear volume), and preserved corrosion resistance addresses the classic trade-off between wear and corrosion protection that has long limited the application of hardfacing alloys on stainless steel components.

The use of TEM in this study is particularly commendable, as it provides nanoscale microstructural information that is inaccessible through optical or scanning electron microscopy alone. The TEM observations would have revealed the size, shape, and interface characteristics of the hard phases (Cr7C3, CrFeB, CrB) within the martensitic matrix, providing fundamental insights into the wear resistance mechanism. Engineers should recognize that understanding microstructure at the nanoscale is essential for rational alloy design and process optimization, rather than relying solely on macroscopic property measurements.

The preservation of corrosion resistance across all current levels is an important finding that deserves further investigation. The study should ideally have included immersion testing in specific corrosive media relevant to the intended application (such as seawater for marine applications or acid solutions for chemical processing) rather than relying solely on potentiodynamic polarization measurements. Polarization data, while informative, may not capture localized corrosion mechanisms such as pitting or crevice corrosion that can be critical in real service conditions.

One practical limitation of this study is that it focuses on a single substrate material (304L stainless steel). The dilution rate, thermal cycle, and resulting microstructure would differ significantly if the Fe90 alloy were applied to carbon steel, low-alloy steel, or other austenitic stainless steels. Engineers applying this technology should conduct their own qualification testing on the specific substrate materials in their application, as the results from this study should be considered as a baseline reference rather than a universal specification.

The plasma surfacing process itself requires specialized equipment and operator training, which can be a barrier to adoption in smaller workshops or field repair operations. However, the process advantages (low dilution, precise composition control, good coating uniformity) justify the investment for high-value components where coating performance is critical to asset reliability. The current optimization findings from this study provide a solid foundation for developing process windows that can be transferred to production environments with appropriate validation.