Plasma Surfacing of Fe90 Alloy on Low Carbon Steel: Process Parameter Optimization

Literature Overview

This study by Lu Hailong and Kang Jiandong, published in Heat Processing Technology in 2012, investigates the plasma arc surfacing of Fe90 alloy (a high-alloy iron-based hardfacing material) onto low carbon steel substrates. The research focuses on the effect of surfacing current on the microstructure, hardness, and wear resistance of the deposited layer. Fe90 alloy is widely used in applications requiring high wear resistance, such as mining equipment, cement mill liners, and material handling components, making this study highly relevant to industrial maintenance and component life extension.

Core Technical Findings

The study systematically varied the plasma arc surfacing current and evaluated the resulting microstructure, hardness distribution, and wear performance. The key findings are summarized below:

The optimal current of 180 A produced a surface hardness of 67.7 HRC and the best wear resistance. The study also found that surface properties were superior to side properties, indicating the effect of the arc interaction zone on the deposited layer quality.

Technical Interpretation

Plasma Arc Surfacing Process Fundamentals

Plasma arc surfacing (PAS) is a high-energy-density welding process that uses a constricted plasma arc to achieve temperatures exceeding 15000°C at the arc core. The process is characterized by:

• High energy density leading to deep, narrow weld pools
• Precise control of heat input through current and travel speed
• Ability to deposit high-alloy materials with controlled dilution
• Reduced thermal distortion compared to conventional arc welding

For Fe90 alloy surfacing, the process parameters must be carefully balanced to achieve complete melting of the alloy powder or wire while minimizing dilution with the base metal. The Fe90 alloy typically contains high levels of Cr, Mo, and C, which form hard carbides (such as Cr7C3 and Mo2C) that provide the wear resistance.

Current Effect on Microstructure and Performance

The relationship between surfacing current and deposited layer properties follows a non-monotonic trend:

1. At low currents (below 180 A), the heat input is insufficient to fully melt the Fe90 alloy, resulting in incomplete melting, unmelted particles, and a coarser microstructure. The hardness is lower because the hard carbide phase is not fully formed or is distributed unevenly.
2. At the optimal current of 180 A, the heat input is sufficient for complete melting and good mixing of the alloy components. The microstructure consists of a fine matrix with uniformly distributed hard carbides, resulting in maximum hardness and wear resistance.
3. At high currents (above 180 A), excessive heat input leads to increased dilution with the low carbon steel base metal, reducing the effective alloy content in the deposited layer. The microstructure becomes coarser due to prolonged high-temperature exposure, and the hardness decreases.

Microstructure Analysis

The OM and SEM analysis would reveal the following microstructural features:

• Matrix phase: M martensite or M + retained austenite, depending on the cooling rate
• Carbide phase: Cr7C3, Mo2C, and possibly VC or WC if vanadium or tungsten is present
• Grain size: Finer at optimal current due to higher cooling rates and better nucleation
• Dilution: Quantified by measuring the Cr and Mo content at the fusion line

The transition from surface to side properties is attributed to the geometry of the plasma arc interaction. The arc core provides the highest energy density at the surface, while the edges receive less energy, resulting in incomplete melting and lower hardness.

Process Optimization Strategy

Parameter Selection Methodology

Based on the study's findings and general plasma surfacing practice, the following parameter selection methodology is recommended:

1. Base parameters: Select wire/powder feed rate and travel speed based on the desired deposition rate and weld geometry.
2. Current optimization: Vary the current in increments of 20-30 A and evaluate hardness, dilution, and microstructure at each level.
3. Verification: Confirm optimal parameters through wear testing and, if required, impact or fatigue testing.

Recommended Parameter Window for Fe90 Surfacing

Quality Assessment Criteria

The following quality criteria should be applied to Fe90 plasma surfacing deposits:

• Surface hardness: ≥ 65 HRC (target 67-70 HRC)
• Dilution: ≤ 30% (measured by Cr content at fusion line)
• Crack-free: No visible or subsurface cracks
• Uniformity: Hardness variation ≤ 5 HRC across the deposit
• Bond strength: ≥ 50 MPa (if required for the application)

Engineering Practice Implications

Application Scenarios

Fe90 plasma surfacing is applicable to the following industrial components:

• Mining equipment: shovels, buckets, wear plates
• Cement industry: mill liners, grinding media
• Material handling: chutes, hoppers, conveyor components
• Power generation: boiler tubes, fan blades
• Agricultural machinery: plowshares, disk blades

Maintenance and Repair Considerations

For field repair applications, the following considerations are important:

1. Surface preparation: Remove all rust, scale, and contaminants by grinding or blasting
2. Preheating: Apply preheat to reduce thermal stress and prevent cracking
3. Layering: Apply multiple thin layers if thick deposits are required
4. Post-weld treatment: Stress relieving may be required for thick deposits or high-stress applications
5. Inspection: Visual and dimensional inspection after each layer; UT or PT for critical applications

Comparison with Alternative Processes

Plasma arc surfacing offers a good balance of hardness, dilution control, and productivity, making it suitable for many industrial applications.

Key Questions and Reflections

The study provides valuable process optimization data but does not address several important aspects. First, the long-term wear performance under actual service conditions was not evaluated. Second, the effect of multiple layers on the microstructure and properties of the top layer was not investigated. Third, the residual stress distribution in the surfacing deposit and its effect on cracking susceptibility is an important consideration that was not covered.

From a practical perspective, the finding that 180 A is optimal is specific to the equipment and consumables used in the study. Other operators should expect to find their own optimal current based on their specific torch geometry, wire/powder feed system, and base metal thickness. The study's emphasis on surface versus side properties highlights the importance of torch alignment and travel technique in achieving uniform deposit quality.

Study Insights and Implications

This research demonstrates that plasma arc surfacing of Fe90 alloy is a viable and effective method for producing high-hardness, wear-resistant deposits on low carbon steel components. The optimal current of 180 A and the resulting hardness of 67.7 HRC provide a clear benchmark for process development and quality assurance. The study's emphasis on the non-monotonic relationship between current and performance underscores the importance of systematic parameter optimization rather than trial-and-error approaches. For engineers involved in component life extension and maintenance, this study provides a solid foundation for developing reliable plasma surfacing procedures for Fe90 and similar iron-based hardfacing alloys.