Effect of Welding Current on Plasma Surfacing Fe90 Microstructure and Properties

Literature Overview

This comprehensive study by Deng Dewei et al. (2017), published in Transactions of Materials and Heat Treatment, investigates the influence of plasma surfacing current on the microstructure, hardness, wear resistance, and corrosion resistance of Fe90 self-fusing alloy powder deposited onto 304L stainless steel substrate. The research was supported by multiple national and provincial funding programs including the National "973" Project (2011CB013402), reflecting its significance in the field of advanced surface engineering. The collaboration between Dalian University of Technology and Shenyang Blower Works Group Co., Ltd. demonstrates a strong application-oriented research approach.

Fe90 Alloy System and Plasma Surfacing Process

Fe90 is a well-known self-fusing alloy system based on the Fe-Ni-Cr-B-Si composition, widely used for surfacing applications requiring combined wear and corrosion resistance. The plasma surfacing (plasma transfer arc welding with powder feed) process offers several advantages for this application:

• Precise control of heat input through current, voltage, and travel speed parameters
• Low dilution with base metal (typically 5-15% depending on process parameters)
• High deposition rate compared to manual methods
• Consistent microstructure due to stable arc characteristics

The study systematically varies the welding current while maintaining other parameters constant, allowing clear attribution of microstructural and property changes to thermal input variation.

Microstructural Characterization and Current Effects

The Fe90 surfacing layer microstructure consists of the following phases identified by OM, XRD, and TEM analysis:

The microstructure is characterized by a martensitic matrix with dispersed hard carbide and boride particles. The relative proportions and morphologies of these phases are strongly influenced by the welding current, which determines the cooling rate and solidification conditions.

Performance Results and Current Optimization

The study reports remarkable performance improvements compared to the 304L base material:

The optimal current of 130 A represents a critical balance point where:

• Below 130 A: Insufficient thermal energy leads to incomplete powder melting, unmelted particles in the deposit, and poor metallurgical bonding. The resulting microstructure contains residual powder morphology and weaker mechanical properties.
• At 130 A: Optimal thermal input achieves complete melting, adequate fluidity for good wetting, and a cooling rate that produces fine martensitic transformation with well-dispersed hard phases.
• Above 130 A: Excessive thermal input causes coarsening of the microstructure, increased dilution with the austenitic base metal, potential softening of the martensite through increased retained austenite, and possible re-solidification cracking due to extended time at high temperatures.

Process-Structure-Property Relationships

The mechanism by which current affects the final properties can be understood through the following chain:

1. Current → Heat input → Cooling rate: Higher current increases heat input, reducing cooling rate in the surfacing layer.
2. Cooling rate → Phase transformation: Slower cooling promotes carbide coarsening, reduces martensite volume fraction, and increases retained austenite content.
3. Phase composition → Hardness and wear: Finer, more uniformly distributed hard phases (at optimal cooling rate) maximize hardness while maintaining adequate toughness.
4. Matrix composition → Corrosion resistance: Since the Fe90 alloy contains significant Cr and Ni content, the corrosion resistance remains relatively stable across the current range studied, as the passive film-forming elements are not significantly depleted by dilution at the currents tested.

Engineering Practice Considerations

For practical implementation of Fe90 plasma surfacing on stainless steel components, the following process considerations are essential:

• Preheating: Generally not required for Fe90 plasma surfacing, but preheating to 100-150°C may be beneficial for thick-section components to reduce thermal gradients and residual stress.
• Interpass temperature control: For multi-pass surfacing, maintaining interpass temperature below 200°C prevents excessive grain growth in previously deposited layers.
• Shielding gas: Argon or helium-argon mixtures are typically used; gas flow rate must be sufficient to prevent oxidation of the molten pool and powder stream.
• Powder feed rate matching: The powder feed rate must be precisely synchronized with the current to maintain consistent dilution and microstructure throughout the deposition.
• Substrate preparation: Surface cleanliness is critical—oxidation, contamination, or surface defects can lead to poor bonding and inclusion formation in the surfacing layer.

Study Insights and Reflections

This study demonstrates the remarkable versatility of the Fe90 alloy system when deposited by plasma surfacing technology. The 3.5-5.2 times hardness improvement over austenitic stainless steel, combined with maintained corrosion resistance, represents an excellent solution for components requiring both wear and corrosion protection in a single surface layer.

The identification of 130 A as the optimal current for this specific configuration is process-specific and should not be directly transferred to other geometries, powder sizes, or substrate materials without appropriate process qualification. Engineers should conduct their own parameter optimization using the methodology established in this paper—systematically varying current while monitoring hardness, microstructure, and wear performance.

The TEM analysis providing nanoscale characterization of the phase distribution adds significant depth to the understanding of wear mechanisms. The interaction between the martensitic matrix and the dispersed (Cr,Fe)₇C₃, CrFeB, and CrB particles creates a composite microstructure where the hard particles resist abrasive attack while the ductile matrix accommodates deformation and prevents crack propagation.

This work has direct relevance to industrial applications such as centrifugal compressor impellers, pump casing surfaces, valve seats, and other critical rotating equipment where surface hardening is required without compromising the corrosion resistance of the base material. The Shenyang Blower Works connection indicates that the research findings have already been applied in industrial practice for wind power equipment manufacturing.