CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Welding Current on Microstructure and Properties of Plasma Cladding Fe90
Literature Overview
This paper by Deng Dewei and colleagues from Dalian University of Technology and Shenyang Blower Works Research Institute investigates the influence of welding current on the microstructure and mechanical properties of Fe90 alloy deposited via plasma transferred arc (PTA) powder cladding. Published in the Journal of Heat Treatment of Materials in 2017, the work was supported by the National 973 Program (2011CB013402) and multiple Liaoning provincial research initiatives. The study addresses a critical engineering challenge: optimizing the PTA process window to achieve the desired balance between dilution control, microstructure refinement, and mechanical performance in high-speed steel and wear-resistant overlay applications.
Core Technical Content
Fe90 alloy, a high-carbon, high-chromium martensitic wear-resistant material, is widely used in severe abrasion environments such as pump impellers, valve seats, and mining equipment. The PTA process offers superior dilution control compared to conventional arc welding methods, typically achieving dilution rates between 3% and 10%. The researchers systematically varied the welding current while maintaining other parameters constant to isolate its effect on the deposited layer.
Key Process Parameters and Their Effects
| Parameter | Typical Range | Effect on Microstructure |
|---|---|---|
| Welding current | 100–250 A | Higher current increases dilution and grain coarsening |
| Powder feed rate | 80–150 g/min | Controls deposition rate and layer thickness |
| Travel speed | 150–400 mm/min | Affects cooling rate and martensite transformation |
| Shielding gas flow | 15–25 L/min | Protects molten pool from atmospheric contamination |
| Substrate preheat | 150–300 °C | Reduces residual stress and cracking tendency |
The study reveals that at lower currents (below 150 A), the dilution rate remains below 5%, preserving the high hardness of the Fe90 deposit (typically 58–62 HRC after proper heat treatment). As current increases beyond 200 A, dilution rises to 8–12%, and the microstructure transitions from fine lenticular martensite with dispersed carbides to coarser martensite with reduced carbide density.
Microstructure Evolution
The researchers employed optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) to characterize the deposited layers. At optimal current settings, the microstructure consists of:
- Fine acicular martensite matrix with retained austenite content of 8–15%
- M₇C₃ and M₂₃C₆ carbides uniformly distributed in the matrix
- Grain size in the range of 20–40 μm, indicating rapid solidification
- Clear demarcation between the cladding layer and the dilution zone
At excessive current levels, the increased heat input promotes:
- Coarsening of carbide particles to 2–5 μm
- Widening of the dilution zone to 0.5–1.0 mm
- Increased retained austenite above 20%, which may compromise dimensional stability
- Potential formation of columnar grains at the fusion boundary
Process Optimization and Engineering Insights
The study demonstrates that the optimal welding current for Fe90 PTA cladding falls in the range of 160–200 A, depending on the substrate material and required layer thickness. This finding aligns with the general principle that PTA processes should operate in the low-heat-input regime to minimize dilution while maintaining adequate wetting and bonding.
Practical Recommendations
- For carbon steel substrates, a current of 180–200 A with powder feed rate of 100–120 g/min provides the best combination of hardness (58–62 HRC) and bonding strength.
- Preheating to 200–250 °C is recommended to reduce thermal stress cracking, particularly on thick sections or high-carbon substrates.
- Interpass temperature should be maintained below 200 °C to preserve the fine microstructure in multi-pass builds.
- Post-weld heat treatment (tempering at 500–550 °C) is essential to reduce residual stress and stabilize retained austenite.
Key Questions and Reflections
The study raises important questions about the scaling of PTA parameters for different substrate geometries. While flat specimens provide clear data, real-world applications involve curved surfaces, varying thicknesses, and complex joints where heat dissipation differs significantly. Engineers must consider that the optimal current for a flat coupon may not directly translate to cylindrical or spherical components.
Additionally, the relationship between current and dilution is not purely linear; powder feed rate and travel speed interact with current to determine the final dilution. The study's single-variable approach, while scientifically rigorous, may not capture the full complexity of industrial PTA operations where multiple parameters are adjusted simultaneously.
The work by Deng and colleagues provides valuable baseline data for PTA parameter selection. However, practical engineers should supplement this with their own qualification trials, particularly when applying Fe90 cladding to critical components such as pump impellers or valve trim where failure consequences are severe. The interplay between microstructure, hardness, and toughness in high-carbon martensitic deposits remains an area requiring continued investigation, especially regarding fatigue performance and thermal cycling resistance.