CLADDING TECHNOLOGY SHANXI CO., LTDPlasma Arc Cladding of Tungsten Carbide Reinforced High-Chromium Cast Iron
Literature Overview
This research investigates the microstructure and mechanical properties of tungsten carbide (WC)-reinforced high-chromium cast iron deposits produced by plasma transferred arc (PTA) powder cladding. The work addresses the persistent challenge of fabricating hardfacing overlays that combine high hardness with acceptable toughness—a requirement critical for components exposed to severe abrasive and erosive wear environments such as mining equipment, cement mills, and slurry pump impellers.
Process Parameters and Their Influence
PTA cladding offers superior control over dilution and microstructure compared to conventional arc welding processes due to the concentrated heat input and rapid solidification rates achievable with a plasma arc. The study examines the effects of key process parameters on the resulting deposit properties.
| Process Parameter | Optimal Range | Effect on Microstructure |
|---|---|---|
| Plasma current | 200–350 A | Controls melt pool size and dilution |
| Powder feed rate | 300–600 g/min | Affects cooling rate and grain size |
| Travel speed | 100–300 mm/min | Influences dilution rate (typically 5–15%) |
| Shielding gas flow | 8–15 L/min (Ar) | Prevents oxidation of WC particles |
| Layer thickness | 1.5–3.0 mm | Balances dilution and bonding quality |
The dilution rate in PTA cladding is typically maintained between 5% and 15%, significantly lower than in SAW or FCAW processes. This low dilution is essential for preserving the integrity of the WC particles and the high-chromium matrix that provides the wear-resistant microstructure.
Microstructural Evolution
The microstructure of the PTA-cladded WC-reinforced high-chromium cast iron consists of several distinct phases:
- Primary WC particles: These remain largely intact as spherical or near-spherical particles (10–100 μm in diameter) embedded in the matrix, providing the primary wear resistance through their extreme hardness (approximately 2400 HV).
- Eutectic carbides: M7C3 type carbides form at grain boundaries and within the interdendritic regions, contributing secondary hardening.
- Martensitic matrix: The rapid solidification rates in PTA produce a predominantly martensitic matrix (B-type and C-type martensite), which provides the base toughness and hardness of the deposit.
- Austenite: Retained austenite content of 5–15% is typically observed, which contributes to toughness through strain-induced transformation during service.
The hardness of the overlay typically ranges from 850 to 1100 HV, with the WC particles themselves measuring above 2300 HV. The combination of hard WC particles in a tough martensitic matrix creates a synergistic effect that significantly improves wear resistance compared to either phase alone.
Wear Performance and Engineering Considerations
The erosive wear resistance of the PTA-cladded overlay was evaluated through standard abrasion testing, demonstrating a wear life 3–5 times greater than conventional high-chromium cast iron without WC reinforcement. The improvement is attributed to the combined mechanisms of micro-cutting resistance from the hard WC particles and ploughing resistance from the hard martensitic matrix.
Key engineering considerations for practical application include:
- WC particle preservation: The plasma arc temperature and heat input must be carefully controlled to prevent excessive dissolution of WC particles. Excessive dissolution leads to the formation of Fe3W3C or Fe2W4C phases, which are softer and less effective for wear resistance.
- Cracking susceptibility: The high carbon and chromium content of the deposit creates susceptibility to cracking, particularly in thick sections. Preheating to 150–250°C and controlled interpass temperature (below 300°C) are recommended.
- Layer bonding: The bond strength between the overlay and substrate must be verified through bending or peel tests. Typical bond strengths exceed 250 MPa when proper process parameters are maintained.
- Thermal cycling effects: Repeated heating and cooling during multi-layer cladding can cause coarsening of carbides and microstructural degradation. Limiting the number of layers to 3–5 passes and maintaining consistent travel speed helps mitigate this issue.
Study Insights
This research underscores the unique advantages of PTA cladding for producing high-performance hardfacing deposits where dilution control is paramount. The ability to maintain WC particle integrity while achieving a fully dense, crack-free overlay represents a significant advancement over conventional hardfacing methods. For engineers specifying PTA cladding for erosive wear applications, the critical success factors are precise control of the powder feed rate relative to the plasma current, consistent travel speed, and appropriate shielding gas coverage to prevent oxidation of the exposed powder and melt pool.
The microstructural analysis reveals that the wear resistance is not solely a function of hardness but depends critically on the morphology and distribution of the WC particles within the matrix. Uniformly distributed WC particles of appropriate size (20–80 μm) in a fully dense martensitic matrix represent the optimal microstructure for erosive wear resistance. This insight should guide both powder selection and process parameter optimization in practical PTA cladding operations.