CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Nickel-Based Alloy Plasma Clad Layers
Literature Overview and Application Context
This study investigates the microstructure, mechanical properties, and corrosion resistance of nickel-based alloy overlay layers produced by plasma transferred arc (PTA) cladding, with particular focus on commonly used alloys such as Inconel 625, Hastelloy C276, and Stellite 6. Nickel-based alloy cladding layers are extensively used in chemical processing, oil and gas, marine, and power generation industries where severe corrosion, erosion, and high-temperature environments demand exceptional material performance.
Microstructural Characteristics of PTA Nickel-Based Overlay Layers
The microstructure of PTA-deposited nickel-based alloy layers is characterized by a columnar dendritic structure growing perpendicular to the substrate surface, with the dendrite spacing and morphology strongly influenced by the cooling rate and alloy composition.
| Alloy Grade | Primary Phase | Secondary Phases | Dendrite Spacing (μm) | Hardness (HV) |
|---|---|---|---|---|
| Inconel 625 | FCC austenite (Ni) | δ-ferrite, Laves phase (Ni₃Nb) | 15–40 | 220–260 |
| Hastelloy C276 | FCC austenite (Ni) | M₂₃C₆ carbides, Mo-rich phases | 20–50 | 200–240 |
| Stellite 6 | FCC austenite (Co-Cr) | M₇C₃ carbides, M₂₃C₆ | 10–30 | 380–450 |
| Monel 400 | FCC austenite (Ni-Cu) | None significant | 25–60 | 160–200 |
δ-Ferrite Control in Inconel 625 Clad Layers
One of the most critical microstructural features in Inconel 625 PTA clad layers is the volume fraction and morphology of δ-ferrite (Ni₃Nb). The δ-ferrite forms during solidification as a result of Nb and Mo enrichment at the dendrite tips, and its presence is essential for preventing hot cracking. However, excessive δ-ferrite (above 25–30%) can significantly reduce ductility and promote intergranular cracking under thermal cycling.
The δ-ferrite content is primarily controlled by:
- Welding current: Higher currents increase heat input and cooling rate variation, affecting Nb partitioning.
- Travel speed: Faster travel speeds increase cooling rates, promoting more δ-ferrite formation.
- Number of layers: Multi-pass builds can redistribute Nb through dilution, reducing δ-ferrite in subsequent passes.
- Substrate composition: The substrate alloy composition affects dilution and consequently the effective alloy composition of the deposit.
The recommended δ-ferrite content for Inconel 625 clad layers is 5–20% by volume, which provides adequate hot cracking resistance while maintaining satisfactory ductility. This can be verified using the ASTM E45 ferroscope method or metallographic image analysis.
Mechanical Properties and Corrosion Performance
The mechanical properties of PTA nickel-based alloy clad layers are significantly influenced by the microstructure, particularly the dendrite spacing, carbide distribution, and phase composition.
| Property | Inconel 625 (PTA) | Hastelloy C276 (PTA) | Stellite 6 (PTA) |
|---|---|---|---|
| Tensile Strength (MPa) | 700–850 | 650–780 | 550–680 |
| Yield Strength (MPa) | 300–400 | 280–380 | 350–450 |
| Elongation (%) | 15–25 | 20–30 | 8–15 |
| Hardness (HV) | 220–260 | 200–240 | 380–450 |
| Crevice Corrosion (ASTM G48) | Excellent | Excellent | Good |
| Pitting Corrosion (ASTM G48) | Excellent | Excellent | Fair |
| Erosion Corrosion | Good | Good | Excellent |
Intergranular Corrosion Susceptibility
A critical concern for nickel-based alloy clad layers is the susceptibility to intergranular corrosion, particularly for alloys susceptible to sensitization. PTA-deposited layers may exhibit intergranular carbide precipitation if the cooling rate is too low or if the layer is exposed to sensitization temperatures (450–850°C) during subsequent thermal cycles. The study recommends that PTA clad layers intended for chloride-containing environments should undergo solution heat treatment at 1100–1150°C followed by water quenching to dissolve intergranular carbides and restore full corrosion resistance.
Dilution Effects and Bonding Quality
The dilution rate in PTA nickel-based alloy cladding typically ranges from 5% to 20%, depending on the process parameters and substrate composition. Higher dilution rates reduce the effective alloy composition of the overlay layer, potentially compromising corrosion resistance. The bond strength between the clad layer and substrate is typically verified by shear testing per ASTM A263 or ASTM A264, with minimum bond strengths of 200 MPa for most applications.
Key Reflections and Concluding Remarks
This literature provides comprehensive insights into the microstructure-property relationships in PTA nickel-based alloy clad layers, with particular emphasis on the critical role of δ-ferrite control, carbide morphology, and dilution management in determining final performance. The most significant practical implication is that PTA process parameters must be optimized not only for deposition efficiency but also for microstructural control, as the cooling rate and thermal cycling history have profound effects on phase distribution, carbide precipitation, and ultimately on corrosion and mechanical performance. Engineers should adopt a systematic qualification approach that includes metallographic examination of the as-deposited and heat-treated microstructures, mechanical property testing across the clad layer thickness, and corrosion testing under simulated service conditions, ensuring that the final clad layer meets the stringent performance requirements of its intended application.