CLADDING TECHNOLOGY SHANXI CO., LTDInconel 625 Alloy Cladding Layer Microstructure and Properties Study Note
Literature Overview and Research Background
This research provides a comprehensive investigation of the microstructural characteristics and mechanical properties of Inconel 625 (UNS N06625) alloy cladding layers deposited by various welding and thermal spray processes. Inconel 625, a nickel-chromium-molybdenum-niobium alloy, is one of the most widely used superalloy cladding materials for pressure vessels, heat exchangers, and other critical components in aggressive chemical and high-temperature service environments. The study examines how the deposition process, microstructure evolution, and heat treatment affect the corrosion resistance, mechanical properties, and long-term stability of the Inconel 625 cladding layer.
Core Technical Findings
The microstructure of Inconel 625 cladding layers is predominantly austenitic, consisting of a solid solution matrix of Ni-Cr-Mo with carbide precipitates (primarily NbC and Nb2C) and, in some cases, δ-ferrite. The microstructure is highly dependent on the deposition process, with plasma arc welding (PAW) and laser cladding producing finer, more uniform microstructures compared to submerged arc welding (SAW) or gas metal arc welding (GMAW).
Microstructural Characteristics by Deposition Process
| Process | Grain Size (μm) | δ-Ferrite (%) | Carbide Size (μm) | Hardness (HB) |
|---|---|---|---|---|
| Plasma arc welding (PAW) | 20–50 | 5–15 | 0.5–2.0 | 220–280 |
| Laser cladding | 10–30 | 3–10 | 0.3–1.5 | 250–320 |
| Submerged arc welding (SAW) | 50–150 | 15–35 | 2.0–5.0 | 180–240 |
| GMAW | 30–80 | 10–25 | 1.0–3.0 | 200–260 |
| Thermal spray (HVOF) | 5–20 | < 2 | 0.2–1.0 | 300–400 |
The δ-ferrite content is a critical microstructural feature that affects the corrosion resistance and mechanical properties of the Inconel 625 cladding layer. Excessive δ-ferrite (above 25%) can lead to intergranular corrosion susceptibility and reduced ductility. The formation of δ-ferrite is primarily influenced by the cooling rate and the Cr/Mo/Nb ratio in the weld metal.
Corrosion Resistance Performance
The corrosion resistance of Inconel 625 cladding layers is evaluated through various testing methods including potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and immersion testing in aggressive solutions.
| Corrosion Test | Medium | Corrosion Rate (mm/y) | Comparison with Bare Steel |
|---|---|---|---|
| Potentiodynamic polarization | 5% HCl | 0.01–0.05 | 50–100× improvement |
| Potentiodynamic polarization | 10% H2SO4 | 0.02–0.08 | 30–60× improvement |
| Potentiodynamic polarization | 20% HNO3 | 0.005–0.02 | 100–200× improvement |
| Immersion test | 3.5% NaCl (ASTM B117) | 0.005–0.02 | 100–300× improvement |
| Pitting resistance (PREN) | 3.5% NaCl + HCl | > 45 | Excellent resistance |
The exceptional corrosion resistance of Inconel 625 is attributed to the formation of a stable, protective chromium oxide film on the surface. The presence of molybdenum and niobium further enhances the pitting and crevice corrosion resistance by promoting the formation of more stable oxide species. The microstructure plays a significant role in determining the corrosion performance, with finer grain structures and lower δ-ferrite content generally providing superior corrosion resistance.
Mechanical Properties and Heat Treatment Effects
The mechanical properties of Inconel 625 cladding layers are characterized by high strength, excellent ductility, and good fatigue resistance. The as-deposited microstructure typically exhibits a combination of solid solution strengthening and precipitation strengthening from NbC carbides.
Mechanical Properties by Condition
| Property | As-Deposited (PAW) | Solution Treated (1050 °C/1h) | Age Treated (870 °C/8h + 620 °C/8h) |
|---|---|---|---|
| Tensile strength (MPa) | 700–900 | 650–800 | 800–1000 |
| Yield strength (MPa) | 350–500 | 300–450 | 500–700 |
| Elongation (%) | 20–35 | 30–45 | 15–25 |
| Hardness (HB) | 220–280 | 180–220 | 280–350 |