CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Different Aging Treatments on Hardness and Wear Resistance of Cladding Layers Study Note
Literature Overview
This literature investigates the influence of various aging heat treatment conditions on the hardness distribution and wear resistance of weld overlay (cladding) layers. The study is particularly relevant to nickel-based alloy overlays such as Inconel 625 and Hastelloy C276, which are widely used in corrosion-resistant pressure vessel applications. The central premise is that the as-deposited microstructure of the cladding layer may not represent its optimal condition, and post-weld aging treatments can significantly modify the hardness, phase composition, and wear resistance of the overlay.
Core Technical Points
As-Deposited Microstructure
The as-deposited microstructure of nickel-based alloy cladding layers typically consists of a dendritic solidification structure with primary austenite (gamma phase) and secondary phases such as delta ferrite, carbides, and intermetallic compounds. For Inconel 625 cladding deposited by multi-wire SAW, the as-deposited hardness typically ranges from 220–260 HV, with a relatively uniform distribution across the overlay thickness. The delta ferrite content in the as-deposited state is generally low (below 5%), which is beneficial for corrosion resistance but may not provide optimal wear resistance.
The microstructural evolution during welding is influenced by the welding sequence, interpass temperature, and welding parameters. Multi-pass welding introduces thermal cycling that can partially homogenize the microstructure, but the final heat input during the last pass dominates the as-deposited condition. The literature emphasizes that understanding the as-deposited condition is essential for designing effective aging treatment parameters.
Aging Treatment Variations
The literature examines four aging treatment conditions, each designed to target different aspects of microstructural modification:
| Treatment Condition | Temperature | Duration | Cooling Method | Target |
|---|---|---|---|---|
| Solution treatment | 1150°C | 1 hour | Air cool | Dissolve secondary phases |
| Low-temperature aging | 700°C | 4 hours | Air cool | Precipitate strengthening |
| Intermediate aging | 850°C | 2 hours | Air cool | Partial dissolution + precipitation |
| High-temperature aging | 950°C | 2 hours | Furnace cool | Phase homogenization |
Hardness Distribution Analysis
The hardness measurements reveal distinct trends depending on the aging treatment condition. The following table summarizes the key findings:
| Treatment | Surface Hardness (HV) | Mid-Layer Hardness (HV) | Fusion Boundary Hardness (HV) | Hardness Uniformity |
|---|---|---|---|---|
| As-deposited | 250 | 245 | 230 | Moderate |
| Solution treatment | 210 | 205 | 195 | High |
| Low-temperature aging (700°C) | 380 | 365 | 340 | Moderate |
| Intermediate aging (850°C) | 310 | 300 | 285 | High |
| High-temperature aging (950°C) | 230 | 225 | 215 | High |
The low-temperature aging at 700°C produces the highest hardness due to the precipitation of fine gamma-prime (gamma') and carbide phases. However, this condition may compromise ductility and toughness, which are critical for pressure vessel applications. The intermediate aging at 850°C offers a good balance between hardness and toughness, with a hardness of approximately 300 HV and acceptable elongation values.
Microstructural Evolution During Aging
Metallographic analysis reveals significant microstructural changes during aging. At 700°C, fine precipitates of Ni3Nb (gamma') and NbC carbides form within the austenite matrix, providing substantial precipitation strengthening. The precipitate size is in the range of 5–50 nanometers, which is below the critical size for coherent strengthening. At 850°C, partial dissolution of the delta ferrite phase occurs, and the remaining precipitates coarsen slightly, resulting in a more uniform hardness distribution. At 950°C, extensive dissolution of secondary phases occurs, leading to a softer but more homogeneous microstructure.
The delta ferrite content is particularly important in nickel-based alloy cladding. Excessive delta ferrite (above 10%) can lead to intergranular corrosion in chloride-containing environments. The aging treatments can modify delta ferrite content: low-temperature aging preserves delta ferrite, while intermediate and high-temperature aging partially dissolve it. For pressure vessel applications where corrosion resistance is paramount, maintaining delta ferrite below 5% is critical.
Wear Resistance Testing
Test Methodology and Results
Wear resistance is evaluated using a ball-on-disc tribometer under dry sliding conditions. The test parameters include a normal load of 50 N, a sliding speed of 0.3 m/s, and a counterface of 52100 bearing steel (HRC 60). The wear rate is determined from the volume loss of the overlay material over a total sliding distance of 500 m.
| Treatment Condition | Wear Rate (mm³/N·m) | Relative Wear Life | Hardness (HV) |
|---|---|---|---|
| As-deposited | 1.85×10⁻⁶ | 1.0× | 250 |
| Solution treatment | 2.45×10⁻⁶ | 0.76× | 210 |
| Low-temperature aging (700°C) | 0.62×10⁻⁶ | 3.0× | 380 |
| Intermediate aging (850°C) | 0.85×10⁻⁶ | 2.2× | 310 |
| High-temperature aging (950°C) | 2.10×10⁻⁶ | 0.88× | 230 |
The low-temperature aging condition provides the best wear resistance, consistent with its highest hardness. However, the intermediate aging condition offers a more balanced combination of wear resistance, toughness, and corrosion resistance, making it more suitable for pressure vessel applications where multiple performance criteria must be met simultaneously.
Corrosion Resistance Considerations
Aging treatments can also affect the corrosion resistance of the overlay layer. The literature reports intergranular corrosion testing according to ASTM G28 Practice A, which simulates a severe acid-regenerating environment. The results show that low-temperature aging at 700°C slightly increases susceptibility to intergranular corrosion due to the preservation of delta ferrite and the potential for chromium depletion at grain boundaries. Intermediate aging at 850°C partially mitigates this issue by dissolving some delta ferrite, while high-temperature aging at 950°C provides the best corrosion resistance but at the cost of hardness and wear resistance.
Engineering Practice Integration
In pressure vessel fabrication, the selection of aging treatment conditions depends on the specific application requirements:
- For hydrogenation reactors where wear resistance is critical and corrosion resistance is secondary, low-temperature aging at 700°C is recommended.
- For heat exchangers and storage tanks where corrosion resistance is the primary concern, intermediate aging at 850°C or high-temperature aging at 950°C is preferred.
- For applications requiring a balance of wear resistance and corrosion resistance, intermediate aging at 850°C provides the best compromise.
The aging treatment must be integrated into the overall heat treatment schedule for the pressure vessel. For clad-plate pressure vessels, the PWHT is typically performed at 590–620°C for carbon steel substrates, which is below the aging temperatures studied in this literature. This means that the aging treatment must be performed either before the vessel is assembled and welded, or as a separate heat treatment step after assembly. The latter approach requires careful planning to avoid over-tempering the base metal or causing distortion.
Key Questions and Reflections
The literature raises an important question about the interaction between aging treatment and subsequent mechanical properties. While hardness and wear resistance improve with low-temperature aging, the impact on impact toughness and fracture resistance is not fully characterized. For pressure vessel applications, impact toughness at the fusion boundary is a critical acceptance criterion according to ASME VIII Div.1 and GB/T 150. The aging treatment should not compromise the toughness of the overlay at the fusion boundary, which is the most critical region for stress concentration and crack initiation.
Another reflection concerns the practicality of aging treatments for large pressure vessels. Aging at 700–950°C requires a large furnace capable of accommodating the entire vessel, which may not be available for large-diameter or long vessels. In such cases, localized aging using induction heating or resistance heating may be considered, but the temperature uniformity and control are challenging. The literature does not address this practical limitation, which is a significant gap for engineers working on large-scale pressure vessel fabrication.
Study Insights and Implications
The study provides valuable quantitative data on the effects of aging treatments on cladding layer properties, which is essential for optimizing the post-weld heat treatment schedule in bimetal pressure vessel fabrication. The key insight is that aging treatment is a powerful tool for tailoring the properties of the overlay layer to meet specific application requirements, but it must be carefully balanced against other performance criteria such as corrosion resistance, toughness, and dimensional stability.
The intermediate aging condition at 850°C emerges as the most versatile option for pressure vessel applications, providing a good balance of hardness, wear resistance, and corrosion resistance. For engineers working in the field, this literature underscores the importance of systematic aging treatment optimization and the need for comprehensive property characterization beyond hardness measurements. In conclusion, aging treatment is a critical process step in cladding layer optimization, and its parameters must be carefully selected based on the specific application requirements and the overall quality control plan for the pressure vessel.