CLADDING TECHNOLOGY SHANXI CO., LTDEffects of Different Aging Treatments on Hardness and Wear Resistance of Cladding Layer Study Notes
Literature Overview
Aging treatment is a fundamental post-welding heat treatment for age-hardenable cladding deposits, particularly those based on nickel-based superalloys, aluminum alloys, and certain stainless steels. The literature under review systematically examines how different aging parameters—temperature, time, and cooling rate—affect the hardness and wear resistance of cladding layers deposited by various welding processes. This systematic investigation provides critical guidance for optimizing the post-welding heat treatment of cladded components to achieve the desired balance between hardness, toughness, and wear resistance.
The study is particularly relevant to high-performance cladding applications where the as-welded microstructure, characterized by a supersaturated solid solution with dissolved strengthening phases, must be tempered through controlled aging to precipitate fine secondary phases that provide maximum hardness and wear resistance.
Metallurgical Fundamentals of Aging in Cladding Deposits
The aging process in cladding deposits involves the controlled precipitation of strengthening phases from a supersaturated solid solution. In the as-welded condition, the rapid cooling from the weld pool temperature traps alloying elements in solid solution, creating a metastable microstructure that is relatively soft but highly ductile. Aging at elevated temperatures (typically 400–700 °C for Ni-based alloys, 150–200 °C for Al-based alloys) allows the supersaturated elements to diffuse and form fine precipitates that obstruct dislocation motion, thereby increasing hardness and strength.
The key metallurgical mechanisms involved in aging include:
- Solute atom diffusion: Alloying elements migrate to favorable sites in the crystal lattice to form nuclei of new phases
- Precipitate nucleation: Critical nuclei form when the local concentration of solute atoms exceeds the solubility limit
- Precipitate growth: Nuclei grow by absorbing additional solute atoms from the surrounding matrix
- Coarsening (Ostwald ripening): Small precipitates dissolve and larger precipitates grow, reducing the total number of precipitates and increasing their average size
The aging treatment must be carefully controlled to maximize the number density of fine precipitates while avoiding excessive coarsening, which would reduce hardness and wear resistance.
| Aging Parameter | Typical Range (Ni-based) | Typical Range (Al-based) | Effect on Hardness |
|---|---|---|---|
| Aging temperature | 400–700 °C | 150–200 °C | Higher T → coarser precipitates → lower hardness |
| Aging time | 4–24 hours | 2–8 hours | Longer time → coarsening → peak hardness then decline |
| Cooling rate | Furnace cool or air cool | Air cool | Faster cool → finer precipitates → higher hardness |
| Pre-aging solution | 1050–1150 °C / 2–4 h | 480–530 °C / 1–2 h | Dissolves existing precipitates for re-aging |
| Peak hardness | 40–50 HRC | 120–160 HV | Achieved at optimal aging temperature and time |
| Over-aged hardness | 30–38 HRC | 80–110 HV | Reduced due to precipitate coarsening |
Systematic Aging Parameter Study
The literature presents a systematic investigation of aging parameters for a typical Inconel 625 cladding deposit produced by plasma transferred arc (PTA) welding. The as-welded deposit exhibited a hardness of 28–32 HRC, primarily due to solid solution strengthening from the dissolved Nb and Mo atoms. Aging at 700 °C for 8 hours produced a peak hardness of 42–45 HRC, representing a 35–40% improvement over the as-welded condition.
The aging response was characterized by a classic over-aging curve, with hardness increasing rapidly during the initial aging period, reaching a peak at the optimal time, and then declining due to precipitate coarsening. The optimal aging time was found to be highly sensitive to temperature, with lower temperatures requiring longer times to achieve peak hardness.
| Aging Temperature (°C) | Optimal Time (h) | Peak Hardness (HRC) | Hardness at 2× Optimal Time (HRC) |
|---|---|---|---|
| 550 | 24 | 38–40 | 36–38 |
| 600 | 12 | 40–42 | 38–40 |
| 650 | 8 | 42–44 | 39–41 |
| 700 | 8 | 42–45 | 38–40 |
| 750 | 4 | 40–42 | 36–38 |
The wear resistance, measured by pin-on-disc testing against a 100Cr6 steel counterface, showed a similar trend to hardness, with the peak wear resistance occurring at the same aging condition that produced peak hardness. However, the wear resistance improvement was more pronounced (up to 60% improvement) than the hardness improvement, suggesting that factors beyond hardness—such as microstructure uniformity, precipitate size distribution, and interfacial bonding—also contribute to wear resistance.
Microstructural Evolution During Aging
Metallographic and electron microscopy examination of the aged cladding deposits revealed distinct microstructural features at different aging stages. In the under-aged condition, the microstructure consisted of a supersaturated solid solution with isolated nuclei of γ' (Ni3(Al,Ti,Nb)) precipitates that were too small to be resolved by optical microscopy but detectable by transmission electron microscopy (TEM).
At the peak-aged condition, the γ' precipitates had grown to a size of 5–20 nm with a number density of 10^22–10^23 per cm³. These fine precipitates were distributed uniformly throughout the matrix, providing maximum resistance to dislocation motion. The matrix hardness was maximized at this condition, and the wear resistance was correspondingly highest.
In the over-aged condition, the γ' precipitates had coarsened to 50–200 nm with a significantly reduced number density (10^20–10^21 per cm³). The larger precipitates were less effective at obstructing dislocation motion, resulting in reduced hardness and wear resistance. Additionally, the over-aged condition often exhibited microcracking at grain boundaries due to the formation of brittle intermetallic phases (such as Laves phase or μ phase) at the grain boundaries.
| Aging Stage | Precipitate Size (nm) | Number Density (cm^-3) | Hardness (HRC) | Wear Resistance Index |
|---|---|---|---|---|
| As-welded | None (supersaturated) | 0 | 28–32 | 1.0 (baseline) |
| Under-aged | 2–5 | 10^21–10^22 | 35–38 | 1.5–2.0 |
| Peak-aged | 5–20 | 10^22–10^23 | 42–45 | 2.5–3.5 |
| Over-aged | 50–200 | 10^20–10^21 | 36–39 | 1.8–2.2 |
| Severely over-aged | 200–500 | 10^19–10^20 | 30–33 | 1.2–1.5 |
Wear Mechanism Analysis
The wear mechanism of the cladding layer varies significantly with the aging condition. In the as-welded and under-aged conditions, the primary wear mechanisms are adhesive wear and micro-ploughing, where the relatively soft matrix deforms plastically under the sliding contact, leading to material transfer and abrasive wear from the counterface.
At the peak-aged condition, the wear mechanism transitions to primarily abrasive wear, where the hard γ' precipitates resist deformation and the wear is limited to the matrix material between the precipitates. The fine precipitates also serve as wear debris traps, where they are embedded in the surface and contribute to a protective tribolayer that further reduces wear.
In the over-aged condition, the wear mechanism reverts to a combination of abrasive and adhesive wear, with the coarsened precipitates providing less resistance to dislocation motion. Additionally, the presence of brittle intermetallic phases at grain boundaries can lead to intergranular fracture and accelerated material loss during sliding contact.
Engineering Practice and Optimization
The aging treatment of cladding deposits requires careful consideration of several practical factors. First, the substrate material must be evaluated for its response to the aging temperature. If the substrate is a carbon steel or low-alloy steel, aging at 650–700 °C may cause undesirable softening or grain growth in the substrate, potentially compromising the structural integrity of the component. In such cases, a lower aging temperature (550–600 °C) with a longer aging time may be selected to compromise between deposit hardness and substrate integrity.
Second, the component geometry must be considered for thermal distortion during aging. Large, complex components may experience significant distortion if heated uniformly to the aging temperature. Solution treatment and aging in a vacuum furnace with controlled ramp rates (≤100 °C/h) can minimize distortion, but this increases processing time and cost.
A practical case study involved the refurbishment of a hydrogenation reactor catalyst basket support plate that had been cladded with Inconel 625 by PTA welding. The as-welded deposit exhibited a hardness of 30 HRC and showed unacceptable wear after 6 months of service. After aging at 650 °C for 8 hours, the deposit hardness increased to 43 HRC and the service life extended to over 24 months, representing a 4× improvement in wear resistance. Metallographic examination confirmed the presence of fine γ' precipitates throughout the deposit with no cracking at the fusion line.
Study Insights and Implications
The systematic study of aging effects on cladding deposits provides valuable quantitative data for optimizing post-welding heat treatment procedures. The key finding is that the aging response is highly material-dependent, with different alloy systems exhibiting different optimal aging parameters and different sensitivity to over-aging.
The literature also highlights the importance of microstructural characterization in understanding the aging response. TEM examination of precipitate size and distribution provides the most accurate prediction of mechanical properties, while hardness testing serves as a practical screening tool for quality control.
The broader implication is that aging treatment is not merely a post-processing step but an integral part of the cladding process design. The selection of cladding alloy, welding parameters, and aging treatment must be coordinated to achieve the desired final properties. This integrated approach to cladding process design is essential for achieving optimal performance in demanding industrial applications.