CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Rare Earth La2O3 Addition on Microstructure and High-Temperature Oxidation Performance of CMT Cladded Inconel 625
Literature Overview
The 2023 study by Wang Xiaoling, Zhu Kaiyang, Gu Xianfeng, and colleagues from Guangdong Provincial Academy of Sciences and related institutions investigates the influence of rare earth La2O3 addition on the microstructure and high-temperature oxidation resistance of Inconel 625 deposited via Cold Metal Transfer (CMT) welding. This research is significant because Inconel 625 is a critical overlay material for high-temperature and corrosive environments in power generation, petrochemical, and aerospace industries, and CMT welding has emerged as a promising low-dilution cladding technique. The introduction of rare earth oxides as micro-alloying additions represents an area of active research aimed at further enhancing the already excellent properties of Ni-based superalloys.
CMT Cladding Process Characteristics
Cold Metal Transfer (CMT) welding, also known as Cold Metal Transfer or Short Arc CMT, is a variant of Gas Metal Arc Welding (GMAW) that employs a pulsed current with a separate short-circuit current source. The process is characterized by extremely low heat input, minimal spatter, and precise control over the deposition rate. For cladding applications, CMT offers several advantages over conventional GMAW:
| Parameter | CMT Cladding | Conventional GMAW |
|---|---|---|
| Heat input | 0.5–2.0 kJ/mm | 3.0–8.0 kJ/mm |
| Dilution rate | 5–15% | 20–40% |
| Deposition rate | 0.5–3.0 kg/h | 3.0–8.0 kg/h |
| Wire feed speed | 1–5 m/min | 5–15 m/min |
| Arc voltage | 12–18 V | 18–28 V |
| Microstructure | Fine, refined | Coarser, more columnar |
| HAZ width | < 1 mm | 2–5 mm |
The low dilution rate achieved by CMT is particularly important when cladding Inconel 625 onto carbon steel or low-alloy steel substrates, as it preserves the Ni-based alloy composition and prevents excessive carbon and iron contamination of the overlay layer.
Effect of La2O3 on Microstructure
The addition of rare earth La2O3 to the Inconel 625 filler wire (or as a coating on the wire surface) influences the solidification behavior and microstructure through several mechanisms:
- Heterogeneous nucleation: La2O3 particles act as nucleation sites for the austenite (γ) phase during solidification, promoting a finer and more equiaxed grain structure. This refinement reduces the tendency for microsegregation and improves mechanical uniformity.
- Modification of δ-ferrite: Inconel 625 solidifies with a primary γ-austenite matrix and may contain secondary δ-ferrite depending on cooling rate. La2O3 addition has been shown to reduce the δ-ferrite fraction by modifying the γ/δ phase boundary and promoting preferential γ nucleation. This is beneficial because δ-ferrite can act as a preferential site for carbide precipitation and intergranular cracking.
- Carbide modification: The L12-type Ni3Nb and B2-type Nb-rich intermetallics are the primary strengthening phases in Inconel 625. La2O3 addition can modify the morphology and distribution of these carbides, potentially reducing their size and promoting a more uniform dispersion.
- Oxide dispersion strengthening: Some of the La2O3 may remain as fine oxide particles within the solidified microstructure, providing additional precipitation hardening and pinning of grain boundaries.
The microstructural evolution can be summarized as follows:
| Microstructural Feature | Without La2O3 | With La2O3 Addition |
|---|---|---|
| Grain size | 50–150 μm | 30–80 μm |
| δ-ferrite fraction | 5–15% | 1–5% |
| Carbide size | 2–10 μm | 1–5 μm |
| Carbide distribution | Semi-continuous network | Dispersed, isolated |
| Oxide particles | Absent | 0.1–1 μm, uniformly distributed |
| Columnar grain ratio | High (>70%) | Reduced (<40%) |
High-Temperature Oxidation Performance
The primary objective of adding La2O3 is to enhance the high-temperature oxidation resistance of the Inconel 625 overlay. The mechanism involves the formation of a more adherent and protective chromium oxide (Cr2O3) scale at the surface during high-temperature exposure. Rare earth elements are known to modify oxide scale growth through the following mechanisms:
- Scale adhesion improvement: La3+ ions segregate to the oxide scale/metal interface, reducing the number of available grain boundary diffusion paths for oxygen and metal cations. This slows scale growth and reduces spallation.
- Scale structure modification: The presence of rare earth ions promotes the formation of a finer-grained, more columnar oxide scale with fewer cracks and voids.
- Thermodynamic stabilization: La2O3 has a higher melting point (2415°C) and greater thermodynamic stability than Cr2O3 (melting point 2435°C, but decomposes at lower temperatures), which can help stabilize the protective oxide layer.
Typical oxidation testing results for Inconel 625 with and without La2O3 addition at 1000°C in air:
| Test Condition | Weight Gain (mg/cm²) | Scale Thickness (μm) | Scale Adhesion |
|---|---|---|---|
| Inconel 625 (base) | 15–25 | 5–10 | Moderate spallation |
| Inconel 625 + 0.5% La2O3 | 8–15 | 3–6 | Good adhesion |
| Inconel 625 + 1.0% La2O3 | 5–10 | 2–4 | Excellent adhesion |
| Inconel 625 + 2.0% La2O3 | 6–12 | 3–5 | Good adhesion (optimal range exceeded) |
The results indicate an optimal La2O3 addition level of approximately 0.5–1.0 wt% for maximum oxidation resistance improvement. Excessive addition may lead to La-rich phase formation that is detrimental to mechanical properties.
Process Parameters and Quality Control
For CMT cladding of Inconel 625 with La2O3 addition, the following process parameters are recommended:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Shielding gas | 98% Ar + 2% CO2 or pure Ar | Minimizes oxidation, stable arc |
| Gas flow rate | 15–20 L/min | Adequate protection, minimal turbulence |
| Travel speed | 200–400 mm/min | Balances deposition rate and penetration |
| Wire feed speed | 2–4 m/min | Controls heat input and dilution |
| Electrode stick-out | 10–15 mm | Stable arc, reduced spatter |
| Interpass temperature | < 150°C | Prevents grain coarsening |
| Number of passes | 3–5 | Achieves target thickness (1–3 mm) |
Quality control for the cladded overlay should include:
- Metallographic examination: Verify microstructure, δ-ferrite content (target < 5%), and absence of hot cracks.
- Hardness testing: Vickers hardness should be in the range of 220–280 HV for Inconel 625 overlay.
- Bond strength test: Peel test or shear test to verify overlay-to-substrate adhesion (minimum 200 MPa for critical applications).
- Corrosion testing: Salt spray, intergranular corrosion (ASTM A263 Practice E or F), and high-temperature oxidation tests.
- Chemical analysis: Confirm Ni, Cr, Mo, Nb, and La content within specification limits.
Engineering Applications and Implications
The findings of this study have direct implications for the following engineering applications:
- Power plant boiler tubes: Inconel 625 CMT cladding is increasingly used to protect boiler tubes from high-temperature corrosion and oxidation in ultra-supercritical coal-fired power plants. The La2O3-enhanced variant could extend tube life in the most severe service conditions.
- Hydrogenation reactor internals: In petrochemical hydrogenation reactors operating at 300–500°C under hydrogen atmosphere, Inconel 625 overlay provides excellent resistance to hydrogen attack and corrosion. Improved oxidation resistance from La2O3 addition would enhance performance during startup and shutdown cycles when air is present.
- Gas turbine components: Turbine blade coatings and hot-section components benefit from rare earth-modified Ni-based alloys. The CMT process offers a potential repair and re-coating method for turbine components in service.
Key Reflections
The integration of rare earth micro-alloying with advanced welding processes represents a paradigm shift in surface engineering. Rather than relying solely on alloy design at the bulk material level, the field is moving toward targeted micro-additions that optimize specific properties without compromising others. The CMT process, with its low dilution and fine microstructure, provides an ideal platform for exploiting the benefits of rare earth additions.
However, practical implementation faces challenges. The cost of La2O3 addition to filler wire is relatively low, but the qualification and qualification testing required for each specific application can be substantial. Furthermore, the long-term stability of the La-modified oxide scale under cyclic thermal conditions requires further investigation. Creep-oxidation interaction at elevated temperatures and the effect of cyclic thermal loading on the modified microstructure are areas that warrant continued research.
This study exemplifies the convergence of materials science, welding technology, and surface engineering. The ability to tailor the microstructure and properties of a cladding layer through both process parameters and alloy additions opens new possibilities for extending the service life of critical components in high-temperature and corrosive environments. The practical adoption of these findings will depend on the development of standardized qualification procedures and the availability of La2O3-containing Inconel 625 filler wire in commercial form.