CLADDING TECHNOLOGY SHANXI CO., LTDNickel-Based Alloy Hardfacing Process for Ultra-High Temperature Service Conditions
Literature Overview
This research by Hou Fengwei, Zhao Jian, Wu Binbin, Li Wenqing, Chao Genming, Yu Chengliang, and Qian Jinyuan was published in 2022 in Chemical Machinery. The work was supported by the National Natural Science Foundation of China (Grant No. 52175067) and the Zhejiang Provincial Key R&D Program (2021C01021). The study addresses the development of nickel-based alloy hardfacing processes and preparation methods for components operating under ultra-high temperature conditions, targeting applications in aerospace propulsion systems, high-temperature chemical reactors, and advanced thermal management equipment.
Technical Requirements and Material Challenges
Ultra-high temperature service environments impose extreme demands on surface materials. The operating temperatures can exceed 1000°C in certain applications, such as supersonic combustion ramjet engines, high-temperature gas turbines, and plasma reactors. At these temperatures, conventional overlay materials suffer from rapid oxidation, creep deformation, and thermal fatigue cracking. The following table presents the key performance requirements for ultra-high temperature overlay applications:
| Performance Parameter | Requirement | Test Method |
|---|---|---|
| Oxidation resistance | Weight gain < 0.5 mg/cm² at 1200°C for 100 h | Thermogravimetric analysis (TGA) |
| Thermal fatigue life | > 5000 cycles (RT to 1100°C) | Thermal cycling test |
| Hardness retention | ≥ 400 HV at 1000°C after 100 h | High-temperature indentation |
| Spallation resistance | No spallation at 1100°C for 50 h | Visual + metallographic |
| Bond strength | ≥ 30 MPa after thermal cycling | Tensile shear test |
| Dilution control | ≤ 25% base metal dilution | Optical emission spectroscopy |
Process Development and Innovation
The researchers developed a multi-strategy approach to achieve the required performance levels. The primary innovation involves a composite overlay structure that combines a transition layer with a functional top layer. The transition layer, typically composed of a nickel-chromium-iron alloy, provides a gradual compositional gradient between the base material and the functional overlay, reducing residual stresses and improving bond strength. The functional top layer incorporates refractory metal carbides such as tungsten carbide (WC) or titanium carbide (TiC) to enhance hardness and wear resistance at elevated temperatures.
The welding process employed is plasma transferred arc (PTA) cladding, which offers superior control over dilution rates and allows precise regulation of heat input. The PTA process parameters optimized in this study include:
- Arc current: 200–350 A
- Arc voltage: 18–25 V
- Travel speed: 80–150 mm/min
- Powder feed rate: 0.8–1.5 kg/h
- Shielding gas flow: 15–25 L/min (argon)
- Preheat temperature: 200–300°C
The powder composition was carefully designed to include alloying elements such as chromium (20–30%), aluminum (4–8%), and yttrium (0.5–2%) to promote the formation of a stable, adherent oxide scale at high temperatures. The addition of yttrium in particular was found to significantly improve the oxidation resistance by modifying the morphology of the oxide layer, promoting a more protective and less porous oxide structure.
Microstructural Analysis and Performance Evaluation
Metallographic examination of the overlay structure revealed a well-defined layered microstructure. The transition layer exhibited a dendritic microstructure with interdendritic precipitation of carbides and intermetallic compounds. The functional top layer showed a columnar grain structure oriented perpendicular to the substrate surface, which is favorable for thermal shock resistance as it allows crack deflection along the grain boundaries.
The oxidation resistance was evaluated through isothermal oxidation tests at 1000°C, 1100°C, and 1200°C in air. The results demonstrated that the optimized overlay exhibited significantly lower weight gain compared to conventional nickel-based overlay materials. At 1100°C for 100 hours, the weight gain was approximately 0.3 mg/cm², compared to 1.8 mg/cm² for a standard Inconel 625 overlay. This improvement is attributed to the formation of a thin, adherent, and self-healing oxide layer enriched in chromium and aluminum oxides.
Thermal fatigue testing was conducted by cycling the specimen between room temperature and 1100°C at a heating rate of 5°C/s and cooling rate of 5°C/s. The overlay maintained structural integrity for over 5000 cycles without spallation or significant cracking, demonstrating excellent thermal fatigue resistance. Post-test metallographic examination revealed that the primary crack initiation sites were at the overlay-substrate interface, but the cracks propagated slowly due to the high fracture toughness of the transition layer.
Engineering Application and Process Standardization
The developed process has been validated on prototype components for high-temperature gas turbine applications. The overlay was applied to turbine blade shroud segments and combustion chamber liner sections. Field trials demonstrated that the service life of the overlay-protected components was extended by a factor of 3–5 compared to uncoated or conventionally coated components. The process has been documented in a detailed welding procedure specification (WPS) that includes all critical parameters, inspection requirements, and acceptance criteria.
The standardization effort included the development of welder qualification procedures specific to PTA cladding with the optimized powder composition. Welders must demonstrate consistent control of dilution rate, surface finish, and bond quality across multiple test specimens before being certified for production work. The qualification specimens are subjected to destructive testing including hardness profiling, metallographic examination, and tensile shear bond strength testing.
Key Technical Insights
This research highlights several important principles for ultra-high temperature overlay applications. First, the composite overlay structure approach—combining a transition layer with a functional top layer—is superior to single-layer overlays because it addresses both adhesion and performance requirements simultaneously. Second, the addition of rare earth elements such as yttrium provides disproportionate improvements in oxidation resistance relative to their concentration in the alloy. Third, process control of dilution rate is critical because even small variations in dilution can significantly alter the phase composition and properties of the overlay layer.
The use of PTA cladding as the primary process is well justified for this application due to its inherent advantages in dilution control, deposition rate, and surface quality. However, for larger components where PTA is impractical, alternative processes such as laser cladding or hot-wire TIG may be considered, provided that equivalent dilution control can be achieved.
Summary
This literature presents a comprehensive approach to developing nickel-based alloy hardfacing processes for ultra-high temperature service conditions. The composite overlay structure, optimized powder composition with rare earth additions, and PTA cladding process collectively deliver exceptional oxidation resistance, thermal fatigue life, and bond strength. The research demonstrates that systematic process development combined with rigorous microstructural characterization and performance testing can achieve significant improvements in overlay performance for the most demanding thermal environments. Engineers working on high-temperature component protection should consider the composite layer approach and rare earth modification as proven strategies for extending service life.