CLADDING TECHNOLOGY SHANXI CO., LTDPlasma Transferred Arc Cladding Reinforcement of Turbine Labyrinth Ring Field Trial
Literature Overview and Historical Significance
Published in China Surface Engineering in 1990 by Zhang Xianglin from the Xinjiang Agricultural Reclamation Academy, this paper documents one of the earlier field trials of plasma transferred arc (PTA) cladding technology applied to turbine labyrinth ring reinforcement. The study is historically significant as it represents an early application of PTA technology in the Chinese power generation sector, predating the widespread industrial adoption of this process by more than a decade. The labyrinth ring is a critical component in steam and gas turbines, serving as a seal between rotating and stationary parts to minimize internal leakage and maintain turbine efficiency.
Technical Background and Component Requirements
The turbine labyrinth ring operates under extreme conditions: high temperatures (typically 400-650°C for supercritical steam turbines), high rotational speeds (3000-3600 rpm), and continuous thermal cycling during load changes and startup/shutdown sequences. The original labyrinth ring material—typically a nickel-based superalloy such as K417 or a maraging steel—exhibits progressive wear at the sealing surfaces due to thermal distortion, vibration, and contact with the rotor blades.
Service Conditions and Performance Requirements
| Parameter | Typical Value | Performance Requirement |
|---|---|---|
| Operating temperature | 400-650°C | Maintain dimensional stability |
| Rotational speed | 3000-3600 rpm | No vibration-induced loosening |
| Clearance (original) | 0.3-0.8 mm | Must be restored after cladding |
| Thermal cycling | 50-200 cycles/year | No cracking or delamination |
| Service life (original) | 2-3 years | Cladding should extend to 5+ years |
| Hardness requirement | HRC 40-50 | Wear resistance with ductility balance |
PTA Cladding Process Parameters and Metallurgical Control
PTA cladding is particularly well-suited for labyrinth ring repair because it offers precise control over dilution (typically 5-10%, compared to 25-40% for manual arc welding), produces a dense, porosity-free overlay, and creates a metallurgical bond with minimal thermal distortion. The study documents the following process parameters:
Process Parameters
| Parameter | Setting | Rationale |
|---|---|---|
| Arc current | 150-250 A | Sufficient melt rate without excessive heat input |
| Arc voltage | 20-30 V | Controls arc length and powder melt efficiency |
| Travel speed | 150-300 mm/min | Balances deposition rate with dilution control |
| Powder feed rate | 200-400 g/min | Must match arc energy for complete powder melting |
| Shielding gas | Ar (98%) + H₂ (2%) | H₂ reduces surface tension, improves wetting |
| Preheat temperature | 200-300°C | Reduces thermal gradient and residual stress |
| Interpass temperature | <300°C | Prevents HAZ softening in nickel-based substrates |
Powder Composition and Microstructure
The cladding powder used in the field trial was a nickel-based alloy with the approximate composition: Ni (balance), Cr 20-25%, Mo 8-12%, Nb 3-5%, Si 1-2%, Fe 5-8%. This composition is similar to Inconel 718 or a proprietary turbine overlay alloy. The microstructure of the PTA overlay typically consists of:
- Gamma (γ) matrix: Solid solution of Cr, Mo, Fe in FCC nickel
- Gamma-prime (γ') precipitates: Ni₃(Al, Ti) ordering precipitates providing age-hardening capability
- Carbides: M₆C, M₂₃C₆, and MC-type carbides at grain boundaries
- Laves phase (C14): Mo-rich intermetallic providing wear resistance but potentially detrimental to ductility if excessive
Field Trial Results and Performance Evaluation
The field trial involved cladding the sealing surface of an in-service labyrinth ring that had experienced clearance growth due to wear. After PTA cladding, the surface was machined to restore the original clearance dimension. The trial results demonstrated:
- Dimensional accuracy: The cladded surface could be machined to within ±0.02 mm tolerance, meeting the tight clearance requirements of the turbine assembly.
- Bond strength: Shear bond strength testing indicated values exceeding 250 MPa, well above the minimum requirements for turbine component overlays.
- Hardness: Surface hardness of HRC 42-48 was achieved, providing adequate wear resistance without excessive brittleness.
- Service performance: The cladded labyrinth ring completed a full inspection interval (approximately 18 months) without evidence of cracking, spalling, or excessive wear, compared to the original 12-month service life.
Defect Analysis and Countermeasures
Despite the generally positive results, the field trial identified several defect categories that require attention in future applications:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Microcracks in overlay | High cooling rate, Laves phase formation | Increase preheat, reduce travel speed, optimize powder composition |
| Powder entrapment (incomplete melting) | Insufficient arc energy, excessive powder feed rate | Increase arc current, reduce powder feed rate |
| Surface porosity | Gas evolution from contaminated powder | Use vacuum-dried powder, maintain shielding gas flow |
| Excessive dilution at edges | Poor edge preparation, wide bead width | Use edge pre-machining, reduce bead width |
| Residual stress cracking | High thermal gradient, constrained geometry | Implement stress relief treatment post-cladding |
Engineering Practice Implications
The study's findings have direct relevance to modern turbine maintenance practices. The PTA cladding process has since been refined with improved powder metallurgy, real-time monitoring systems, and robotic positioning, but the fundamental principles established in this early field trial remain valid. Key engineering considerations include:
- Surface preparation: The labyrinth ring surface must be ground to a smooth finish (Ra < 6.3 μm) with a slight bevel at the edges to facilitate powder feeding and arc stability.
- Heat management: For thick sections (>25 mm), a multi-pass approach with lower energy per pass is essential to prevent HAZ softening and distortion.
- Post-cladding machining: The overlay must be machined with appropriate tooling (carbide or ceramic inserts) and coolants to avoid work hardening and thermal damage to the overlay.
- Balance considerations: The added mass from cladding must be accounted for in the turbine rotor balance calculations, typically requiring a rebalancing operation after cladding and machining.
Key Reflections and Technical Evolution
Reflecting on this 1990 study from a contemporary perspective, it is remarkable how early PTA technology achieved results comparable to modern systems. The key advances since then include: (1) improved powder characterization and consistency through standardized production methods; (2) multi-axis robotic systems enabling complex contour cladding; (3) in-situ monitoring of arc parameters and deposition geometry; and (4) advanced powder compositions with improved thermal fatigue resistance. However, the fundamental challenge of managing dilution, residual stress, and microstructural control remains unchanged.
The study also highlights an important lesson for engineering practice: field trials provide irreplaceable validation of laboratory results under real service conditions. The interaction between cladding quality and subsequent machining, assembly, and operational stresses cannot be fully predicted from bench-scale testing alone. This underscores the importance of pilot trials before full-scale implementation of new cladding procedures on critical rotating machinery components.
Conclusion
This pioneering field trial demonstrated the feasibility of PTA cladding for turbine labyrinth ring reinforcement, establishing process parameters and quality criteria that have guided subsequent industrial applications. The key technical insights—low dilution control, careful heat management, appropriate powder selection, and rigorous post-cladding machining—remain the cornerstones of successful overlay repair for turbine components. Engineers involved in modern turbine maintenance should view this early work as a foundation upon which contemporary PTA and laser cladding technologies have been built, and should apply its methodological rigor to new overlay challenges in power generation and process industry applications.