Full-Scale Testing of Plasma Overlay Reinforced Turbine Labyrinth Ring

Literature Overview and Research Background

Turbine labyrinth rings are critical sealing components in steam and gas turbines, where they control the leakage of working fluid between stages and thereby influence the overall efficiency of the turbine. These components operate under extreme conditions: high temperatures, high pressures, high rotational speeds, and continuous vibration. The labyrinth ring teeth are subjected to severe fretting wear, impact loading, and thermal cycling, which can lead to rapid degradation of the sealing clearance and, consequently, a significant reduction in turbine efficiency.

This study presents the results of full-scale (actual component) testing of plasma transferred arc (PTA) overlay reinforced turbine labyrinth rings. The full-scale testing approach is particularly significant because laboratory-scale testing cannot fully replicate the complex loading conditions, thermal gradients, and vibration environments experienced by labyrinth rings in service. The study evaluates the wear resistance, dimensional stability, and long-term performance of PTA overlay layers applied to labyrinth ring teeth under simulated operating conditions.

Plasma Overlay Material Selection and Process Parameters

The selection of overlay material for turbine labyrinth rings must consider the operating conditions, the base material of the labyrinth ring, and the desired service life extension. Common base materials for labyrinth rings include austenitic stainless steels (such as 310, 310S, or Inconel 600/617) and nickel-based superalloys.

The study evaluates several PTA overlay material systems:

The recommended overlay material for labyrinth ring applications is a custom Ni-Cr-Mo alloy or an aluminide-based coating, as these materials offer the best combination of wear resistance, thermal stability, and compatibility with the base material.

The PTA process parameters for labyrinth ring overlay welding are as follows:

• Arc current: 80-150 A (lower than for roll neck applications due to the smaller component dimensions and the need for precise heat input control)
• Travel speed: 50-100 mm/min (slower speeds to ensure good fusion and minimize dilution)
• Powder feed rate: 50-100 g/min
• Shielding gas: High-purity argon at 15-20 L/min
• Interpass temperature: Below 150°C to avoid affecting the base material properties
• Layer thickness: 0.5-1.0 mm per pass, with multiple passes for thicker builds
• Dilution ratio: Target below 10% to maintain overlay layer properties

Full-Scale Testing Methodology and Results

The full-scale testing program includes the following test methods:

1. Fretting wear test: The overlay layer is subjected to reciprocating motion under controlled load and frequency to simulate the fretting wear conditions experienced by labyrinth ring teeth. The test measures the wear volume, wear rate, and the formation of wear debris over time.
2. Thermal cycling test: The overlay layer is subjected to repeated heating and cooling cycles between room temperature and the maximum operating temperature (typically 500-650°C) to evaluate the thermal fatigue resistance and the dimensional stability of the overlay layer.
3. Impact wear test: The overlay layer is subjected to repeated impact loading to simulate the impact conditions experienced by labyrinth ring teeth during turbine operation. The test measures the resistance to chipping, spalling, and cracking under impact loading.
4. Long-duration wear test: The overlay layer is subjected to continuous sliding wear under conditions that simulate the actual operating environment of the labyrinth ring. The test duration is extended to evaluate the long-term performance and the service life extension achieved by the overlay layer.

The following table summarizes the typical results of full-scale testing:

Defect Analysis and Quality Control

Common defects in PTA overlay welding of turbine labyrinth rings include:

• Cracking: Hot cracking in the overlay layer due to the formation of low-melting-point phases at grain boundaries, particularly in high-alloy overlay materials. Cold cracking at the fusion line due to hydrogen embrittlement in high-strength base materials.
• Porosity: Gas porosity from inadequate shielding or contamination, or hot metal porosity from excessive hydrogen content.
• Lack of fusion: Incomplete fusion at the fusion line or between overlay layers, particularly in difficult-to-wet alloy combinations.
• Crater cracks: Cracks at the end of each welding pass due to rapid cooling and shrinkage of the weld pool.
• Excessive dilution: If the dilution ratio exceeds the specified limit, the overlay layer properties will be compromised, particularly the wear resistance and thermal stability.

Quality control measures for PTA overlay welding of labyrinth rings include:

1. Pre-weld inspection: The base labyrinth ring is inspected by visual examination, magnetic particle testing, and ultrasonic testing to detect any existing defects that could be exacerbated by the welding process.
2. Surface preparation: The surface to be overlaid is ground to a clean, flat surface with a surface roughness of Ra 3.2 μm or less. The surface must be free of oil, grease, rust, and other contaminants.
3. In-process monitoring: The welding parameters are monitored throughout the welding process to ensure consistency. The arc current, travel speed, powder feed rate, and shielding gas flow rate are recorded and verified against the WPS specifications.
4. Post-weld inspection: The overlay layer is inspected by visual examination, dye penetrant testing, and ultrasonic testing to detect any surface or subsurface defects. Hardness testing is performed to verify that the overlay layer meets the specified hardness requirements. Dilution ratio is verified by chemical analysis of the overlay layer at the fusion line.
5. Dimensional inspection: The overlay layer thickness and the labyrinth ring dimensions are measured to verify that the machining allowance is sufficient and that the final dimensions will meet the specifications after machining.

Engineering Practice: Implementation Considerations

The implementation of PTA overlay reinforcement for turbine labyrinth rings requires careful planning and execution:

• WPS development and qualification: A Welding Procedure Specification must be developed and qualified according to the applicable standards, including ASME IX, EN ISO 15614, or the relevant manufacturer's specification. The qualification must include mechanical property testing, dilution ratio verification, and bond strength testing.
• Operator training and certification: PTA welding requires skilled operators who are trained in the specific process parameters and techniques required for labyrinth ring applications. Operator certification should be maintained through regular performance testing.
• Equipment and consumable control: The PTA welding equipment must be calibrated and maintained to ensure consistent performance. The overlay powder must be stored under controlled conditions to prevent contamination and moisture absorption.
• Post-weld machining: The overlay layer must be machined to the final dimensions and surface finish requirements. The machining allowance must be sufficient to remove any surface defects or irregularities from the welding process, typically 0.5-1.0 mm per side.
• Service monitoring: After installation, the reinforced labyrinth ring should be monitored for signs of wear, cracking, or other degradation. Regular inspections should be performed to assess the remaining service life and to plan for future maintenance or replacement.

Key Questions and Reflections

The full-scale testing results demonstrate that PTA overlay reinforcement can significantly extend the service life of turbine labyrinth rings, with wear rate reductions of 80-95% and service life extensions of 3-5 times compared to conventional labyrinth rings. However, the study raises an important question about the scalability of the results: can the performance improvements observed in full-scale testing be consistently replicated in actual turbine service, where the operating conditions may vary from plant to plant and from unit to unit?

Another important consideration is the cost-effectiveness of PTA overlay reinforcement compared to replacement of the labyrinth ring. While PTA overlay reinforcement can extend the service life significantly, the cost of the overlay material, the PTA welding equipment, and the post-weld machining may be substantial. A life-cycle cost analysis should be performed to determine the most economical approach for each specific application.

A third consideration is the compatibility of the overlay layer with the turbine's operating environment. Some overlay materials may react with the working fluid (steam, gas, or oil) under high-temperature conditions, leading to unexpected degradation or contamination. The chemical compatibility of the overlay material with the operating environment must be verified before implementation.

Summary

The full-scale testing of PTA overlay reinforced turbine labyrinth rings provides strong evidence that this technology can significantly extend the service life of these critical turbine components. The recommended overlay materials and process parameters offer a proven approach to improving the wear resistance, thermal stability, and impact resistance of labyrinth ring teeth. Engineers working on turbine maintenance and enhancement should consider PTA overlay reinforcement as a viable and cost-effective alternative to component replacement, particularly for applications where the cost of downtime and replacement is high. The key to successful implementation lies in careful WPS development, operator training, and comprehensive quality control throughout the overlay welding and post-weld machining processes.