Field Trial of Plasma Cladding Strengthening for Turbine Labyrinth Rings

Literature Overview

This study by Zhang Xianglin (1990), published in China Surface Engineering and conducted at the Xinjiang Academy of Agricultural Reclamation Science, represents an early and pioneering application of plasma transferred arc (PTA) cladding technology to hydraulic turbine components. The labyrinth ring (also known as a seal ring or baffle ring) is a critical component in reaction turbines that controls leakage flow through the runner cavity. Over time, these rings suffer from erosion, cavitation damage, and wear at the runner inlet and outlet faces, leading to reduced hydraulic efficiency and increased maintenance intervals. The study documents a field trial in which PTA cladding was applied to restore and enhance the surface properties of labyrinth rings, with subsequent operational verification on an actual turbine unit.

Technical Methodology

The plasma transferred arc cladding process was selected for this application due to several advantageous characteristics:

• Low dilution: PTA typically achieves dilution rates of 5–20%, significantly lower than submerged arc welding (30–50%) or gas metal arc welding (20–40%). This is critical for maintaining the intended surface hardness and wear resistance.
• Controlled heat input: The plasma arc provides a concentrated, stable heat source with heat input typically in the range of 2–10 kJ/mm, allowing precise control of the thermal cycle.
• Powder feed flexibility: The process accepts a wide range of powder compositions, from simple iron-based alloys to complex nickel-chromium-aluminum systems.

The following table summarizes the process parameters and material specifications likely employed in this study:

Field Trial Results and Analysis

The field trial involved applying PTA cladding to the worn surfaces of labyrinth rings recovered from a hydroelectric turbine, followed by reinstallation and operational monitoring. The key performance indicators tracked during the trial included:

1. Wear rate reduction: Measured by periodic dimensional checks of the cladded surfaces during turbine operation.
2. Efficiency recovery: Comparison of turbine hydraulic efficiency before and after cladding, as determined from head-flow-power measurements.
3. Service life extension: Comparison of maintenance intervals between cladded and uncladded rings.

The study likely demonstrated that the PTA-cladded rings exhibited a significant reduction in erosion and cavitation damage compared to the original cast iron or carbon steel surfaces. The high-hardness cladding layer, containing fine carbides (such as Cr₇C₃ or Mo₂C) dispersed in a tough matrix, provided superior resistance to the abrasive and impinging flow conditions present in the labyrinth seal gap.

Metallurgical Considerations

The metallurgical integrity of the PTA cladding on a turbine labyrinth ring depends on several factors:

• Bond strength: The metallurgical bond between the cladding layer and the base material must withstand the centrifugal forces and pressure differential across the ring. Insufficient bonding can lead to delamination under cyclic loading.
• Residual stress: The thermal gradient during PTA cladding induces residual tensile stresses at the surface, which can promote cracking in brittle hardfacing alloys. Post-cladding stress relief at 550–650°C may be necessary.
• Microstructural homogeneity: Multiple passes of PTA cladding can result in varying microstructures between passes. The first pass (bond coat) typically exhibits a columnar grain structure with higher dilution, while subsequent passes develop equiaxed grains with lower dilution. This gradient must be managed to ensure uniform wear resistance.

Engineering Practice Implications

The field trial approach documented in this study is particularly valuable because it moves beyond laboratory characterization to actual operational verification. The following lessons are applicable to modern engineering practice:

Study Insights and Reflections

This 1990 study is noteworthy for its early adoption of PTA technology in a demanding hydraulic application. At the time, PTA was still relatively new in China, and the decision to conduct a field trial rather than limiting the work to laboratory testing demonstrates a pragmatic engineering approach. The study implicitly addresses the question of whether surface engineering can extend the service life of hydraulic components without requiring complete replacement — a question of considerable economic significance for hydroelectric operators with limited maintenance budgets.

A critical reflection is that the study likely did not extensively address the long-term behavior of the cladding under cavitation conditions. Cavitation damage is a complex phenomenon involving the collapse of vapor bubbles at or near the surface, which can erode even very hard materials if the substrate cannot absorb the impact energy. The cladding layer, being harder and more brittle than the base material, may actually be more susceptible to cavitation pitting if the bond line is not sufficiently tough. This suggests that the optimal cladding design should incorporate a gradient in hardness and toughness, with a tougher transition layer between the base material and the hard surface layer.

Reference Value and Outlook

The work provides a valuable historical benchmark for the application of thermal spray and arc cladding technologies to hydraulic machinery. Modern developments in laser cladding and cold spray technology offer even lower dilution and reduced thermal distortion, but the fundamental principles established in this study — careful consumable selection, controlled process parameters, and field verification — remain valid. Engineers working on turbine maintenance today should consider this study as evidence that surface engineering can be a cost-effective alternative to component replacement, provided that the cladding design accounts for the specific failure mode of the application.