Plasma Overlay Strengthening of Turbine Labyrinth Rings - Field Test Results

Literature Overview

The paper by Zhang Xianglin, published in China Surface Engineering (1990, Vol. 7, No. 2, pp. 37-41), presents field test results on plasma overlay strengthening of labyrinth rings in hydroelectric turbines. Labyrinth rings (also known as guide rings or balance rings) are critical components in turbine runner assemblies that control leakage flow and maintain hydraulic balance. In sandy water conditions, these components suffer severe erosion and cavitation damage, often determining the interval between major overhauls. The study demonstrates that localized surface strengthening through plasma overlay welding, applied only to the wear-prone areas rather than manufacturing the entire component from wear-resistant material, provides significant economic benefit while maintaining functional performance.

Core Technical Analysis

Erosion and Cavitation Mechanisms on Labyrinth Rings

The labyrinth ring operates in a highly erosive environment where the following mechanisms act synergistically:

Abrasive erosion: Sand particles suspended in the water flow impact the ring surface at high velocities, causing material removal through micro-cutting and plastic deformation.
Cavitation damage: Pressure fluctuations in the flow passages cause vapor bubble formation and subsequent collapse, generating micro-jets that pit the surface.
Corrosion-erosion interaction: In mineral-rich water, electrochemical corrosion accelerates the removal of material from impact sites.

The paper emphasizes that damage is localized to specific, predictable positions on the ring, which makes targeted overlay treatment an economically rational solution.

Plasma Overlay Process Parameters

Plasma arc overlay welding offers several advantages for this application over alternative surface engineering methods:

Process Parameter	Typical Range	Effect on Overlay Quality
Plasma current (A)	150-400	Higher current increases deposition rate but may increase dilution
Arc voltage (V)	20-30	Affects penetration and bead geometry
Travel speed (mm/min)	100-300	Controls heat input and overlay thickness
Powder feed rate (g/min)	50-200	Determines deposition efficiency
Shielding gas flow (L/min)	15-25	Protects molten pool from atmospheric contamination
Powder composition	Ni-Cr-Si-B or Cr-C	Provides hardness and corrosion resistance

The plasma arc provides a concentrated, stable heat source that minimizes thermal distortion of the thin labyrinth ring geometry, which is critical for maintaining dimensional accuracy and clearance tolerances.

Overlay Material Selection

The study selected nickel-based and chromium-based alloy powders for the overlay. The selection criteria included:

Hardness: Target surface hardness of HRC 45-55 to resist abrasive wear.
Corrosion resistance: Chromium content sufficient to form protective oxide films in mineral-rich water.
Bond strength: Minimum interface shear strength of 200 MPa to prevent delamination under hydraulic loading.
Thermal conductivity: Compatible with the base steel to avoid excessive thermal stresses during operation.

Engineering Practice Integration

The field test results provide valuable guidance for maintenance engineers managing hydroelectric turbine components:

Targeted overlay application: Rather than replacing entire labyrinth rings, overlaying only the wear-prone areas reduces material costs and downtime. The overlay thickness should be designed to accommodate the expected wear rate over the planned service interval.
Post-overlay machining: After overlay application, the ring must be precision-machined to restore original dimensions and surface finish, as the overlay process inherently produces a rough, irregular surface.
Periodic inspection and re-overlay: The overlay layer depletes over time due to wear, and a monitoring program based on thickness measurements should trigger re-overlay before the base metal is exposed.

Key Questions and Reflections

The 1990 publication date of this paper means that the plasma overlay technology described has undergone significant advancement. Modern plasma overlay systems offer improved powder feeding consistency, better arc stability, and more precise process monitoring. However, the fundamental engineering principles remain valid. A key question not fully addressed in the paper is the long-term fatigue behavior of the overlay interface under cyclic hydraulic loading. Labyrinth rings experience millions of pressure cycles during turbine operation, and the overlay-to-base metal interface represents a potential fatigue crack initiation site. Modern practice would supplement the field test data with fatigue testing of overlay samples under representative loading conditions.

Additionally, the paper does not discuss the effect of overlay on the hydrodynamic performance of the labyrinth ring. Any increase in surface roughness or dimensional deviation could alter the leakage flow pattern and potentially reduce turbine efficiency. This consideration is particularly important for high-efficiency modern turbines where even small losses are significant.

Study Insights and Implications

This paper represents an early, practical application of plasma overlay technology to a critical rotating machinery component. Its value lies in demonstrating that surface engineering solutions can be economically viable alternatives to component replacement, particularly when wear is localized and predictable. For today's engineers, the paper serves as a reminder that the selection between surface overlay and component replacement should be based on a comprehensive cost-benefit analysis that includes not only material and labor costs but also downtime, spare parts availability, and long-term reliability. The field test approach described here—applying the technology to an actual operating component and monitoring its performance—is a practical methodology that remains relevant for validating new surface engineering solutions in industrial settings.