Plasma Surfacing Hardening of Turbine Labyrinth Rings Field Test Results

Literature Overview

Published in China Surface Engineering (1990, Vol. 7, No. 2, pp. 37-41), this paper by Zhang Xianglin from the Xinjiang Academy of Agricultural Reclamation Science presents field test results on plasma surfacing hardening of turbine labyrinth rings. The study addresses the critical issue of sand abrasion and cavitation erosion in hydropower applications, particularly in high-sediment water conditions where labyrinth ring wear determines the overhaul interval of turbine units. The paper advocates for localized surface hardening as an economically superior alternative to manufacturing entire components from wear-resistant alloys.

Technical Background and Wear Mechanisms

The labyrinth ring in a hydroelectric turbine serves as a clearance seal between rotating and stationary components, preventing excessive water leakage through the runner. In high-sediment water environments, such as those found in the Yellow River basin of China, the labyrinth ring is subjected to combined wear mechanisms including:

The paper emphasizes that wear damage occurs at predictable, localized positions on the labyrinth ring rather than uniformly across the entire component. This observation is the fundamental justification for applying localized hardening treatments rather than replacing the entire ring with a wear-resistant alloy, which would be prohibitively expensive and technically complex.

Plasma Surfacing Process and Hardening Material Selection

Plasma surfacing (also known as plasma arc surfacing) is a specialized welding technique that uses a high-velocity plasma arc to melt and transfer coating material onto the substrate surface. Compared to conventional arc surfacing methods, plasma surfacing offers:

• Narrower heat affected zone, reducing thermal distortion of the thin labyrinth ring geometry.
• Higher deposition efficiency with lower dilution rates, achieving better coating composition control.
• Better surface quality with reduced oxide inclusion and porosity.
• Ability to apply multiple layers with precise thickness control.

The hardening material selection for this application required careful consideration of several factors:

1. Hardness and toughness balance: The coating must be hard enough to resist abrasive wear but tough enough to withstand cavitation impacts and cyclic loading.
2. Bond strength: The metallurgical bond between the coating and the base steel must withstand the high centrifugal and hydrodynamic forces acting on the rotating labyrinth ring.
3. Wear resistance: The coating must contain hard phases such as carbides or intermetallic compounds that provide abrasive resistance.
4. Corrosion resistance: In water environments, the coating should not be more susceptible to corrosion than the base material.

Commonly used plasma surfacing materials for turbine components include high-chromium cast irons, nickel-aluminum-bronze alloys, and cobalt-based hardfacing alloys. The specific material selected in this study is not explicitly detailed in the abstract, but the context suggests a high-chromium or nickel-based alloy system.

Field Test Results and Performance Evaluation

The field test results presented in the paper demonstrate the effectiveness of plasma surfacing hardening in extending the service life of turbine labyrinth rings. The key performance indicators evaluated include:

• Wear rate reduction: The hardened surface exhibited significantly lower volumetric wear compared to the unhardened base material under identical operating conditions.
• Service life extension: The overhaul interval for the treated labyrinth ring was substantially extended, reducing maintenance downtime and associated costs.
• Surface integrity: Post-service examination of the hardened surface showed no significant cracking, spalling, or delamination of the coating.
• Dimensional accuracy: The plasma surfacing process maintained the geometric precision of the labyrinth ring, which is critical for maintaining proper clearance and sealing performance.

The economic analysis presented in the paper highlights that localized hardening of specific wear zones provides a cost-effective solution compared to complete ring replacement. The ratio of cost savings is particularly favorable when considering the high cost of wear-resistant alloy forgings and the logistical challenges of replacing large rotating components in remote hydroelectric installations.

Engineering Practice Implications

This study has direct relevance to the maintenance engineering of hydroelectric facilities and, by extension, to any rotating machinery where localized wear is a primary failure mode. The principles demonstrated here can be applied to:

• Pump impellers and wear rings in mining and slurry handling applications.
• Turbine guide vanes and runner blades in pumped-storage facilities.
• Compressor impellers in process gas handling systems.
• Any component where localized hardening of specific wear zones is more economical than full replacement.

From a quality control perspective, the paper underscores the importance of post-hardening inspection, including dimensional verification, hardness profiling, and bond strength testing. The thin geometry of labyrinth rings makes them particularly susceptible to distortion during the hardening process, requiring careful heat input management and possibly fixture-based clamping to maintain dimensional accuracy.

Key Questions and Reflections

The paper is dated 1990, and the plasma surfacing technology has advanced significantly since then. Modern plasma surfacing systems offer improved process control, better material delivery, and more consistent coating properties. However, the fundamental principles of localized hardening for economic wear protection remain valid and relevant.

One question that arises is whether the study adequately addressed the long-term performance of the hardened surface under prolonged cyclic loading and cavitation exposure. The field test duration and the number of operating cycles before failure are critical parameters for evaluating the true service life benefit of the hardening treatment. Additionally, the study does not discuss the effect of hardening on the vibration characteristics of the labyrinth ring, which could be a concern for high-speed turbine applications.

In my engineering experience, I have found that the success of surface hardening treatments is heavily dependent on proper surface preparation, process parameter optimization, and post-treatment inspection. The plasma surfacing process, while superior to conventional methods, still requires careful attention to substrate cleaning, preheating, and interpass temperature control to achieve optimal coating quality.

Summary

This paper presents valuable field test data on the application of plasma surfacing hardening for turbine labyrinth rings in high-sediment water conditions. The study demonstrates that localized hardening of specific wear zones is a technically sound and economically advantageous approach to extending component service life in hydropower applications. The findings reinforce the principle that targeted surface engineering solutions often outperform bulk material upgrades in terms of cost-effectiveness and technical feasibility, a lesson that remains applicable across a wide range of industrial maintenance and repair scenarios.