Zhuojin Pipe Fitting Co., LtdPlasma Surfacing Hardening of Turbine Labyrinth Rings: Field Trial Results
Literature Overview
This paper by Zhang Xianglin, published in China Surface Engineering (1990, Vol. 7, No. 2, pp. 37-41), presents field trial results of plasma surfacing hardening applied to turbine labyrinth rings in hydroelectric power stations. The study addresses the critical issue of sand abrasion and cavitation erosion affecting turbine runner components, particularly in waterways with high sediment content. The labyrinth ring, as a key sealing component, is a primary determinant of whether a major overhaul is required. The author demonstrates that localized hardening at specific wear locations offers a highly cost-effective alternative to replacing entire components with wear-resistant materials.
Technical Background and Engineering Challenge
Turbine runner components in hydroelectric installations are subjected to severe erosive environments, particularly in waterways carrying suspended sediment. The labyrinth ring, which forms a critical seal between rotating and stationary components, experiences progressive material loss at predictable locations. Traditional approaches to addressing this problem include:
- Manufacturing entire components from wear-resistant special steels, which increases cost significantly
- Developing new bulk hardening methods, which are technologically complex and expensive
- Performing localized surface hardening at specific wear locations, which offers the best cost-benefit ratio
The plasma surfacing technique was selected because it provides excellent control over the dilution rate, allows the use of a wide range of hard alloy compositions, and produces a dense, well-bonded surface layer with minimal heat input to the base metal.
Key Technical Parameters
| Parameter | Specification |
|---|---|
| Plasma power | 15-25 kW |
| Plasma gas | Argon with 5% H2 |
| Powder feed rate | 150-250 g/min |
| Powder composition | Ni-Cr-B-Si alloy with WC particles |
| Travel speed | 30-60 mm/min |
| Layer thickness | 0.5-1.0 mm per pass |
| Number of passes | 2-3 passes |
| Bond strength | >400 MPa |
| Hardness (as-deposited) | 55-65 HRC |
| Dilution rate | <15% |
Field Trial Methodology and Results
The field trial was conducted on an operational turbine labyrinth ring in a sediment-rich waterway. The following methodology was employed:
- Surface preparation: The wear-damaged area was ground to remove all eroded material and exposed fresh base metal. The surface was cleaned with acetone to remove contaminants.
- Plasma surfacing: The Ni-Cr-B-Si/WC alloy powder was deposited using a single-pass plasma transfer arc (PTA) process. The powder was fed into the plasma arc via a gravity-fed nozzle positioned at a 90° angle to the substrate.
- Post-treatment: The deposited layer was machined to the required dimensions and then heat-treated at 900°C for 1 hour followed by air cooling to optimize the carbide distribution.
- Performance evaluation: The hardened ring was returned to service and monitored for a period of 18 months.
Performance Comparison
| Metric | Before Hardening | After Hardening | Improvement Factor |
|---|---|---|---|
| Service life | 6 months | >18 months | >3× |
| Material loss rate | 0.15 mm/month | 0.03 mm/month | 5× reduction |
| Maintenance cost | High (full replacement) | Low (local repair) | ~70% savings |
| Downtime | 2 weeks per overhaul | Minimal | Significant reduction |
Integration with Engineering Practice
The successful field trial of plasma surfacing for turbine labyrinth ring hardening has broad implications for the maintenance and repair of rotating machinery in the power generation industry. Key engineering considerations include:
- Wear location mapping: Before applying surface hardening, it is essential to accurately identify the specific wear locations through inspection and analysis. This ensures that hardening is applied only where needed, minimizing cost and thermal distortion.
- Powder composition optimization: The Ni-Cr-B-Si/WC alloy composition should be selected based on the specific erosion mechanism (abrasion, cavitation, or combination). For predominantly abrasive wear, higher WC content is beneficial; for cavitation erosion, higher Ni content improves toughness.
- Dimensional control: Plasma surfacing produces a relatively thick deposit that requires machining. The dimensional tolerance of the final component must be considered when planning the surfacing operation to avoid excessive material removal that would compromise the bond strength.
Key Questions and Reflections
A significant question raised by this study is the long-term reliability of plasma-surfaced layers under cyclic loading conditions. While the 18-month field trial demonstrated excellent performance, the long-term behavior under continuous vibration and thermal cycling requires further investigation. The bond strength between the deposit and the base metal is critical, and any degradation of this bond over time could lead to catastrophic failure.
Another reflection concerns the scalability of this technology. While the field trial was conducted on a single component, the technology could potentially be applied to entire turbine runner assemblies, offering a comprehensive solution for erosion-resistant refurbishment. The key challenge would be maintaining dimensional accuracy across large surfaces, which requires sophisticated plasma surfacing equipment with computer-controlled motion systems.
Study Insights and Implications
The most significant insight from this study is that localized surface hardening, when properly designed and executed, can dramatically extend the service life of critical components at a fraction of the cost of replacement. The plasma surfacing technique, with its low dilution rate and excellent control over the deposit composition, is particularly well-suited for this application. The field trial results provide compelling evidence that this technology is ready for widespread adoption in the hydroelectric power industry. This study also highlights the importance of field trials in validating laboratory-developed technologies, as real-world conditions often reveal challenges that are not apparent in controlled testing environments.