Penetrant Testing of Overlay Layers on Large Hydro Turbine Runner Crowns

Literature Overview

This paper, published in 2005 in Welding, was co-authored by Yin Liying, Xu Li, and Song Yanzhong from the Harbin Welding Technology Training Center and Huludao Binhai Hydropower Large Component Manufacturing Co., Ltd. The study addresses the application of penetrant testing (PT) to inspect overlay layers on the crown of large hydro turbine runners, a critical component in hydroelectric power generation where the integrity of the weld overlay directly impacts structural safety and operational reliability.

Technical Background

Hydro turbine runners are large, rotating components that convert hydraulic energy into mechanical energy. The runner crown—the upper surface of the runner—is often subjected to cavitation erosion, water flow erosion, and impact loading from entrained debris. To protect against these degradation mechanisms, a wear-resistant overlay layer is applied to the crown surface using various welding processes.

The overlay layer on turbine runner crowns typically serves one or more of the following purposes:

1. Cavitation erosion resistance: The overlay material must be hard and tough enough to resist the formation and collapse of cavitation bubbles.
2. Abrasive wear resistance: In rivers with high sediment content, the overlay must withstand continuous particle impact.
3. Corrosion resistance: In waters with high dissolved oxygen or aggressive chemical species, the overlay must provide a barrier against corrosion.
4. Repair of existing damage: The overlay may be applied to repair cavitation pitting or erosion damage on the runner crown.

Common Cladding Processes for Runner Crowns

The choice of cladding process depends on the size, geometry, and accessibility of the runner crown:

• Submerged arc welding (SAW): The most common process for large runner crowns due to its high deposition rate and ability to produce sound, continuous welds.
• Flux-cored arc welding (FCAW): Used for smaller areas or in positions where SAW equipment is impractical.
• Gas metal arc welding (GMAW): Employed for repair welding and in situations requiring precise control over the deposit geometry.
• Plasma transferred arc (PTA): Used for high-quality, low-dilution overlays on critical components.

The overlay material selection for hydro turbine runners typically includes:

Penetrant Testing Methodology

Penetrant testing is a surface-sensitive non-destructive testing (NDT) method that is particularly effective for detecting surface-breaking defects such as cracks, laps, and lack of fusion in overlay layers. The PT procedure for turbine runner crown overlays follows the principles outlined in JB/T 4730.5 and ASME Section V Article 6.

PT Procedure Steps

1. Pre-cleaning: The surface must be cleaned to remove all slag, spatter, paint, and other contaminants that could interfere with penetrant entry into surface defects. Mechanical cleaning (grinding, wire brushing) followed by solvent cleaning is typical.
2. Applicator application: A white or colored applicator is applied to enhance the contrast of the penetrant indication.
3. Penetrant application: A liquid penetrant is applied to the cleaned surface and allowed to dwell for a specified time (typically 10–60 minutes, depending on the penetrant type and defect size).
4. Excess removal: Excess penetrant is removed from the surface using a solvent-wetted cloth or paper towel, taking care not to wipe away penetrant from defect indications.
5. Developer application: A dry powder or wet developer is applied to draw the trapped penetrant out of surface defects, creating visible indications.
6. Inspection: The developed surface is inspected under appropriate lighting (visible light for fluorescent penetrants, UV-A light for fluorescent penetrants) for indications.
7. Evaluation and documentation: Indications are evaluated against acceptance criteria and documented according to the applicable code or specification.

Inspection Parameters and Acceptance Criteria

The PT inspection of overlay layers must be performed with attention to the following parameters:

Defect Types and PT Sensitivity

The types of defects that PT can detect in overlay layers include:

• Surface cracks: These are the most critical defects, as they can propagate under cyclic loading and lead to catastrophic failure. PT has high sensitivity for cracks, with detection limits as small as 0.05 mm wide for fluorescent penetrant systems.
• Lack of fusion: Incomplete bonding between the overlay and base metal, or between successive overlay passes. PT can detect surface-breaking lack of fusion but not internal lack of fusion that does not break the surface.
• Laps and folds: Metal folds or laps that occur during the cladding process, particularly in multi-pass overlays. These can act as crack initiation sites.
• Porosity: Gas porosity in the overlay layer. While individual pores may be acceptable, clusters of porosity or porosity at the bond line are typically unacceptable.
• Inclusions: Slag or oxide inclusions in the overlay layer. These can reduce the fatigue life of the component.

Limitations of PT for Overlay Inspection

PT has several limitations that must be considered when evaluating overlay layers:

1. Surface sensitivity: PT only detects surface-breaking defects. Internal defects such as subsurface porosity or internal lack of fusion will not be detected.
2. Surface preparation sensitivity: The quality of PT results is highly dependent on the surface cleanliness and preparation. A poorly prepared surface can mask real defects or produce false indications.
3. Geometry sensitivity: Complex geometries, such as the curved surfaces of a turbine runner crown, can make it difficult to apply penetrant and developer uniformly.
4. Material sensitivity: Porous or rough surfaces can absorb penetrant and produce false indications, reducing the signal-to-noise ratio.

Engineering Practice and Quality Assurance

The inspection of overlay layers on hydro turbine runner crowns is a critical quality assurance activity. The following practices are recommended:

1. 100% inspection of critical areas: The entire overlay surface should be inspected, with particular attention to the bond line, the leading edges of the runner crown, and areas of geometric discontinuity.
2. Multiple inspection methods: PT should be supplemented with other NDT methods such as ultrasonic testing (UT) for internal defects and magnetic particle testing (MT) for ferromagnetic materials.
3. Welding procedure qualification: The cladding procedure must be qualified in accordance with NB/T 47014 or ASME IX before production use.
4. Operator qualification: PT operators must be certified to Level II or higher in accordance with the applicable standard.
5. Documentation and traceability: All inspection results must be documented and traceable to the specific component, welder, and inspection operator.

Case Study: Defect Detection in Runner Crown Overlay

In a typical application, PT inspection of a runner crown overlay might reveal the following defects:

• Cracks at the bond line: These are often caused by excessive thermal stresses during cooling. The countermeasure is to reduce the heat input per pass, increase the preheat temperature, and apply a post-weld stress relief treatment.
• Lack of fusion between passes: This is often caused by insufficient overlap between adjacent passes or by the presence of slag between passes. The countermeasure is to ensure proper pass overlap (minimum 50%) and complete slag removal between passes.
• Porosity clusters: These may be caused by moisture contamination of the flux or filler material. The countermeasure is to use properly stored and dried consumables.

Study Insights and Implications

This 2005 study represents a practical application of NDT technology to the quality assurance of hydro turbine components. The authors' focus on PT as the primary inspection method reflects the surface-sensitive nature of the defects most likely to occur in overlay layers—cracks, laps, and lack of fusion at the surface or near-surface.

A key insight from this work is the importance of surface preparation in PT inspection quality. The curved, often rough surfaces of turbine runner crowns present unique challenges for penetrant application and developer coverage. The use of appropriate applicators and developers, combined with thorough surface cleaning, is essential for achieving reliable inspection results.

Modern practitioners should note that while PT remains a valuable inspection method for overlay layers, advanced techniques such as phased array ultrasonic testing (PAUT) and terahertz imaging are increasingly being used to supplement or replace PT for more comprehensive defect detection. However, PT's simplicity, low cost, and high sensitivity for surface cracks ensure its continued relevance in overlay inspection practice.