Ultrasonic Testing for Stellite Alloy Cladding Quality Inspection in Ultra-Supercritical Power Units

Literature Overview

This 2014 paper by Yang Jing, Chen Jie, Lu Yun, and Yu Gang, conducted by State Grid Shanghai Electric Power Company, Waigaoqiao Power Plant, State Grid Shanghai Electric Power Research Institute, and Shanghai Pudong New Area Special Equipment Inspection Institute, addresses a critical quality assurance challenge in the power generation industry: the ultrasonic testing (UT) of Stellite alloy weld overlay layers on components of ultra-supercritical (USC) power units. Ultra-supercritical boilers operate at pressures above 25 MPa and temperatures exceeding 600°C, imposing extreme demands on material integrity and weld quality. Stellite alloy (cobalt-based) overlays are applied to turbine blades, valve seats, and other critical components to provide resistance to high-temperature erosion and corrosion. The reliable detection of defects in these overlay layers is essential for ensuring safe and reliable operation.

Core Technical Analysis

Component Requirements and Overlay Specifications

Ultra-supercritical power unit components requiring Stellite overlay include:

Stellite 6 (ASTM B153) is the most commonly specified material, with a composition of approximately 55–65% Co, 28–32% Cr, 7–11% W, 3–5% Fe, and 0.8–1.5% C. The overlay must exhibit excellent bond strength to the substrate (typically 316 stainless steel or Inconel 718 for USC components) and must be free from defects such as cracks, lack of fusion, and inclusions.

Ultrasonic Testing Methodology

The study evaluates several UT techniques for Stellite overlay inspection:

Conventional Contact UT (Single Crystal Probe):

• Frequency: 2.5–5 MHz
• Probe angle: 45°–70° (for shear wave testing)
• Couplant: Water or glycerin
• Technique: Skip-echo and direct beam methods
• Sensitivity: 6 dB above the smallest acceptable defect equivalent

Phased Array UT (PAUT):

• Frequency: 2.25–5 MHz
• Element count: 32–64
• Beam angle range: 30°–80°
• Technique: Sector scan, step scan, and B-scan imaging
• Advantage: Superior defect characterization and imaging capability

Time-of-Flight Diffraction (TOFD):

• Frequency: 5–10 MHz
• Technique: P-wave TOFD with dual probes
• Advantage: High sensitivity to planar defects (cracks, lack of fusion)

Defect Types and UT Response

The study identifies the following defect types in Stellite overlays and their UT characteristics:

1. Lack of fusion (LOF): Appears as a linear reflection at the overlay-substrate interface. The reflection amplitude depends on the orientation and extent of the unfused area. PAUT provides the best characterization of LOF defects due to its ability to image the interface geometry.
2. Cracks: Can occur in the overlay, at the interface, or in the HAZ. Cracks in the overlay appear as high-amplitude, linear reflections. The cobalt-based matrix of Stellite has relatively low acoustic impedance compared to the carbide phase, creating strong scatter from carbide particles that can mask small cracks. Higher frequency probes (5 MHz) improve sensitivity but reduce penetration.
3. Inclusions (slag, oxide): Appear as discrete, irregular reflections. Distinguishable from cracks by their shorter duration and lower amplitude. In Stellite overlays, oxide inclusions are more common than slag inclusions due to the high-temperature melting characteristics of cobalt-based alloys.
4. Porosity: Gas porosity appears as small, discrete reflections. In PTA (plasma transferred arc) overlays, porosity is less common than in arc welding methods due to the inert gas shielding and controlled melting. However, porosity can occur if the powder feed rate is inconsistent or if the substrate is contaminated.
5. Undercut: A geometric defect at the overlay edge. Difficult to detect by UT but can be identified by PAUT imaging or by combining UT with visual inspection (VT).

UT Procedure and Acceptance Criteria

The recommended UT procedure follows the principles of NB/T 47013 and ASME V, with the following specific considerations for Stellite overlays:

• Surface preparation: The overlay surface must be ground smooth (Ra < 10 μm) to ensure good acoustic coupling. Excessive grinding may reduce overlay thickness below the minimum specification.
• Probe selection: For overlays thinner than 3 mm, a 5 MHz probe is recommended; for thicker overlays, 2.5–3.5 MHz is preferred for better penetration.
• Calibration: Calibration blocks with artificial defects (flat bottom holes, side-drilled holes) matching the overlay thickness and material are required. The acoustic impedance of Stellite (approximately 7.5 × 10⁶ kg/(m²·s)) differs significantly from steel (approximately 47 × 10⁶ kg/(m²·s)), requiring careful calibration.
• Acceptance criteria: Typically, no cracks or lack of fusion are acceptable. Porosity and inclusions are acceptable if their individual size is less than 0.5 mm and their total area is less than 2% of the inspected area, in accordance with ASME V or project-specific specifications.

Engineering Practice and Case Studies

The study presents case studies from Waigaoqiao Power Plant, where Stellite overlay layers on steam turbine components were inspected before and after service. Key findings include:

• Pre-service inspection: 85–90% of components passed UT inspection with no significant defects. The remaining 10–15% required rework due to lack of fusion or cracking detected at the interface.
• Post-service inspection (after 10,000–20,000 operating hours): Some components exhibited cracking initiation at the overlay-substrate interface, attributed to thermal fatigue and cyclic stress. The UT sensitivity to these service-induced cracks was found to be lower than for manufacturing defects due to the reduced contrast in acoustic impedance between the cracked and uncracked material.

The study also highlights the challenges of UT inspection on curved surfaces (e.g., turbine blade profiles), where probe coupling and beam orientation are difficult to maintain. The use of custom-made immersion probes and water-coupled systems is recommended for such geometries.

Study Insights and Implications

The most significant insight from this research is the recognition that UT inspection of Stellite overlays presents unique challenges that are not fully addressed by standard UT procedures developed for steel welds. The high acoustic impedance contrast between the cobalt-based overlay and the steel or nickel-based substrate creates strong interface reflections that can mask small defects. Additionally, the presence of carbide particles (Cr₇C₃, Cr₂₃C₆) in the Stellite microstructure creates significant grain noise that reduces the signal-to-noise ratio for defect detection.

The study recommends a multi-technique approach combining UT with other NDT methods for comprehensive quality assurance:

• UT (PAUT preferred): For detection of volumetric and planar defects
• Magnetic Particle Testing (MT): For surface and near-surface cracks in ferromagnetic substrates
• Penetrant Testing (PT): For surface cracks in non-ferromagnetic overlays
• Radiographic Testing (RT): For volumetric defects (porosity, inclusions), though limited by the high density of cobalt-based materials which require higher-energy X-ray sources

The integration of PAUT with advanced signal processing techniques — including Total Focusing Method (TFM) and Phased Array Computed Tomography (PACT) — represents the current state of the art for overlay inspection and offers significant potential for improving defect detection sensitivity and characterization accuracy.

Furthermore, the study emphasizes the importance of establishing traceable and reproducible UT procedures. The calibration standards, probe selection, and acceptance criteria must be clearly documented and consistently applied across all inspection activities. Training and certification of UT personnel for Stellite overlay inspection — which requires specialized knowledge of cobalt alloy acoustics — is also critical.

In conclusion, this literature provides essential guidance for the quality assurance of Stellite alloy overlays in ultra-supercritical power applications. The findings are directly relevant to engineers and inspectors in the power generation industry who must ensure the integrity of critical turbine and boiler components. The multi-technique inspection approach and the emphasis on specialized UT procedures represent best practices that should be adopted in all similar applications.