CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Ultrasonic Testing in Quality Inspection of Stellite Alloy Overlay for Ultra Supercritical Units
Overview and Significance
Ultra-supercritical (USC) power units operating at pressures above 25 MPa and temperatures exceeding 600°C impose extreme demands on component materials and fabrication quality. Stellite alloy overlay is widely applied to critical components such as turbine casings, valve seats, and hot section components to provide resistance to erosion, corrosion, and high-temperature oxidation. The quality of the Stellite overlay directly affects the safety and reliability of these critical components. Ultrasonic testing (UT) has emerged as the primary non-destructive testing method for evaluating the integrity of Stellite overlay layers, and this study note examines its application, capabilities, and limitations.
Stellite Alloy Overlay Characteristics
Common Stellite Alloys in USC Applications
| Alloy Designation | Composition (wt%) | Hardness (HRC) | Typical Application |
|---|---|---|---|
| Stellite 6 | Co-28Cr-6W-5Mo-3Fe | 40-45 | Turbine casing erosion protection |
| Stellite 21 | Co-28Cr-6W-5Mo-3Fe | 40-45 | Valve seat sealing |
| Stellite 25 | Co-25Cr-13Mo-3Ni | 40-45 | High temperature erosion |
| Stellite 31 | Co-28Cr-6W-5Mo-3Fe | 40-45 | Severe erosion environments |
| Stellite 60 | Co-25Cr-13Mo-3Ni | 35-40 | High temperature oxidation |
Overlay Fabrication Methods
Stellite overlay for USC components is typically applied using one of the following methods:
- Electroslag welding (ESW): For thick overlay layers (6-15 mm) on large components
- Submerged arc welding (SAW): For medium thickness overlays (3-8 mm) on medium-sized components
- Plasma transferred arc (PTA) cladding: For thin, precise overlays (1-5 mm) on critical components
- Laser cladding: For localized repairs and thin overlays (0.5-3 mm)
The choice of method depends on the component geometry, required overlay thickness, and the criticality of the application.
Ultrasonic Testing Principles and Methods
Basic UT Principles for Overlay Inspection
Ultrasonic testing of Stellite overlay layers relies on the detection of impedance mismatches at material interfaces. The acoustic impedance of Stellite alloys differs from that of the base metal substrate, creating reflectivity at the interface. The key parameters for UT of overlay layers are:
| Parameter | Typical Value | Purpose |
|---|---|---|
| Transducer frequency (MHz) | 2.5-5.0 | Balance between resolution and penetration |
| Beam angle (degrees) | 0° (normal) or 45°-70° (angled) | Detect different defect types |
| Pulse repetition frequency (PRF) | 200-1000 Hz | Optimize signal-to-noise ratio |
| Gain (dB) | 20-40 | Adjust sensitivity for defect detection |
| Couplant | Water, oil, or glycerin | Ensure acoustic coupling |
Specific UT Methods for Stellite Overlay
Normal Beam Inspection
Normal beam (0°) inspection is used to detect planar defects parallel to the surface, such as lack of bonding, delamination, and stratification. The technique requires careful calibration using reference blocks with known defect sizes. For Stellite overlays, the acoustic velocity in the overlay material is approximately 5800 m/s (longitudinal) and 3200 m/s (shear), which differs from typical carbon steel values of 5900 m/s and 3230 m/s respectively. This velocity difference must be accounted for in depth calculations.
Angled Beam Inspection
Angled beam (shear wave) inspection is used to detect defects at various orientations, including cracks, porosity, and inclusions. The typical beam angles used for Stellite overlay inspection are:
- 45°: For detecting near-surface defects and shallow cracks
- 60°: For detecting mid-depth defects and general bonding quality
- 70°: For detecting deep defects near the substrate interface
The choice of beam angle depends on the overlay thickness and the expected defect orientation.
Phase Array Ultrasonic Testing (PAUT)
PAUT technology has revolutionized the inspection of Stellite overlays by providing real-time imaging capabilities and flexible beam steering. The key advantages of PAUT for Stellite overlay inspection include:
- Real-time A-scan, B-scan, and C-scan imaging
- Beam steering and focusing without physical transducer movement
- Improved defect characterization and sizing
- Reduced inspection time by 30-50% compared to conventional UT
Typical PAUT parameters for Stellite overlay inspection include a 16-element or 32-element transducer with element pitch of 1.0-1.5 mm, element width of 1.0-1.5 mm, and center frequency of 2.5-5.0 MHz.
Defect Detection Capabilities and Limitations
Detectable Defects
| Defect Type | Detection Method | Minimum Detectable Size | Reliability |
|---|---|---|---|
| Lack of bonding | Normal beam UT | 0.5 mm diameter | High |
| Delamination | Normal beam UT | 1.0 mm diameter | High |
| Cracks (surface-breaking) | Angled beam UT | 0.2 mm length | Medium-High |
| Cracks (internal) | Angled beam UT | 0.5 mm length | Medium |
| Porosity | Normal beam UT | 0.5 mm diameter | Medium |
| Inclusions | Normal beam UT | 1.0 mm equivalent | Low-Medium |
| Stratification | Normal beam UT | 2.0 mm thickness | High |
Limitations and Challenges
The UT inspection of Stellite overlays faces several challenges that must be understood:
- Grain structure effects: The coarse grain structure of Stellite alloys, particularly in ESW deposits, can cause significant grain scattering that reduces signal-to-noise ratio and limits defect detectability. This is particularly problematic for frequencies above 5 MHz.
- Interface reflections: The impedance mismatch at the Stellite-steel interface creates strong reflections that can mask nearby defects. The use of phased array techniques with appropriate beam angles helps mitigate this issue.
- Geometry complexity: USC components often have complex geometries with curves, corners, and varying thicknesses that challenge UT inspection. Special probes and scanning techniques are required for these geometries.
- Surface preparation: The rough surface of as-welded Stellite overlays requires grinding or machining before UT inspection. The surface roughness should be Ra ≤ 6.3 μm for reliable UT results.
- Temperature effects: For in-service inspection of USC components, the elevated operating temperature affects UT signal characteristics. Compensation for temperature-induced velocity changes is necessary.
Calibration and Acceptance Criteria
Calibration Procedures
The calibration of UT equipment for Stellite overlay inspection follows a systematic procedure:
- Reference block preparation: Fabricate reference blocks with known defect sizes (flat bottom holes, side-drilled holes, or artificial cracks) in material similar to the Stellite overlay.
- Velocity measurement: Measure the longitudinal and shear wave velocities in the Stellite alloy using pulse-echo or through-transmission techniques.
- Gain setting: Set the gain to produce a reference signal from the smallest acceptable defect size.
- Distance-amplitude calibration: Establish the distance-amplitude curve (DAC) or time-corrected gain compensation (TCGC) for accurate defect sizing.
Acceptance Criteria
The acceptance criteria for Stellite overlay inspection in USC applications are typically defined by the relevant code or specification. For ASME Section VIII, the following criteria are commonly applied:
| Defect Type | Maximum Acceptable Size | Maximum Number Per Area |
|---|---|---|
| Lack of bonding | 3 mm equivalent diameter | 3 per 100 cm² |
| Porosity | 2 mm equivalent diameter | 5 per 100 cm² |
| Cracks | None acceptable | 0 |
| Stratification | 1 mm thickness | None acceptable |
Engineering Practice and Case Studies
Case Study 1: Turbine Casing Overlay Inspection
A 1000 MW USC turbine casing with Stellite 6 overlay on the high-pressure cylinder was inspected using PAUT. The overlay thickness was 8 mm, applied by ESW in 4 passes. The inspection revealed:
- Two areas of lack of bonding, each approximately 50 mm × 20 mm, located near the weld start and stop positions
- Multiple small porosity indications (0.5-1.5 mm) distributed throughout the overlay
- No cracks or stratification detected
The lack of bonding areas were repaired by grinding back to sound material and re-applying the overlay using PTA cladding. The porosity was within acceptable limits and did not require repair.
Case Study 2: Valve Seat Overlay Inspection
A main steam valve with Stellite 21 overlay on the valve seat was inspected using conventional UT. The overlay thickness was 3 mm, applied by SAW. The inspection identified:
- A 15 mm long crack near the valve seat edge, detected using 60° angled beam
- Multiple indications of dilution exceeding 20% in the first pass, detected by hardness mapping and spectroscopic analysis
The crack was repaired by TIG welding with matching Stellite 21 filler metal, followed by re-inspection using both UT and MT.
Study Insights and Conclusions
The application of ultrasonic testing to Stellite alloy overlay inspection in USC applications is a mature but challenging practice that requires specialized knowledge and equipment. The key insights from this study are:
- PAUT technology provides significant advantages over conventional UT for Stellite overlay inspection, particularly in terms of imaging capability and inspection efficiency.
- The grain structure of Stellite alloys is the primary limitation on UT detectability, and frequency selection must balance resolution requirements with signal-to-noise considerations.
- A combination of UT with other NDT methods (MT, PT, hardness testing, and spectroscopic analysis) provides the most comprehensive quality assessment.
- Calibration procedures must be carefully designed for Stellite overlay materials, accounting for the unique acoustic properties and microstructure.
- Acceptance criteria should be tailored to the specific application, considering the criticality of the component and the consequences of undetected defects.
The continued development of advanced UT techniques, including electromagnetic acoustic transducers (EMAT) and laser ultrasonics, promises to further improve the inspection capabilities for Stellite overlays in USC applications. Engineers should stay current with these developments and consider their application in critical inspection scenarios.