CLADDING TECHNOLOGY SHANXI CO., LTDExperimental Study on Stellite Alloy Weld Overlay on Supercritical Main Steam Inlet Pipe Inserts
Literature Overview
This study addressed the critical challenge of applying Stellite alloy weld overlay to supercritical main steam inlet pipe inserts in power generation equipment. Supercritical and ultra-supercritical power plants operate at steam pressures exceeding 22.1 MPa and temperatures above 565°C, creating extreme conditions for pipe insert components that are subjected to high-velocity steam erosion, thermal fatigue, and corrosive attack. The study investigated the feasibility, process parameters, microstructure, and performance of Stellite 6 and Stellite 21 alloy overlay layers deposited on P91 (9Cr-1Mo-V) steel pipe insert substrates using plasma transferred arc (PTA) welding.
Operating Conditions and Material Selection
The supercritical main steam inlet pipe inserts operate under the following conditions, which necessitate the application of hardfacing overlay:
| Operating Parameter | Value | Effect on Component |
|---|---|---|
| Steam Pressure | 23.0–25.0 MPa | High mechanical stress |
| Steam Temperature | 565–585°C | Thermal fatigue, oxidation |
| Steam Velocity | 60–80 m/s | Erosive wear |
| Cycle Operation | 1–2 cycles/day | Thermal cycling fatigue |
| Design Life | 20–30 years | Long-term reliability |
The material selection for the overlay layer was based on the following criteria: resistance to high-temperature oxidation, erosion resistance at elevated temperatures, compatibility with the P91 base metal, and resistance to thermal fatigue cracking. Both Stellite 6 and Stellite 21 were evaluated, with Stellite 6 selected for its superior high-temperature strength and Stellite 21 for its enhanced carbide content and erosion resistance.
Welding Process Development
The PTA welding process was selected for the overlay application due to its advantages in dilution control, deposition efficiency, and microstructure quality. The following process parameters were optimized through systematic trial welding:
| Parameter | Optimized Value | Rationale |
|---|---|---|
| Arc Current | 180–220 A | Sufficient for full melting without excessive dilution |
| Arc Voltage | 16–20 V | Stable arc, good wetting |
| Wire Feed Speed | 2.5–3.5 m/min | Controlled deposition rate |
| Travel Speed | 150–200 mm/min | Balanced heat input |
| Shielding Gas | Argon (99.99%) | Inert atmosphere, no contamination |
| Gas Flow Rate | 15–20 L/min | Adequate protection of weld pool |
| Interpass Temperature | 150–250°C | Prevent cracking, reduce residual stress |
| Number of Passes | 3–4 | Achieve 2.0–3.0 mm overlay thickness |
| Preheat Temperature | 200–300°C | Reduce thermal gradient, prevent cracking |
The overlay thickness was specified as 2.0–3.0 mm to provide adequate erosion resistance while maintaining the structural integrity of the pipe insert. The multi-pass welding sequence was designed to minimize residual stress and ensure uniform overlay thickness across the insert surface.
Microstructure and Phase Analysis
The metallographic examination of the overlay layers revealed the following microstructural features:
| Feature | Stellite 6 Overlay | Stellite 21 Overlay |
|---|---|---|
| Matrix Structure | Dendritic austenite | Dendritic austenite |
| Primary Carbides | Cr7C3, W2C | Cr7C3, Cr23C6 |
| Grain Size | 30–50 μm | 25–40 μm |
| Dilution Zone Width | 0.3–0.5 mm | 0.3–0.5 mm |
| Transition Zone | Narrow, gradual composition change | Narrow, gradual composition change |
| Cracking Tendency | Low | Low |
The dilution zone at the overlay/base metal interface was found to be critical for long-term performance. The study revealed that the dilution rate was maintained at 5–8% for both Stellite alloys, which is within the acceptable range for maintaining the overlay layer properties. The transition zone exhibited a gradual composition gradient from the P91 base metal to the Stellite overlay, with no sharp interface that could serve as a crack initiation site.
Performance Testing Results
The overlay layers were subjected to a comprehensive suite of performance tests to evaluate their suitability for supercritical main steam service:
| Test Method | Stellite 6 Result | Stellite 21 Result | Acceptance Criteria |
|---|---|---|---|
| Hardness (HV) | 220–250 | 320–380 | ≥ 200 HV |
| Tensile Strength (MPa) | 580–620 | 540–580 | ≥ 500 MPa |
| Elongation (%) | 18–22 | 15–19 | ≥ 12% |
| Bond Strength (MPa) | 350–400 | 320–370 | ≥ 250 MPa |
| Thermal Shock (50 cycles, 800→25°C) | No cracking | No cracking | No visible defects |
| Erosion Test (1000 h, 60 m/s steam) | 0.05 mm loss | 0.03 mm loss | ≤ 0.1 mm |
| Intercritical Corrosion (ASTM A263) | Pass | Pass | No intergranular attack |
Both Stellite 6 and Stellite 21 overlay layers met all acceptance criteria, with Stellite 21 demonstrating superior erosion resistance due to its higher carbide content. However, Stellite 6 exhibited better toughness and thermal fatigue resistance, making it the preferred choice for applications where thermal cycling is the dominant failure mode.
Quality Control and Inspection Protocol
A comprehensive quality control protocol was developed for the production of Stellite overlay pipe inserts:
- Pre-weld inspection: Visual examination and magnetic particle testing (MT) of the base metal surface to ensure freedom from defects.
- In-process monitoring: Real-time monitoring of welding parameters (current, voltage, wire feed speed, travel speed) with automatic recording and alarm systems.
- Post-weld NDT: Magnetic particle testing (MT) of each pass to detect surface cracks; ultrasonic testing (UT) of the overlay/base metal interface to detect lack of fusion; radiographic testing (RT) for critical areas to detect internal porosity.
- Mechanical testing: Hardness testing at multiple locations, bond strength testing per ASTM E2339, and tensile testing of overlay layer coupons.
- Heat treatment: Post-weld heat treatment at 760°C for 2 hours to relieve residual stresses and improve the microstructure of the dilution zone.
- Final inspection: Dimensional verification, surface roughness measurement (Ra ≤ 3.2 μm), and final visual and MT inspection.
Study Insights and Reflections
This study addresses a highly critical application in the power generation industry, where the reliability of supercritical main steam pipe inserts directly impacts plant availability and safety. The successful application of Stellite alloy overlay using PTA welding demonstrates that advanced hardfacing technology can significantly extend the service life of critical components in extreme operating environments. The finding that Stellite 21 provides superior erosion resistance while Stellite 6 offers better thermal fatigue performance highlights the importance of material selection based on the dominant failure mechanism in the specific application. The comprehensive quality control protocol developed in this study serves as a valuable template for similar overlay applications in the power industry. Engineers should note that the long-term performance of the overlay layer in service will be influenced by factors beyond the initial welding quality, including operational parameters, maintenance practices, and the overall thermal cycling history of the component. Regular in-service inspection and monitoring of overlay thickness reduction is essential to ensure continued protection against erosion and to plan timely maintenance interventions.