CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Microstructure and Properties of Inconel 625 Cladded Boiler Water-Cooled Wall
Literature Overview
This 2013 publication by Sun Huanhuan, Liu Aiguo, and Meng Fanling investigates the microstructure and mechanical properties of Inconel 625 alloy overlay cladding applied to boiler water-cooled wall tubes. The research from Shenyang University of Technology addresses a critical corrosion resistance challenge in coal-fired power boiler systems, where water-cooled wall tubes are exposed to severe corrosion from ash deposits, acid gases, and high-temperature flue gas environments. The application of Inconel 625 overlay provides an economical corrosion protection strategy compared to full alloy tube replacement.
Core Technical Content
The study examines the metallurgical behavior of Inconel 625 weld overlay deposits on carbon steel water-cooled wall tubes under various welding conditions. Inconel 625 (UNS N06625) is a nickel-chromium-molybdenum superalloy known for exceptional resistance to oxidation, corrosion, and stress corrosion cracking at elevated temperatures.
Material System
| Component | Specification |
|---|---|
| Base material | 20G carbon steel (GB/T 5310) or SA-210C |
| Base tube dimensions | φ32×4 mm or φ51×5 mm |
| Overlay material | Inconel 625 (ERNiCrMo-3 equivalent) |
| Inconel 625 composition | Ni-22Cr-9Mo-3Nb-1Ti (balance) |
| Overlay process | TIG (GTAW) or plasma arc welding |
| Overlay thickness | 0.3-1.5 mm |
| Service temperature | 450-650°C |
| Corrosive environment | SO2, HCl, H2S in flue gas; molten ash |
Microstructural Analysis
The overlay microstructure exhibits the following characteristics:
| Feature | Description | Significance |
|---|---|---|
| Columnar grains | Directional growth from interface to surface | Normal for single-pass thin overlays |
| γ matrix | FCC nickel-based solid solution | Provides ductility and corrosion resistance |
| Laves phase (M6C) | Ni3Mo-based intermetallic | Can form at grain boundaries if cooling rate is low |
| NbC/TiC precipitates | Fine carbides (0.1-0.5 μm) | Strengthening phases; potential cracking sites |
| Segregation at grain boundaries | Mo, Nb, Ti enrichment | Risk of Laves phase formation during slow cooling |
Mechanical Properties
| Property | Overlay (Inconel 625) | Base (20G) | Interface Region |
|---|---|---|---|
| Tensile strength (Rm) | 700-850 MPa | 410-490 MPa | 500-650 MPa |
| Yield strength (Rp0.2) | 350-500 MPa | 245-305 MPa | 350-450 MPa |
| Elongation (A5) | 30-40% | 20-25% | 15-25% |
| Hardness (HV10) | 250-300 | 130-160 | 180-250 |
| Impact energy (23°C) | 100-200 J | 80-150 J | 50-100 J |
Dilution Effects on Performance
The dilution of base metal into the overlay significantly affects the final properties:
| Dilution Level | Cr Content (overlay) | Nb Content (overlay) | Hardness | Corrosion Resistance |
|---|---|---|---|---|
| <10% | >20% | >2.5% | 260-280 HV | Excellent |
| 10-20% | 18-20% | 2.0-2.5% | 240-270 HV | Good |
| 20-30% | 15-18% | 1.5-2.0% | 220-260 HV | Moderate |
| >30% | <15% | <1.5% | 200-240 HV | Poor |
Welding Parameter Optimization
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Current | 80-150 A | Control heat input; limit dilution |
| Arc voltage | 12-18 V | Maintain arc stability |
| Travel speed | 50-150 mm/min | Balance deposition rate with dilution |
| Shielding gas | 100% Ar or Ar/He (70/30) | Prevent oxidation; control arc characteristics |
| Wire diameter | 0.8-1.2 mm | Fine wire for thin overlay control |
| Preheat | Not required (thin tube) | Avoid distortion; tube wall is thin |
| Interpass temperature | ≤150°C | Prevent excessive grain growth |
Engineering Practice Integration
Application Assessment
Inconel 625 overlay cladding on water-cooled wall tubes is most effective in:
- Furnace wall tubes — where ash deposition and internal corrosion are primary concerns.
- Superheater inlet tubes — exposed to high-temperature flue gas with corrosive species.
- Boiler economizer tubes — subject to external corrosion from acid dew point conditions.
- Areas with high ash loading — where molten ash corrosion (sulfidation) is aggressive.
Service Performance Data
Based on field experience with Inconel 625 overlaid tubes:
| Service Condition | Tube Life Improvement | Failure Mode Reduction |
|---|---|---|
| Bituminous coal firing | 2-4x life extension | Internal corrosion reduced 80-90% |
| Coal with high sulfur | 3-6x life extension | Sulfidation failure eliminated |
| Mixed fuel (coal/gas) | 2-3x life extension | General corrosion reduced significantly |
| High-ash coal | 4-8x life extension | Ash erosion-corrosion dramatically reduced |
Quality Control and Acceptance Criteria
| Inspection Method | Criteria | Frequency |
|---|---|---|
| Visual inspection | No cracks, porosity, undercut >0.5 mm | 100% |
| Dye penetrant testing | No linear indications | 100% of joints |
| Hardness testing | Overlay: 240-320 HV; Interface gradient smooth | 1 per 10 m |
| Thickness measurement | Minimum 0.3 mm at thinnest point | 1 per 5 m |
| Hydrostatic test | No leakage at 1.5x design pressure | 100% per pressure boundary |
| Metallographic (witness) | No Laves phase >5% at grain boundaries | Per heat batch |
Study Insights and Reflections
This research highlights the practical application of advanced alloy overlay technology to conventional boiler components, demonstrating that targeted corrosion protection through weld overlay can extend service life dramatically without requiring complete component replacement. The Inconel 625 alloy provides excellent corrosion resistance through its high chromium and molybdenum content, forming a stable chromium oxide passive film that protects against both internal and external corrosion mechanisms.
The critical finding regarding dilution effects underscores the importance of welding parameter control in achieving acceptable overlay performance. Engineers must recognize that excessive dilution degrades both the corrosion resistance and mechanical properties of the overlay, potentially rendering the cladding ineffective. Multi-pass welding with controlled dilution per pass is essential for achieving the required overlay composition.
The economic case for Inconel 625 overlay cladding is compelling when compared to full alloy tube replacement. While the initial overlay cost represents 15-25% of the cost of replacing tubes with full Inconel 625 material, the service life extension of 2-8 times provides significant return on investment. This approach is particularly attractive for existing boiler retrofits where tube replacement would require extensive shutdown time and structural modification.
The research also highlights an important consideration for future boiler designs: the integration of overlay cladding as a design feature rather than a repair measure. By specifying Inconel 625 overlay in the original design for critical tube locations, designers can achieve the corrosion protection benefits while maintaining the structural cost-effectiveness of carbon steel base tubes. This hybrid approach represents an optimal balance between performance and economy in modern boiler design philosophy.