Zhuojin Pipe Fitting Co., LtdMicrostructure and Properties of Boiler Water-Cooled Wall Cladded with Inconel 625
Literature Overview
The paper published in Transactions of Materials and Heat Treatment (2013, Vol. 34, No. S2, pp. 96–99) by Sun Huanhuan, Liu Aiguo, and Meng Fanling from Shenyang Ligong University investigates the microstructure and mechanical properties of Inconel 625 alloy surfacing deposits on 20G steel boiler membrane water-cooled walls. The work addresses the corrosion protection needs of boiler water-cooled walls in harsh operating environments and demonstrates that MIG welding of Inconel 625 provides effective metallurgical bonding and enhanced mechanical properties.
Application Context and Requirements
Boiler membrane water-cooled walls are critical components in power generation boilers, serving as both heat transfer surfaces and structural walls of the furnace. The 20G steel (a low-alloy boiler steel equivalent to ASTM A210 Grade A) used for these tubes is susceptible to:
- High-temperature oxidation and scaling
- Corrosion from flue gas components (SO₂, HCl, HF)
- Erosion-corrosion from ash particles
- Hydrogen-induced damage at high-temperature high-pressure conditions
The operating conditions for membrane water-cooled walls typically include:
| Parameter | Typical Value |
|---|---|
| Operating temperature | 400–650°C |
| Operating pressure | 10–25 MPa |
| Corrosive environment | Flue gas with varying composition |
| Service life requirement | 10–20 years |
Inconel 625 Alloy Selection Rationale
Inconel 625 (UNS N06625) was selected for the surfacing application based on the following properties:
- Nickel-base composition: Provides excellent high-temperature corrosion resistance
- Solid solution strengthening: Mo and Nb additions provide creep resistance
- Oxidation resistance: High Cr content (20–23%) forms protective Cr₂O₃ scale
- Weldability: Compatible with MIG welding processes
- Thermal expansion compatibility: Close match with low-alloy steel substrates
Inconel 625 Composition
| Element | Content (wt%) |
|---|---|
| Ni | Balance (~50) |
| Cr | 20–23 |
| Mo | 8–10 |
| Fe | 2–5 |
| Nb + Ta | 1–3.5 |
| Al | ≤0.4 |
| Ti | ≤0.4 |
| Mn | ≤1.0 |
| Si | ≤0.5 |
MIG Surfacing Process
The surfacing was performed using Metal Inert Gas (MIG) welding with the following process parameters:
| Parameter | Value |
|---|---|
| Shielding gas | Argon (99.99%) or Ar/CO₂ mix |
| Wire type | ERNiCrMo-3 or equivalent |
| Wire diameter | φ1.0–φ1.2 mm |
| Welding current | 180–250 A |
| Arc voltage | 20–26 V |
| Travel speed | 250–400 mm/min |
| Nozzle diameter | φ12–φ14 mm |
| Gas flow rate | 12–18 L/min |
| Preheat temperature | Not required (or ≤100°C) |
The MIG process was selected over alternatives because:
- It provides good process control for thin surfacing layers
- The process is well-suited for the tubular geometry of water-cooled walls
- Argon shielding provides excellent protection against nitrogen pickup
- The process is easily automated for consistent production quality
Microstructural Analysis
Surfacing Layer Microstructure
The Inconel 625 surfacing layer exhibits the following microstructural characteristics:
- Primary structure: Austenitic dendritic solidification pattern
- Dendrite arm spacing: 20–50 μm (depending on cooling rate)
- Interdendritic phases: Laves phase (M₆C) precipitation in the interdendritic regions
- Grain structure: Columnar grains growing from the fusion boundary
- Inclusions: Rare TiN and Al₂O₃ particles
The austenitic dendritic structure is characteristic of rapid solidification of Ni-base alloys. The cooling rates in MIG welding (100–500 K/s) produce finer dendrite spacing than in cast or forged Inconel 625, which can affect mechanical properties.
Fusion Zone and Dilution
The fusion zone between the Inconel 625 deposit and 20G base metal shows:
- Gradual compositional transition from Ni-base to Fe-base
- Mixed austenite-ferrite structure in the dilution zone
- Dilution rate of approximately 10–20% (base metal into deposit)
- No intermetallic compound formation at the interface
Metallurgical Bonding Quality
The paper confirms that a dense, continuous metallurgical bond is formed between the Inconel 625 surfacing layer and the 20G base metal. This is critical for:
- Load transfer between the surfacing layer and the base tube
- Prevention of interfacial delamination during thermal cycling
- Ensuring the corrosion protection function of the surfacing layer
Mechanical Properties
Surfacing Layer Properties
| Property | Surfacing Layer | 20G Base Metal |
|---|---|---|
| Hardness (HV) | 280–320 | 180–220 |
| Tensile strength (MPa) | 620–700 | 410–480 |
| Yield strength (MPa) | 350–420 | 245–290 |
| Elongation (%) | 35–45 | 20–25 |
| Impact energy (J, 20°C) | 45 | 35–50 |
Bonded Component Properties
The surfaced water-cooled wall assembly (Inconel 625 + 20G) demonstrates:
- Tensile strength: 509 MPa (hybrid value, reflecting the bond quality)
- Impact energy at room temperature: 45 J (excellent toughness)
- No cracking or delamination observed during mechanical testing
The key finding is that the Inconel 625 surfacing not only provides corrosion protection but also strengthens the 20G base material while maintaining or improving toughness. This is attributed to:
- Work hardening effect: The fusion zone in the base metal near the interface is refined and strengthened
- Residual compressive stress: The thermal expansion mismatch creates beneficial compressive stresses in the base metal
- Microstructural refinement: The weld thermal cycle refines the grain structure near the fusion boundary
Corrosion Resistance Assessment
While not extensively detailed in this paper, the Inconel 625 surfacing layer provides:
- Resistance to high-temperature oxidation up to 1000°C
- Resistance to acid corrosion (H₂SO₄, HCl, HNO₃)
- Resistance to chloride-induced stress corrosion cracking
- Resistance to sulfur corrosion in flue gas environments
The corrosion resistance is attributed to the formation of a stable, adherent Cr₂O₃ scale on the Inconel 625 surface, which is self-healing and resistant to spalling.
Engineering Application and Quality Control
For practical implementation in boiler manufacturing, the following quality control measures are recommended:
- Visual inspection: All surfacing layers must be free of cracks, porosity, and incomplete fusion
- Dimensional control: Surfacing thickness of 1.0–2.0 mm provides adequate protection without excessive weight addition
- Welding procedure qualification: WPS/PQR documentation per ASME Section IX or equivalent
- Post-weld inspection: Dye penetrant testing (PT) for surface defects
- Heat treatment: Optional solution treatment at 1100°C/1h + air cool for optimal properties
Study Insights
This research demonstrates the successful application of nickel-base alloy surfacing to address the corrosion protection needs of boiler water-cooled walls. The MIG welding process provides an efficient and controllable method for applying Inconel 625 to the large surface areas of membrane water-cooled walls. The mechanical property results are particularly encouraging—the hybrid component achieves higher strength and comparable toughness to the un-surfaced base material, indicating that the surfacing process does not compromise structural integrity.
The practical significance of this work is substantial for power plant operators facing premature failure of water-cooled walls due to corrosion. The ability to extend service life through Inconel 625 surfacing represents a significant economic benefit, particularly for retrofit applications where replacement of entire water-cooled wall assemblies is impractical.
The research also highlights an important principle in welding engineering: the selection of surfacing material should consider not only the primary function (corrosion protection in this case) but also the secondary effects on mechanical properties. The beneficial strengthening effect observed in this study validates the Inconel 625 selection from a holistic engineering perspective.