CLADDING TECHNOLOGY SHANXI CO., LTDINCONEL 690 Strip Electroslag Cladding Process and Overlay Properties
Literature Overview
The research by Zhang Maolong, Yan Changgen (Shanghai Boiler Works Co., Ltd.), and You Junfu (Shanghai Institute of Testing Technology), published in Boiler Technology in 1997, addresses the application of strip electroslag welding (ESW) for Inconel 690 nickel-based alloy overlay. This work is historically significant as it represents early systematic investigation of electroslag cladding for nuclear-grade nickel alloys in China's power generation sector.
Core Technical Content
Inconel 690 (UNS N06690) is a nickel-chromium-molybdenum alloy specifically designed for nuclear steam generator tube applications. Its composition is characterized by:
| Element | Content (wt%) |
|---|---|
| Ni | Balance (≥67%) |
| Cr | 27.0–31.0 |
| Mo | 3.0–4.0 |
| Fe | 1.0–2.5 |
| Mn | ≤1.0 |
| Si | ≤1.0 |
| C | ≤0.08 |
| N | ≤0.05 |
The strip electroslag cladding process uses a continuous strip of Inconel 690 as the filler metal, fed into the slag pool formed by the arc between a consumable electrode (typically a copper strip or graphite electrode) and the workpiece. The molten slag provides thermal insulation and protects the weld pool from atmospheric contamination.
Process Parameters
| Parameter | Typical Range |
|---|---|
| Welding current | 300–600 A |
| Arc voltage | 12–20 V |
| Strip width | 10–20 mm |
| Strip thickness | 1.5–3.0 mm |
| Travel speed | 100–300 mm/min |
| Slag composition | CaF2/CaO/Al2O3/SiO2 system |
| Preheat temperature | 150–300°C |
| Interpass temperature | 150–250°C |
Microstructural Characteristics
The electroslag cladding process produces a distinctive microstructure due to the unique thermal conditions. The high heat input and slow cooling rate characteristic of ESW result in:
- Coarse equiaxed dendritic structure throughout the overlay
- Interdendritic spacing of 50–150 μm
- Presence of delta ferrite in the matrix (controlled by Mo and Nb additions)
- Possible precipitation of carbides at grain boundaries during slow cooling
- Homogeneous solid solution matrix with minimal segregation
The delta ferrite content is a critical parameter for Inconel 690, as it affects both mechanical properties and corrosion resistance. For nuclear applications, delta ferrite content is typically controlled to be below 5% to maintain adequate pitting resistance.
Mechanical Properties and Quality Assessment
The overlay properties achieved through strip ESW of Inconel 690 typically include:
| Property | Target Value |
|---|---|
| Tensile strength | ≥620 MPa |
| Yield strength | ≥275 MPa |
| Elongation | ≥30% |
| Hardness | 180–220 HV |
| Impact energy (room temperature) | ≥100 J |
Quality assessment for nuclear applications requires comprehensive testing including:
- Radiographic testing (RT) for internal defects
- Ultrasonic testing (UT) for laminations and lack of fusion
- Magnetic particle testing (MT) for surface defects
- Hardness mapping across the overlay cross-section
- Chemical analysis of the overlay for dilution verification
- Intergranular corrosion testing (ASTM A263)
- Stress rupture testing for long-term performance
Engineering Application Context
Inconel 690 was developed specifically for pressurized water reactor (PWR) steam generator tubes to address the issues of stress corrosion cracking (SCC) and flow-accelerated corrosion (FAC) experienced by earlier alloys such as Inconel 690 (the original version) and Incoloy 800. The strip ESW cladding process was investigated for repairing or overlaying critical components in nuclear power plants.
The application scenarios include:
- Repair of steam generator tube sheet penetrations
- Overlay of reactor pressure vessel internals
- Cladding of heat exchanger surfaces in nuclear service
- Restoration of worn or corroded nuclear-grade components
Key Insights and Reflections
The strip electroslag cladding approach offers several advantages for thick overlay applications. The high deposition rate (typically 5–15 kg/h) makes it economical for building substantial overlay thicknesses. The slag pool provides excellent thermal control, reducing residual stresses and distortion compared to arc welding processes.
However, the process has significant limitations that must be carefully managed:
- High heat input promotes grain coarsening and may require post-weld heat treatment
- Dilution with base metal can be substantial, particularly in the first pass
- The process is difficult to apply in tight geometries or complex shapes
- Slag inclusion defects are a persistent concern requiring careful slag composition control
- The process requires significant setup time and specialized equipment
From a metallurgical standpoint, the slow cooling rate of ESW is a double-edged sword. While it reduces residual stresses and promotes a more homogeneous microstructure, it also increases the risk of carbide precipitation and delta ferrite formation at grain boundaries. Post-weld solution heat treatment (typically 1050–1100°C for 1–2 hours followed by air cooling) may be necessary to restore the desired single-phase microstructure.
The work by Zhang et al. represents an important contribution to the understanding of electroslag cladding for nickel-based alloys in the nuclear industry. The findings inform current practices in nuclear component repair and maintenance, where Inconel 690 overlay remains a critical technology for ensuring long-term reliability of steam generators and related equipment.
In summary, strip electroslag cladding of Inconel 690 provides a viable method for producing thick, high-quality nickel alloy overlays for nuclear applications, though careful attention to process parameters, dilution control, and post-weld treatment is essential for achieving the required metallurgical quality and service performance.