Zhuojin Pipe Fitting Co., Ltd90 mm Wide Strip Electrode Stainless Steel Double-Layer Electroslag Overlay Welding on 2.25Cr-1Mo-0.25V Steel
Literature Overview
This paper by Li Xiaoqing, Liu Zhiying, and Zhang Kun, published in the "Pressure Vessel" journal (2006, Vol. 23, No. 5, pp. 17-21), presents a comprehensive process development study on 90 mm wide strip electrode electroslag overlay welding of stainless steel onto 2.25Cr-1Mo-0.25V steel. The work was conducted at China Second Heavy Machinery Group Corporation, a major manufacturer of large-scale pressure vessels and nuclear equipment. The research addresses the challenge of achieving high-quality, thick stainless steel overlay layers with controlled dilution and excellent hydrogen blistering resistance.
The 2.25Cr-1Mo-0.25V steel (equivalent to P91 or similar grade) is widely used in high-pressure, high-temperature applications such as supercritical boiler headers, reactor pressure vessels, and nuclear steam generator components. The addition of 0.25% V provides enhanced creep strength at elevated temperatures. However, this alloy has limited corrosion resistance in certain environments, necessitating a stainless steel overlay layer for components exposed to corrosive media.
Process Development Approach
Electroslag Overlay Welding Selection Rationale
Electroslag welding (ESW) was selected for this application due to several advantages:
| Advantage | Description |
|---|---|
| High deposition rate | 90 mm strip electrode enables very high deposition rates, suitable for thick overlay layers |
| Low dilution | The slag pool shields the molten weld pool from excessive base metal mixing |
| Consistent quality | The electroslag process is inherently stable with minimal spatter |
| Thick layer capability | Multiple passes can build up thick overlay layers efficiently |
| Low residual stress | The slow cooling rate and slag insulation reduce residual stresses |
The 90 mm strip electrode width is notably large, enabling each pass to cover a substantial area. This reduces the number of passes required and minimizes the total heat input cycles on the substrate.
Double-Layer Strategy
The paper describes a double-layer overlay strategy, which is a common approach for achieving the required corrosion resistance with controlled dilution:
| Layer | Purpose | Material |
|---|---|---|
| First layer (bond layer) | Transition between base metal and final overlay | Modified stainless steel with controlled dilution tolerance |
| Second layer (final layer) | Provide the required corrosion resistance | Standard austenitic stainless steel (e.g., 309L or 310L) |
The double-layer approach is necessary because the dilution from the first layer onto the 2.25Cr-1Mo-0.25V substrate can be significant. By using a modified composition for the first layer and a standard composition for the second layer, the final surface composition can be controlled to meet corrosion resistance requirements.
Magnetic Field Control Technology
A distinctive feature of this research is the use of magnetic field control (磁控) for weld bead management. The application of a transverse magnetic field to the electroslag weld pool can:
- Control bead width: The Lorentz force acts on the electric current in the molten pool, influencing the shape and width of the weld bead.
- Improve bead profile: Magnetic field control can flatten the bead profile, reducing the need for excessive overlap between adjacent passes.
- Enhance slag flow: The magnetic field influences the electromagnetic stirring of the slag pool, improving the stability of the electroslag process.
- Reduce defects: By controlling the weld pool geometry, the magnetic field can reduce the formation of slag inclusions and porosity.
Magnetic Field Parameter Optimization
| Parameter | Typical Range | Effect |
|---|---|---|
| Magnetic field strength | 0.1-0.5 T | Higher strength provides more pronounced bead control |
| Field orientation | Transverse to weld direction | Maximizes the effect on bead width |
| Field coverage | Full bead width | Ensures uniform control across the entire bead |
The magnetic field control technology is particularly valuable in electroslag welding because the large weld pool is more susceptible to instability and geometric variation. By applying a controlled magnetic field, the operators can maintain consistent bead geometry throughout the welding process, even as the substrate geometry changes.
Hydrogen Blistering Resistance
The hydrogen blistering test is a critical quality requirement for overlay welded components in certain service environments. Hydrogen blistering occurs when atomic hydrogen diffuses into the overlay layer and accumulates at microstructural interfaces, forming blisters that compromise the corrosion resistance and structural integrity of the overlay.
Hydrogen Blistering Test Results
The paper reports that the overlay layers passed the hydrogen blistering test, which is a significant achievement for electroslag overlay welding. The hydrogen blistering resistance is influenced by:
| Factor | Influence on Hydrogen Blistering |
|---|---|
| Carbon content | Lower carbon reduces carbide interfaces where hydrogen can accumulate |
| Ferrite content | Excessive ferrite can provide pathways for hydrogen diffusion |
| Grain size | Finer grains reduce the diffusion distance for hydrogen |
| Inclusion content | Inclusions can act as hydrogen traps, promoting blistering |
| Residual stress | Tensile residual stresses can promote blister initiation |
The electroslag process, with its slow cooling rate and slag insulation, naturally produces a coarser but more homogeneous microstructure with fewer inclusions. This is advantageous for hydrogen blistering resistance, as the reduced inclusion content minimizes hydrogen trapping sites.
Process Parameter Determination
Electrode and Flux Selection
| Component | Specification |
|---|---|
| Strip electrode width | 90 mm |
| Strip electrode material | Austenitic stainless steel (composition optimized for low dilution) |
| Flux type | Electroslag welding flux, low-hydrogen type |
| Flux composition | SiO₂-Al₂O₃-CaF₂ system |
Welding Parameter Optimization
| Parameter | Typical Value | Optimization Consideration |
|---|---|---|
| Welding current | 5000-8000 A | Higher current increases deposition rate but may increase dilution |
| Arc voltage | 35-45 V | Affects slag pool viscosity and weld pool geometry |
| Travel speed | 300-600 mm/min | Slower speed increases heat input and dilution |
| Strip electrode stick-out | 50-80 mm | Affects arc stability and heat distribution |
| Magnetic field strength | 0.1-0.3 T | Optimized for bead width control |
The large current values (5000-8000 A) are characteristic of electroslag welding and enable the high deposition rates that make this process economically attractive for thick overlay layers. The voltage range of 35-45 V corresponds to the arc length and slag pool conditions required for stable electroslag welding.
Quality Verification
Physical and Mechanical Testing
| Test | Purpose | Acceptance Criteria |
|---|---|---|
| Spectroscopic analysis | Verify overlay composition | C, Cr, Ni, Mo within specified ranges |
| Hardness testing | Assess microstructure and dilution | Hardness within specified range (typically 150-250 HV for austenitic) |
| Tensile testing | Verify mechanical properties | Adequate strength and ductility |
| Bend testing | Assess ductility and lack of cracking | No cracking on the bend face |
| Hydrogen blistering test | Verify resistance to hydrogen damage | No blisters after specified exposure time |
Microstructural Analysis
The microstructural analysis would reveal:
- First layer: Mixed ferrite-austenite microstructure due to dilution from the 2.25Cr-1Mo-0.25V substrate
- Second layer: Predominantly austenitic microstructure with controlled ferrite content (typically 2-10% δ-ferrite)
- Bond interface: Metallurgical bond with no lack of fusion or cracks
The controlled ferrite content in the second layer is important because it provides resistance to solidification cracking while maintaining the corrosion resistance of the austenitic matrix.
Key Reflections and Study Insights
The most significant technical contribution of this paper is the demonstration that 90 mm wide strip electrode electroslag welding can produce high-quality stainless steel overlay layers with excellent hydrogen blistering resistance on 2.25Cr-1Mo-0.25V steel. The combination of magnetic field control and double-layer strategy represents a sophisticated approach to managing dilution and ensuring final overlay quality.
The magnetic field control technology is particularly noteworthy because it provides a unique means of controlling weld geometry in electroslag welding. This technology has potential applications beyond overlay welding, including in the electroslag welding of thick sections where bead geometry control is critical.
A practical consideration is the equipment requirement. The 90 mm strip electrode electroslag welding system requires specialized equipment including a large-capacity power supply, strip electrode feed mechanism, flux delivery system, and magnetic field generation equipment. This represents a significant capital investment that must be justified by the production volume and quality requirements.
The hydrogen blistering resistance achievement is particularly important for nuclear applications, where the overlay layers must withstand prolonged exposure to hydrogen-containing environments. The electroslag process's natural advantage in producing inclusion-free overlay layers is a significant quality benefit.
Reference Value and Outlook
This paper provides a comprehensive technical reference for electroslag overlay welding of stainless steel onto high-alloy steels. The combination of wide strip electrode, magnetic field control, and double-layer strategy offers a powerful tool for achieving thick, high-quality overlay layers with controlled dilution. Future developments could include the integration of online dilution monitoring, the extension of magnetic field control to more complex geometries, and the application of this technology to other high-value components requiring corrosion-resistant overlay. The work demonstrates that advanced welding technologies can meet the demanding quality requirements of nuclear and high-pressure equipment manufacturing.