CLADDING TECHNOLOGY SHANXI CO., LTDStrip Electrode Electroslag Cladding Process and Its Applications
Overview and Background
Electroslag welding (ESW) with strip electrode is a highly efficient cladding process that utilizes the heat generated by the resistance of the electric current passing through the molten slag pool to melt the base metal and the strip electrode. This process is particularly suited for depositing thick cladding layers on large components such as pressure vessels, heat exchangers, and structural plates. The study provides a comprehensive overview of the ESW strip electrode cladding process, including its principles, process parameters, equipment requirements, and industrial applications.
The key advantage of ESW strip electrode cladding over other welding-based cladding methods is its extremely high deposition rate, which can exceed 10 kg/h, compared to 1–3 kg/h for conventional SAW or GMAW cladding. This makes it economically attractive for applications requiring thick overlay layers, such as hydrogenation reactors, storage tanks, and large-diameter pipes.
Core Technical Content
The ESW strip electrode cladding process involves a continuous strip electrode that is fed into a molten slag pool, where it is melted by the heat of the slag and the electric current. The molten metal flows into the mold, which is typically formed by a copper backing plate and a water-cooled mold. The process is characterized by a high heat input, a slow travel speed, and a relatively low dilution rate compared to other arc welding processes.
The study discusses the following key process parameters:
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding current | 600–1200 A | Depends on strip width and thickness |
| Travel speed | 50–150 mm/min | Lower speed for thicker deposits |
| Slag voltage | 30–45 V | Maintained for stable slag pool |
| Strip electrode width | 25–100 mm | Wider strips for larger components |
| Strip electrode thickness | 3–6 mm | Thicker strips for higher deposition rate |
| Slag composition | CaF₂-based or SiO₂-based | Affects wetting and fluidity |
| Preheating temperature | 200–400 °C | Required for thick sections |
The study emphasizes the importance of slag composition in ESW cladding. The slag must have appropriate viscosity and fluidity to ensure stable operation and proper wetting of the base metal. A typical slag composition includes 40–60% CaF₂, 10–20% SiO₂, 5–10% Al₂O₃, and the remainder being flux additives. The slag must be preheated to a temperature of 400–600 °C before use to ensure proper fluidity and to remove moisture.
The study also discusses the concept of "multi-layer cladding" in ESW, where multiple layers of strip electrode are deposited to achieve the desired thickness and composition. The dilution rate in ESW is typically 20–40% for the first layer and decreases to 10–20% for subsequent layers. The multi-layer approach allows for gradual refinement of the overlay composition and ensures adequate bond strength at the interface.
Applications and Case Studies
The study presents several industrial applications of ESW strip electrode cladding:
| Application | Base Metal | Overlay Material | Layer Thickness | Key Requirement |
|---|---|---|---|---|
| Hydrogenation reactor | 16MnR | 309 stainless steel | 10–20 mm | Hydrogen resistance |
| Storage tank | Q345R | 316L stainless steel | 5–10 mm | Corrosion resistance |
| Heat exchanger shell | 15CrMoR | 321 stainless steel | 8–15 mm | High-temperature corrosion |
| Large-diameter pipe | 20# steel | 304 stainless steel | 3–8 mm | Internal corrosion protection |
| Structural plate | Q345B | 309 stainless steel | 5–12 mm | Surface hardening |
The study highlights the hydrogenation reactor as a particularly challenging application due to the combined effects of high pressure, high temperature, and hydrogen embrittlement. The ESW strip electrode cladding provides a thick, uniform overlay layer that effectively protects the base metal from hydrogen attack while maintaining the structural integrity of the reactor.
The study also discusses the inspection requirements for ESW cladding, including ultrasonic testing (UT) for bond strength verification, magnetic particle testing (MT) for surface defect detection, and radiographic testing (RT) for internal defect detection. The acceptance criteria for bond strength are typically in accordance with ASTM A263 or EN 10028-7, with a minimum bond strength of 250 MPa.
Process Challenges and Defect Analysis
ESW strip electrode cladding, while highly efficient, presents several unique challenges:
| Challenge | Description | Solution |
|---|---|---|
| High heat input | Can cause excessive grain growth and distortion | Control travel speed, use multi-layer approach |
| Slag inclusion | Entrapped slag in the weld metal | Optimize slag composition, ensure proper cooling |
| Cracking | Hot cracking or cold cracking in the overlay | Preheat, use appropriate consumables |
| Uneven deposition | Non-uniform thickness due to travel speed variation | Use automated travel control |
| Interface weakness | Poor bonding at the base metal-overlay interface | Ensure adequate heat input and surface preparation |
The study recommends a systematic approach to defect prevention, including thorough surface preparation, proper preheating, controlled travel speed, and post-weld heat treatment. The post-weld heat treatment temperature and duration depend on the base metal and the overlay material, but a typical PWHT for carbon steel and low-alloy steel substrates is at 600–650 °C for 1–2 hours per 25 mm of thickness.
Study Insights and Reflections
This study provides a thorough understanding of the ESW strip electrode cladding process, which is often underutilized in favor of more conventional cladding methods. The key insight is that ESW cladding is not merely a faster alternative to SAW or GMAW cladding but a fundamentally different process with unique advantages and challenges.
The high deposition rate of ESW cladding makes it economically attractive for applications requiring thick overlay layers, but this advantage must be balanced against the higher heat input and the associated risks of distortion and cracking. Engineers must carefully evaluate the specific requirements of each application before selecting ESW cladding as the preferred method.
The study also emphasizes the importance of slag composition and temperature control in ESW cladding. The slag is not merely a shielding agent but a critical process parameter that affects the fluidity, wetting, and overall stability of the welding process. Engineers must pay close attention to slag preparation and handling to ensure consistent and reliable results.
In conclusion, the ESW strip electrode cladding process is a powerful tool for depositing thick, uniform overlay layers on large components. When properly applied, it offers excellent efficiency, quality, and cost-effectiveness. The study serves as an essential reference for engineers working on large-scale bimetallic components in the petrochemical, nuclear, and marine industries.