CLADDING TECHNOLOGY SHANXI CO., LTDStrip Electroslag Welding Cladding Process and Its Applications
Literature Overview
Strip electroslag welding (ESW) cladding, also known as strip cladding or electroslag overlay, represents one of the most productive methods for producing large-area corrosion-resistant and wear-resistant surfaces on thick-section base materials. The literature under review covers the fundamental principles of ESW cladding, process parameter optimization, typical applications in pressure vessel and heat exchanger fabrication, and quality assurance considerations. This study note synthesizes the key technical content and connects it with practical engineering experience.
Core Principles and Process Mechanism
Strip electroslag cladding operates on the principle of electroslag welding, where a continuous strip of cladding material is fed through a slag pool formed by the interaction of flux with the molten metal. The molten slag pool serves as both a heat source and a protective atmosphere, providing deep and uniform penetration into the base metal. The thermal cycle is characterized by high heat input, slow cooling rates, and a large molten pool, which fundamentally distinguishes it from arc-based cladding methods such as submerged arc welding (SAW) or gas metal arc welding (GMAW).
The process involves three main components: the cladding strip (typically stainless steel, nickel-based alloy, or specialty alloy strip), the flux composition (which governs slag fluidity, deoxidation, and alloying), and the base metal preparation. The strip is fed from the rear while the welding head travels along the joint, maintaining a stable slag pool through continuous flux addition.
Key Process Parameters
| Parameter | Typical Range | Influence |
|---|---|---|
| Welding current | 1000–3000 A | Controls penetration depth and travel speed |
| Voltage | 25–40 V | Governs arc stability and heat input |
| Travel speed | 50–300 mm/min | Affects dilution and deposition rate |
| Strip thickness | 1.0–2.0 mm | Determines single-pass deposit thickness |
| Flux composition | CaF₂-based with alloying additions | Controls slag properties and cladding composition |
| Preheat temperature | 100–250 °C | Mitigates cracking risk in high-carbon base metals |
| Interpass temperature | 250–400 °C | Controls cooling rate and microstructure |
| Dilution rate | 5–30% | Critical for maintaining cladding alloy properties |
The dilution rate is perhaps the most critical parameter in ESW cladding. Because of the deep penetration characteristic of the process, the base metal dilution can reach 20–30% in a single pass, which is significantly higher than in arc cladding methods. For applications requiring strict control of the cladding layer composition—such as Hastelloy C276 or Inconel 625 overlays—this high dilution necessitates careful selection of the strip composition to compensate for base metal alloying element dilution.
Microstructure and Metallurgical Considerations
The ESW cladding layer microstructure is typically characterized by a coarse columnar grain structure growing from the fusion boundary into the cladding deposit. The slow cooling rate inherent to the process promotes the formation of:
- Coarse ferrite-austenite structures in duplex stainless steel cladding
- Sigma phase precipitation at grain boundaries in nickel-based alloy overlays
- Carbide precipitation along grain boundaries in austenitic stainless steel deposits
These microstructural features have direct implications for corrosion resistance and mechanical properties. The sigma phase, in particular, is a significant concern for nickel-based alloy cladding layers because it depletes chromium from the surrounding matrix and severely reduces corrosion resistance. Countermeasures include:
- Post-weld solution heat treatment at 1050–1150 °C followed by rapid quenching
- Careful control of interpass temperature to limit sigma phase formation
- Selection of strip compositions with higher chromium and niobium content to compensate for dilution
Applications in Pressure Vessel Fabrication
Strip ESW cladding finds extensive application in the fabrication of bimetal pressure vessels, particularly in:
- Hydrogenation reactors with austenitic stainless steel or nickel-based alloy overlays
- Heat exchanger shell-and-tube components requiring corrosion-resistant surfaces
- Large-diameter columns and towers where SAW cladding would be impractical due to excessive pass count
- Storage tanks for aggressive chemical media
The primary advantage of ESW cladding in pressure vessel fabrication is productivity. For thick base plates (20–80 mm), ESW cladding can achieve a deposition rate of 5–15 kg/h, which is 3–5 times higher than equivalent SAW cladding processes. This translates directly into reduced fabrication time and cost for large components.
However, the deep penetration of ESW also presents challenges for pressure vessel applications. The high heat input can cause:
- Distortion and residual stress in thin-walled vessels
- Reduced toughness in the heat-affected zone (HAZ) of the base metal
- Potential hydrogen-induced cracking in high-hardness base metals
According to NB/T 47002 and ASME Section VIII Division 1, the cladding layer must be inspected by magnetic particle testing (MT) or penetrant testing (PT) for surface and near-surface defects, and by ultrasonic testing (UT) for bond integrity at the fusion boundary. The bond strength test, typically performed by the peel test method per ASTM A263/A264, must demonstrate that failure occurs within the cladding layer rather than at the base metal-cladding interface.
Common Defects and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Cracks in cladding layer | High dilution, sigma phase, hydrogen | Reduce interpass temperature, adjust strip composition |
| Lack of bond | Base metal contamination, insufficient heat | Clean base surface, increase current, ensure proper flux coverage |
| Slag inclusion | Inadequate slag removal between passes | Implement rigorous slag removal procedures |
| Porosity | Flux moisture, base metal contamination | Dry flux, clean base surface |
| Excessive dilution | High current, slow travel speed | Reduce current, increase travel speed, use multiple thinner passes |
Engineering Practice Insights
From practical experience, several observations emerge regarding ESW cladding in pressure vessel fabrication. First, the base metal composition must be carefully considered. For low-carbon steel base metals (e.g., Q345R, SA-516 Gr.70), dilution is relatively benign because the diluting elements (iron, manganese) are compatible with most cladding alloys. However, for low-alloy steels with higher alloy content (e.g., 15CrMo, P91), the dilution effect can significantly alter the cladding layer composition and properties.
Second, the geometry of the component plays a crucial role in process feasibility. ESW cladding is most effective on flat or gently curved surfaces. For highly curved surfaces (such as small-diameter tubes or heavily formed heads), the flux containment becomes problematic, and alternative cladding methods such as PTA or laser cladding may be more appropriate.
Third, the post-weld heat treatment (PWHT) requirements for pressure vessels can interact adversely with the cladding layer. The standard PWHT temperatures for carbon steel and low-alloy steel vessels (540–620 °C) can promote sigma phase formation in stainless steel cladding layers if held for extended periods. This requires careful coordination between the PWHT schedule and the cladding layer metallurgy, sometimes necessitating a separate solution treatment of the cladding layer after vessel PWHT.
Study Insights and Implications
The study of ESW cladding reinforces the fundamental principle that process selection must be driven by the specific requirements of the application, including corrosion environment, mechanical loading, operating temperature, and economic constraints. ESW cladding excels in applications requiring large-area, thick cladding layers on thick base plates, but its limitations in composition control and applicability to complex geometries must be recognized.
The evolution of ESW cladding technology has included improvements in flux composition for better deoxidation and reduced slag inclusion, development of specialized strip compositions for specific service conditions, and integration with automated welding systems for improved consistency. Future developments may focus on hybrid processes combining ESW with arc cladding to achieve the productivity of ESW with the composition control of arc methods.
In conclusion, strip ESW cladding remains an indispensable technology for the fabrication of large-scale bimetal components, and a thorough understanding of its metallurgical behavior, process parameters, and quality requirements is essential for engineers involved in pressure vessel and heat exchanger design and fabrication.