CLADDING TECHNOLOGY SHANXI CO., LTDDouble-Layer Electroslag Weld Overlay Process with Stainless Steel Strip and Flux
Literature Overview and Technical Significance
Electroslag welding (ESW) overlay is a well-established method for producing thick, high-quality weld overlay layers on carbon steel and low-alloy steel substrates, particularly in pressure vessel fabrication, heat exchanger construction, and heavy structural applications. The literature under study focuses on a specific double-layer ESW overlay process that employs a stainless steel strip as the electrode and a specialized flux composition to produce a corrosion-resistant overlay layer. This approach differs from conventional ESW overlay, which typically uses wire electrodes, by leveraging the advantages of strip cladding combined with the high deposition rates and consistent quality of the electroslag process. The double-layer configuration — a base layer for metallurgical bonding and a cap layer for corrosion resistance — addresses the common challenge of achieving both strong interfacial bonding and optimal surface composition in a single welding operation.
Process Configuration and Parameter Optimization
The double-layer ESW overlay process involves two distinct welding stages, each with specific electrode and flux configurations. The first layer (bonding layer) uses a transition alloy strip to ensure metallurgical compatibility between the carbon steel base and the stainless steel cap layer. The second layer (cap layer) uses the final stainless steel strip to achieve the desired corrosion resistance and surface composition. The process is performed in the flat or horizontal position, with the workpiece tilted slightly (5–10°) to promote proper slag pool formation and electrode feeding.
| Parameter | Layer 1 (Bonding) | Layer 2 (Cap) |
|---|---|---|
| Electrode (strip) | 309L / 309 | 308L / 308 |
| Flux type | BNi-2 (Ni-based) | B309 (Fe-based) |
| Current (A) | 600–800 | 500–700 |
| Voltage (V) | 35–40 | 33–38 |
| Travel speed (mm/min) | 80–120 | 100–150 |
| Strip thickness (mm) | 1.5–2.0 | 1.5–2.0 |
| Strip width (mm) | 20–30 | 20–30 |
| Deposition rate (kg/h) | 80–120 | 70–100 |
| Overlay thickness per pass (mm) | 3–5 | 3–5 |
The critical process parameter is the slag pool stability, which is governed by the balance between electrical resistance of the flux, the electromagnetic force acting on the molten pool, and the surface tension of the slag. The BNi-2 flux used for the bonding layer has a higher electrical resistance than the B309 flux used for the cap layer, resulting in a higher voltage and a more stable slag pool at the lower current range. This is intentional, as the bonding layer requires slower deposition to ensure complete melting and mixing at the interface.
Metallurgical Analysis of the Double-Layer Interface
The metallurgical quality of the double-layer ESW overlay is assessed through metallographic examination, hardness mapping, and chemical analysis across the interface. The bonding layer, produced with the 309L strip and BNi-2 flux, develops a microstructure of ferrite-austenite dual-phase matrix with Cr₂₃C₆ and Cr₇C₃ carbides. The cap layer, produced with the 308L strip and B309 flux, exhibits a predominantly austenitic microstructure with dispersed carbides and minimal δ-ferrite content (typically <5%).
The interface between the bonding layer and the cap layer is the most critical region for both mechanical integrity and corrosion performance. The study reports that the interface exhibits a dilution gradient of approximately 0.5–1.0 mm, with the chromium content transitioning from 22–25% (bonding layer) to 18–20% (cap layer). This dilution gradient is acceptable provided the minimum chromium content in the cap layer remains above 18%, which ensures the formation of a continuous chromium oxide passive film and resistance to general and pitting corrosion.
| Region | Cr (%) | Ni (%) | Ferrite Content (%) | Hardness (HV) |
|---|---|---|---|---|
| Base metal | 0.15 | 0.05 | — | 220–260 |
| HAZ | 0.20 | 0.08 | — | 250–280 |
| Bonding layer | 22–25 | 12–14 | 30–40 | 280–320 |
| Interface zone | 20–22 | 10–12 | 15–25 | 290–330 |
| Cap layer | 18–20 | 8–10 | 5–10 | 220–260 |
The hardness profile across the overlay is relatively uniform, with no sharp transitions that could act as crack initiation sites. The slightly higher hardness in the bonding layer is attributed to the higher ferrite content and the presence of harder carbide phases, which is beneficial for wear resistance at the interface where mechanical loading is concentrated.
Standards Compliance and Quality Assurance
The double-layer ESW overlay process must comply with relevant standards for clad plate and weld overlay qualification. The primary standards referenced include ASTM A263 (standard specification for corrosion-resistant clad plate), ASTM A264 (standard specification for corrosion-resistant clad steel plate), NB/T 47002 (technical requirements for steel plates used in pressure vessels), and ASME VIII Div.1 (for pressure vessel applications). The welding procedure qualification follows NB/T 47014 or ASME IX, with additional requirements for intergranular corrosion testing (ASTM A262 Practice E), pitting corrosion resistance testing, and bond strength testing (ASTM E816 or equivalent).
The non-destructive testing (NDT) protocol includes:
- Visual inspection of the entire overlay surface for cracks, undercuts, and surface irregularities
- Magnetic particle testing (MT) of the overlay surface and the weld toe for surface-breaking defects
- Ultrasonic testing (UT) for interfacial bonding quality, using contact methods or immersion techniques
- Radiographic testing (RT) for volumetric defects, particularly porosity and incomplete fusion in the bonding layer
- Penetrant testing (PT) for surface defects in the final cap layer after grinding
A critical quality requirement is the interfacial bond strength, which must exceed 200 MPa for most pressure vessel applications. The ESW process typically achieves bond strengths of 250–350 MPa due to the high heat input and the prolonged molten pool residence time, which promotes complete melting and mixing at the interface.
Engineering Practice and Process Challenges
The practical implementation of the double-layer ESW overlay process faces several challenges that the study addresses through systematic process optimization. The primary challenge is maintaining slag pool stability during the transition from the bonding layer to the cap layer, as the change in flux composition and electrical resistance can cause transient instabilities. The recommended approach is to perform a transition weld of 50–100 mm length using the cap layer flux but with the bonding layer strip, allowing the slag pool to gradually adjust to the new flux composition before switching to the cap layer strip.
Another practical consideration is the management of the cooling rate to prevent cracking in the stainless steel overlay. The high heat input of ESW results in relatively slow cooling, which is generally favorable for preventing solidification cracking in austenitic stainless steels. However, excessive cooling rate in the cap layer — due to high travel speed or excessive preheating of the base metal — can lead to hot cracking along grain boundaries. The study recommends limiting the travel speed to 150 mm/min and maintaining the base metal temperature below 300°C to ensure adequate cooling rate control.
The economic comparison between ESW overlay and alternative cladding methods (explosive cladding, roll-bonded cladding, laser cladding) is also discussed. ESW overlay offers the advantage of producing thick overlay layers (up to 10–15 mm) in a single operation, with high deposition rates (70–120 kg/h) and relatively low material costs. However, it is limited to flat or mildly curved geometries and requires specialized equipment, making it less suitable for complex-shaped components or small-scale production runs.
Study Insights and Conclusions
The double-layer ESW overlay process with stainless steel strip and specialized flux represents a robust and efficient method for producing high-quality corrosion-resistant overlay layers on carbon steel substrates. The process combines the high deposition rates of ESW with the metallurgical advantages of strip electrodes, producing overlay layers with excellent interfacial bonding, uniform composition, and superior corrosion resistance. The key to successful implementation lies in careful parameter control, rigorous flux selection, and comprehensive quality verification through both destructive and non-destructive testing methods. Engineers working in pressure vessel fabrication and heavy equipment manufacturing should consider this process as a viable alternative to explosive cladding or roll-bonded cladding for applications requiring thick overlay layers on flat or mildly curved surfaces, particularly where the economics of high-volume production favor the ESW approach.