CLADDING TECHNOLOGY SHANXI CO., LTDElectroslag Strip Electrode Welding Overlay Flux and Process Research
Literature Overview
This research focuses on the development and optimization of flux formulations specifically designed for electroslag welding (ESW) with strip electrodes used in overlay applications. ESW overlay is widely employed in the fabrication of large-diameter pressure vessels, shafts, and structural components where corrosion resistance or wear resistance is required on thick sections. The flux in ESW serves multiple critical functions: it melts to form a slag pool that provides thermal insulation, stabilizes the arc, deoxidizes the weld metal, and acts as a consumable diluent. The composition of the flux directly determines the chemical composition, mechanical properties, and service performance of the overlay layer.
Flux Composition and Metallurgical Behavior
The study examines flux systems based on manganese-silicon compounds with varying additions of iron powder, aluminum, and deoxidizers. The following table summarizes the key flux formulations evaluated:
| Flux Type | Mn (wt%) | Si (wt%) | Fe Powder (wt%) | Al (wt%) | Purpose |
|---|---|---|---|---|---|
| F-1 | 18-22 | 1.5-2.5 | 40-50 | 0 | Baseline |
| F-2 | 15-18 | 1.0-1.5 | 30-40 | 0.3-0.5 | Enhanced deoxidation |
| F-3 | 20-24 | 2.0-3.0 | 50-60 | 0 | High Mn transfer |
| F-4 | 12-15 | 0.8-1.2 | 20-30 | 0.5-0.8 | Low dilution to base |
The metallurgical interaction between the flux and the molten weld pool governs the partition of alloying elements. Manganese from the flux transfers to the weld metal through diffusion and droplet transfer, while silicon acts as a primary deoxidizer. The iron powder serves as a thermal ballast, increasing the heat input and reducing the cooling rate, which is particularly beneficial for thick overlay sections where controlled solidification is essential.
Process Parameters for ESW Overlay
| Parameter | Range | Optimization Target |
|---|---|---|
| Welding current | 2500-5000 A | Deposition rate vs. penetration |
| Arc voltage | 35-45 V | Slag pool stability |
| Travel speed | 100-250 mm/min | Bead width-to-depth ratio |
| Strip electrode thickness | 4-10 mm | Thermal efficiency |
| Slag pool depth | 15-25 mm | Thermal insulation |
| Electrode angle | 75-85° from horizontal | Arc stability |
| Preheat temperature | 200-400 °C | HAZ hardness control |
Defect Analysis and Countermeasures
The primary defects encountered in ESW overlay welding include:
- Slag inclusion: Caused by insufficient slag pool depth or excessive travel speed. The slag pool must be maintained at a minimum depth of 15 mm to ensure complete coverage of the solidifying metal. Countermeasure: increase current by 10% or reduce travel speed.
- Undercut at the bond line: Results from excessive arc voltage or improper electrode angle. The electrode angle should be maintained at 80° from horizontal to direct the arc force downward, promoting wetting of the base metal.
- Porosity in the upper layer: Occurs when the slag pool becomes too shallow and allows gas entrapment. Maintaining a consistent slag pool depth through current-voltage coordination is essential.
- Cracking at the overlay-base interface: This is the most critical defect. It arises from high carbon and sulfur content in the base metal, combined with excessive cooling rates. The countermeasure involves using a low-carbon, high-toughness transition layer (such as E6010 or equivalent) before applying the final overlay layer, and implementing a post-weld heat treatment cycle of 620-680 °C for 2 hours per 25 mm.
Engineering Practice Considerations
In pressure vessel fabrication, ESW overlay is typically applied to carbon steel or low-alloy steel base materials to provide a corrosion-resistant inner surface. The transition layer is mandatory when the base material has a carbon equivalent (CE) exceeding 0.45. The overlay material selection must comply with the applicable code requirements: for ASME VIII Div.1, the overlay material must satisfy the qualification requirements of ASME IX; for GB/T 150, the weld procedure qualification must follow NB/T 47014.
The study demonstrates that flux F-2 with 0.3-0.5% aluminum addition provides the optimal balance of deoxidation efficiency and mechanical properties for overlay applications involving austenitic stainless steel strip electrodes (such as 309L or 310L). The aluminum effectively reduces oxygen content in the weld metal to below 0.020%, which is critical for maintaining the corrosion resistance of the overlay layer.
Study Insights
The selection of flux composition in ESW overlay is not merely a matter of process efficiency but directly determines the service life of the overlay. A poorly chosen flux can introduce harmful elements such as sulfur and phosphorus into the weld metal, leading to hot cracking and reduced corrosion resistance. Engineers must always perform metallographic examination of the overlay layer, paying particular attention to the microstructure at the bond line and the distribution of inclusions. The research reinforces the principle that flux development must be approached through systematic metallurgical analysis rather than empirical trial-and-error alone.