Zhuojin Pipe Fitting Co., LtdComparison of Strip Electrode Electroslag and Submerged Arc Overlay Welding for Thick-Walled Pressure Vessel Stainless Steel Cladding
Literature Overview
This 1997 paper by Wang Jiachun and Sun Dunwu from the Harbin Welding Research Institute, published in the journal Welding (焊接), presents a comprehensive comparative study of strip electrode electroslag overlay welding (S-ESOW) and strip electrode submerged arc overlay welding (S-SAOW) for stainless steel cladding of thick-walled pressure vessels. The study covers welding process parameters, weld bead geometry, base metal dilution, overlay layer properties, and fusion zone characteristics. The authors conclude that S-ESOW is superior to S-SAOW and can replace S-SAOW for internal surface cladding of thick-walled pressure vessels.
Core Technical Content
Both S-ESOW and S-SAOW are mechanized strip electrode processes designed for high-deposition-rate overlay welding of thick cladding layers on large-diameter pressure vessels. The fundamental difference lies in the arc stabilization mechanism: S-ESOW uses a molten slag pool to stabilize the arc and control heat input, while S-SAOW relies on a granular flux blanket. This difference has profound implications for weld quality, dilution rate, and process control.
Comparative Technical Parameters
| Parameter | Strip Electrode Electroslag (S-ESOW) | Strip Electrode Submerged Arc (S-SAOW) |
|---|---|---|
| Typical current density | 20–30 A/mm² | 15–25 A/mm² |
| Travel speed | 100–300 mm/min | 100–400 mm/min |
| Electrode width | 20–60 mm | 20–60 mm |
| Base metal dilution rate | 5–12% | 15–25% |
| Overlay layer uniformity | Excellent | Good |
| Weld bead profile | Flat, uniform | Slightly convex |
| Slag inclusion tendency | Low | Moderate |
| Process stability | High (slag pool stabilization) | Good (flux blanket) |
| Equipment complexity | Higher | Lower |
The dilution rate is the most critical differentiating factor. In S-ESOW, the molten slag pool acts as a thermal barrier between the arc and the base metal, reducing the heat transferred to the substrate and consequently lowering the dilution rate. Lower dilution means the overlay layer composition more closely matches the intended stainless steel composition, which is essential for achieving the required corrosion resistance.
Dilution Rate Analysis and Its Significance
For stainless steel cladding applications on carbon steel pressure vessels, the dilution rate directly determines whether the overlay layer achieves the required corrosion resistance. Austenitic stainless steels such as 304L or 316L require a minimum chromium content of approximately 18% and sufficient nickel for austenite stabilization. If dilution is too high, the chromium and nickel content in the overlay layer may fall below these thresholds, compromising corrosion resistance.
The authors demonstrate that S-ESOW achieves dilution rates significantly lower than S-SAOW—typically in the range of 5–12% compared to 15–25%. This difference has several practical consequences:
- Corrosion resistance: Lower dilution ensures the overlay layer maintains adequate Cr and Ni content for the intended service environment
- Number of layers required: Lower dilution may reduce the number of overlay layers needed to achieve full corrosion-resistant composition
- Cost efficiency: Fewer layers translate to lower material and labor costs
- Residual stress: Lower heat input per layer reduces residual stresses in the cladding
Fusion Zone Characteristics
The fusion zone is the critical transition region between the base metal and the overlay layer, and its characteristics significantly influence the cladding's resistance to cracking and delamination. In S-ESOW, the lower heat input and more controlled solidification rate produce a narrower fusion zone with a more gradual compositional gradient. This reduces the susceptibility to cracking during welding and subsequent thermal cycling.
The microstructural evolution in the fusion zone is governed by the cooling rate and the dilution of base metal into the overlay. S-ESOW produces fusion zones with finer microstructures due to the lower thermal input, while S-SAOW tends to produce coarser microstructures with more pronounced segregation of alloying elements to grain boundaries.
Engineering Practice and Process Selection
The selection between S-ESOW and S-SAOW for pressure vessel cladding should consider the following factors:
- Vessel diameter and geometry: Both processes are suitable for large-diameter vessels, but S-ESOW's superior bead profile may be advantageous for internal cladding where access is limited
- Required overlay thickness: For thick cladding (50 mm or more), S-ESOW's lower dilution rate provides better composition control in subsequent layers
- Equipment availability: S-ESOW requires more specialized equipment, which may limit applicability in some facilities
- Quality requirements: For critical service applications requiring high corrosion resistance, S-ESOW's lower dilution provides a wider safety margin
The Harbin Welding Research Institute's recommendation to replace S-SAOW with S-ESOW for thick-walled pressure vessel cladding is well-supported by the comparative data presented. However, the recommendation should be qualified by the practical consideration that S-ESOW equipment is less commonly available and requires more specialized operator training.
Key Questions and Reflections
The paper raises an important question about the long-term performance of S-ESOW cladding under cyclic thermal and pressure loading. While the lower dilution rate and superior microstructure are beneficial, the residual stress state in S-ESOW cladding may differ from S-SAOW due to the different heat input profiles. Post-weld heat treatment requirements should be evaluated for each process to ensure adequate stress relief.
Additionally, the paper does not extensively address the economic comparison between the two processes. While S-ESOW may require fewer overlay layers, its equipment costs and consumable costs (specialized slag) may offset the savings. A comprehensive life-cycle cost analysis would strengthen the case for process selection.
Summary
This paper provides a rigorous technical comparison that clearly establishes the superiority of strip electrode electroslag overlay welding over strip electrode submerged arc overlay welding for stainless steel cladding of thick-walled pressure vessels. The lower dilution rate, superior bead geometry, and better microstructural characteristics of S-ESOW make it the preferred process for applications requiring high corrosion resistance in the overlay layer. The work contributes valuable comparative data that can guide process selection decisions in pressure vessel manufacturing and repair operations.