CLADDING TECHNOLOGY SHANXI CO., LTDComparison of Stainless Steel Strip Electroslag Welding Overlay and Submerged Arc Welding Overlay for Thick-Walled Pressure Vessels
Literature Overview
This study provides a systematic comparison between strip electroslag welding (ESW) overlay and submerged arc welding (SAW) overlay for stainless steel cladding on thick-walled pressure vessels. The comparison encompasses process parameters, deposition efficiency, overlay quality, dilution control, and economic factors. Thick-walled pressure vessels, such as hydrogenation reactors with wall thicknesses exceeding 50 mm, require substantial overlay thickness (typically 3-8 mm) to achieve the required corrosion resistance, making the selection of overlay process a critical engineering decision.
Core Technical Points
Process Comparison Matrix
The following table presents a comprehensive comparison of the two overlay methods:
| Parameter | ESW Overlay | SAW Overlay |
|---|---|---|
| Deposition Rate | 8-15 kg/h | 2-5 kg/h |
| Single Pass Thickness | 3-6 mm | 2-4 mm |
| Heat Input | 30-60 kJ/mm | 15-35 kJ/mm |
| Dilution Rate | 10-20% | 15-35% |
| Overlay Surface Quality | Good (requires finishing) | Excellent |
| Equipment Complexity | High (ESW machine required) | Low (standard welding machine) |
| Setup Time | 2-4 hours | 0.5-1 hour |
| Flexibility | Low (geometry constrained) | High (various geometries) |
| Cost per kg of Overlay | Lower (high productivity) | Higher (lower productivity) |
Microstructural Comparison
Metallographic analysis reveals significant differences in the overlay microstructure between the two processes. ESW overlay produces a coarser columnar dendritic structure with larger grain sizes due to the higher heat input and slower cooling rate. The dendrite arm spacing in ESW overlay is typically 50-120 micrometers, compared to 20-60 micrometers in SAW overlay. This difference has implications for mechanical properties and corrosion resistance.
| Microstructural Feature | ESW Overlay | SAW Overlay |
|---|---|---|
| Grain Size | Coarse (ASTM 1-3) | Fine (ASTM 4-6) |
| Dendrite Arm Spacing | 50-120 micrometers | 20-60 micrometers |
| Inclusion Content | Higher (slag inclusions) | Lower |
| Columnar-to-Equiaxed Transition | At 40-60% thickness | At 60-80% thickness |
| Sigma Phase Risk | Higher (slower cooling) | Lower (faster cooling) |
| Retained Austenite | 5-15% (for austenitic SS) | 5-15% (for austenitic SS) |
The higher heat input in ESW overlay increases the risk of sensitization and sigma phase formation, particularly in Cr-Ni austenitic stainless steels such as 304 and 316. This necessitates more careful control of interpass temperatures and may require post-weld solution heat treatment to restore full corrosion resistance.
Dilution and Corrosion Performance
Dilution control is the most critical quality parameter for stainless steel overlay. The study demonstrates that ESW overlay can achieve lower dilution rates (10-20%) compared to SAW overlay (15-35%) when properly parameter-controlled, primarily due to the ability to use thicker strip electrodes and the self-shielding effect of the slag pool. However, the dilution gradient profile differs between the two methods. ESW overlay shows a more uniform dilution profile through the overlay thickness, while SAW overlay exhibits a steeper dilution gradient near the fusion line.
Intergranular corrosion testing (ASTM G153) confirms that both methods produce acceptable overlay layers when dilution is controlled below 20 percent. However, the ESW overlay layer demonstrates slightly better resistance to chloride stress corrosion cracking due to the lower dilution and more homogeneous composition throughout the overlay thickness.
Process Selection Guidelines
Application-Based Selection
The study proposes the following selection criteria for thick-walled pressure vessel overlay:
| Application Condition | Recommended Process | Rationale |
|---|---|---|
| Flat or large-diameter cylindrical shells (>1000 mm) | ESW | High productivity, low cost per unit area |
| Small-diameter shells (<500 mm) | SAW | Better geometric flexibility |
| Overlay thickness > 5 mm | ESW (base) + SAW (finish) | Optimal combination of productivity and quality |
| Tight schedule requirements | ESW | Faster deposition rate |
| Limited equipment availability | SAW | Standard welding equipment suffices |
| High dilution sensitivity (e.g., Inconel overlay) | SAW or PTA | Lower heat input, better dilution control |
Multi-Pass Strategy
For thick overlay requirements, the study recommends a hybrid approach:
- Base passes (3-5 passes) using ESW to achieve bulk deposition rapidly at 8-15 kg/h.
- Transition pass using SAW to reduce the dilution gradient and refine the microstructure.
- Finish pass using GMAW or SAW with a finer electrode to achieve surface quality and dimensional accuracy within specification.
This hybrid approach combines the productivity advantages of ESW with the surface quality benefits of SAW/GMAW, achieving an overall cost reduction of 30-40 percent compared to using SAW alone for the entire overlay.
Quality Control Considerations
Non-Destructive Testing Requirements
The following table outlines the NDT requirements for each overlay method:
| NDT Method | ESW Overlay | SAW Overlay | Purpose |
|---|---|---|---|
| Visual Testing (VT) | 100% | 100% | Surface defects, undercut |
| Magnetic Particle Testing (MT) | 100% | 100% | Surface and near-surface cracks |
| Ultrasonic Testing (UT) | 100% of fusion line | 100% of fusion line | Lack of fusion, delamination |
| Radiographic Testing (RT) | 10-20% (random) | 10-20% (random) | Internal porosity, inclusions |
| Hardness Mapping | Grid pattern at 50 mm spacing | Grid pattern at 50 mm spacing | Dilution verification |
| Chemical Analysis | Dilution zone sampling | Dilution zone sampling | Composition verification |
The study emphasizes that UT examination of the fusion line is particularly critical for ESW overlay, as the higher heat input and larger weld pool increase the risk of lack of fusion at the interface, especially in the first pass. The recommended UT technique is phased array ultrasonic testing (PAUT) with a dual-element probe oriented to detect planar defects parallel to the substrate surface.
Mechanical Property Verification
Tensile testing of overlay coupons demonstrates that ESW overlay layers exhibit slightly lower yield strength (by 5-10 percent) compared to SAW overlay layers, attributable to the coarser microstructure and higher retained austenite content. However, the elongation values are comparable, indicating similar ductility. Hardness mapping reveals that ESW overlay produces a more uniform hardness profile through the overlay thickness, while SAW overlay shows a harder surface layer with a sharper transition to the substrate.
Engineering Case Study
The study includes a case study of a 200 m3 hydrogenation reactor with a 16MnR shell (wall thickness 70 mm) requiring 6 mm of 316L stainless steel overlay. The fabrication employed the hybrid ESW + SAW approach:
- Three ESW passes at 800 A, 36 V, 150 mm/min deposited approximately 5 mm of overlay.
- One SAW pass at 400 A, 28 V, 200 mm/min provided the final 1 mm finishing pass.
- Post-weld heat treatment at 1050 degrees C for 2 hours followed by water quench.
- UT examination of the fusion line confirmed full penetration with no lack of fusion.
- Dilution analysis at the fusion line showed 12-15 percent dilution, well within specification.
- Intergranular corrosion testing passed ASTM G153 requirements.
The total overlay time was reduced by approximately 60 percent compared to an all-SAW approach, with a cost saving of approximately 35 percent. The overlay layer demonstrated excellent corrosion resistance in simulated process conditions (350 degrees C, 3% NaCl, 5% H2S) with no evidence of intergranular corrosion after 1000 hours of exposure.
Study Insights and Implications
The literature provides a clear framework for selecting between ESW and SAW overlay methods for thick-walled pressure vessels. The key insight is that neither method is universally superior; rather, the optimal selection depends on the specific application requirements including geometry, overlay thickness, schedule constraints, and quality criticality. The hybrid ESW + SAW approach represents a practical compromise that leverages the strengths of both methods.
For engineering practitioners, the study highlights several important considerations. First, the qualification procedure under NB/T 47014 or ASME IX must account for the specific process combination used, as parameter changes between ESW and SAW passes require separate qualification. Second, the post-weld heat treatment requirements differ between the two methods, with ESW overlay more frequently requiring solution treatment to mitigate sensitization. Third, the cost-benefit analysis must include not only the direct welding cost but also the inspection, rework, and heat treatment costs, which may favor one method over another depending on the specific application.
Overall, the study contributes valuable comparative data that supports informed process selection decisions in pressure vessel fabrication, ultimately leading to improved quality, reduced costs, and enhanced service reliability.