CLADDING TECHNOLOGY SHANXI CO., LTDStudy on Material Properties and Application of Single-Layer Electroslag Weld Overlay for Pressure Vessels
Literature Overview and Background
Electroslag welding (ESW) is a highly productive welding process widely used for the fabrication of thick-section pressure vessel components. When a corrosion-resistant overlay is required on a thick base plate, electroslag weld overlay (ESW overlay) offers significant advantages in terms of deposition rate and uniformity compared to conventional arc welding processes. The literature under review investigates the material properties and application of single-layer electroslag weld overlay on pressure vessel components, focusing on the challenges of achieving acceptable overlay quality in a single pass and the implications for pressure vessel design and inspection.
Core Technical Findings
Single-layer ESW overlay was performed on carbon steel and low-alloy steel base plates with thicknesses ranging from 60 mm to 150 mm. The overlay material was a 309L or 316L stainless steel electrode with a matching consumable. The key parameters were optimized to achieve full penetration of the overlay layer onto the base metal surface while maintaining acceptable dilution and mechanical properties.
| Parameter | Typical Value |
|---|---|
| Base plate thickness | 60-150 mm |
| Overlay material | 309L or 316L |
| Electrode diameter | 6.0-8.0 mm |
| Welding current | 1000-1800 A |
| Welding voltage | 40-55 V |
| Travel speed | 150-250 mm/min |
| Electrode feed rate | 200-350 mm/min |
| Flux coverage | Continuous, 20-30 mm depth |
| Preheat temperature | 100-150°C |
| PWHT | 620°C × 2h per 25 mm |
| Overlay thickness | 12-20 mm |
The single-layer ESW overlay produced a homogeneous overlay layer with a dilution ratio of 8-15%, which is acceptable for most corrosion service applications. Metallographic examination revealed a coarse-grained austenitic-ferritic microstructure with a ferrite content of 10-18%, which is within the acceptable range for crack resistance. The hardness of the overlay layer was in the range of 200-250 HV, consistent with austenitic stainless steel.
Process Analysis and Standards Considerations
The single-layer approach offers significant productivity advantages over multi-layer ESW overlay, reducing welding time by 40-60%. However, the single-layer process requires careful control of the slag pool depth and composition to ensure consistent penetration and avoid defects such as lack of fusion at the overlay-base interface. The slag composition must be optimized to provide adequate fluidity, deoxidation, and alloying control.
According to ASME VIII Div.1 and NB/T 47002, the single-layer ESW overlay must be qualified through mechanical property testing including tensile tests, bend tests, and impact tests on the overlay material. The overlay layer must also pass intergranular corrosion testing if used in chloride-containing environments. The literature emphasized that the single-layer ESW overlay is suitable for pressure vessel applications where the overlay thickness is limited to 20 mm or less and the service conditions do not require exceptional overlay hardness or wear resistance.
Engineering Practice and Defect Analysis
The following defects and their countermeasures were identified in the literature:
- Lack of fusion at the overlay-base interface: Caused by insufficient preheat or excessive travel speed. Increasing preheat to 150°C and reducing travel speed by 10-15% ensures adequate penetration.
- Undercut at the edge of the overlay: Results from inadequate edge coverage of the slag pool. Using a backing bar or edge guide ensures consistent slag pool shape and prevents undercut formation.
- Porosity in the overlay layer: Associated with moisture in the flux or electrode coating. Strict flux drying and electrode storage procedures are essential.
- Cracking in the overlay layer: Can occur if the ferrite content is too low or the cooling rate is too high. Adjusting the slag composition to increase ferrite content and maintaining adequate preheat prevent cracking.
The literature provided a case study of a single-layer ESW overlay applied to a hydrogenation reactor shell with a base plate thickness of 100 mm. The overlay layer was 15 mm thick and deposited using 316L electrode. The qualification tests confirmed acceptable mechanical properties and corrosion resistance, and the vessel passed hydrostatic testing and in-service inspection after two years of operation.
Study Insights and Reflections
This literature demonstrates that single-layer ESW overlay is a viable and economical alternative to multi-layer ESW overlay for pressure vessel applications. The key advantage is the significant reduction in welding time and labor cost while maintaining acceptable overlay quality. However, engineers must recognize that the single-layer approach has inherent limitations in terms of overlay thickness, microstructure homogeneity, and defect sensitivity. The single-layer ESW overlay is most appropriate for applications where the overlay thickness is moderate (12-20 mm) and the service conditions are not extremely severe. For applications requiring thicker overlay layers or superior microstructural control, multi-layer ESW overlay or alternative processes such as plasma transferred arc (PTA) cladding may be more suitable. The literature reinforces the importance of welding procedure qualification and the need for process-specific optimization to ensure the reliability of ESW overlay in pressure vessel fabrication.