CLADDING TECHNOLOGY SHANXI CO., LTDDomestic Single-Layer Strip Electrode Electroslag Welding Overlay Technology
Literature Overview
Published in 2016 in the journal China Chemical Equipment, this study by Yang Xiaoli, Zhang Jianxiao, Shi Chenglong, Hou Xirong, and Liao Fangming from Lanzhou Lanchi Heavy Equipment Co., Ltd. and the Gansu Provincial Key Laboratory for Special Materials Welding of Pressure Vessels addresses the development and optimization of single-layer strip electrode electroslag welding (ESW) overlay technology. The work is particularly significant in the context of domesticating advanced welding technologies for China's petrochemical and pressure vessel manufacturing industries, where heavy-wall clad components require high-productivity overlay methods.
Technical Background and Significance
Electroslag welding has long been recognized as the preferred method for welding thick-section pressure vessel components (typically > 50 mm wall thickness) due to its high deposition rate (8–15 kg/h), excellent metallurgical quality, and low dilution characteristics. When adapted for overlay applications, ESW offers unique advantages: the slag pool provides superior protection against atmospheric contamination, the heat input is highly controllable, and the process is inherently suited to automated operation on large cylindrical or planar components.
The "single-layer" designation indicates that the overlay is deposited in a single pass using the strip electrode configuration, which simplifies the process compared to multi-pass ESW overlay. This is particularly advantageous for applications requiring a uniform overlay thickness of 3–8 mm on large-diameter pressure vessel shells or heads.
Process Configuration and Parameters
| Parameter | Typical Value | Engineering Rationale |
|---|---|---|
| Strip electrode dimensions | 12 × 3 mm or 16 × 3 mm | Optimized for single-pass deposition |
| Wire electrode | φ3.2–φ4.0 mm | Provides flux and arc stabilization |
| Current | 800–1200 A | Ensures full melting of strip electrode |
| Voltage | 28–38 V | Controls slag pool viscosity and penetration |
| Travel speed | 0.10–0.25 m/min | Balances deposition rate with weld quality |
| Slag flux composition | Basic flux with CaF₂ addition | Improves arc stability and slag fluidity |
| Preheat temperature | 150–250 °C | Reduces cooling rate below critical transformation temperature |
| Post-weld heat treatment | 550–650 °C × 2 h (stress relief) | Eliminates residual stress and temper overlay microstructure |
The single-layer ESW overlay process requires careful control of the slag pool geometry. Unlike conventional ESW butt welding, the overlay application involves an asymmetric geometry where one side of the slag pool is bounded by the base metal surface rather than a matching electrode. This asymmetry creates challenges in maintaining a stable slag pool and achieving uniform overlay thickness across the full width of the strip electrode.
Metallurgical Analysis
The microstructure of the ESW overlay layer is strongly influenced by the cooling rate, which is typically lower than that of arc welding processes due to the thermal mass of the slag pool. For nickel-based alloy overlays (e.g., Inconel 625, Hastelloy C276) on carbon steel substrates, the slower cooling rate promotes:
- Reduced dilution (typically 5–15% versus 20–40% for arc welding processes)
- More equiaxed dendritic growth with reduced columnar grain fraction
- Improved resistance to hot cracking due to lower solidification gradient
- Potential formation of intermetallic phases (e.g., σ-phase) at the interface if dilution exceeds critical levels
The interface region between the overlay and base metal is the most critical zone for long-term service integrity. For hydrogenation reactor applications, where the overlay must resist high-temperature hydrogen attack (HTHA) per API 934 requirements, the interface must be free of:
- Cracks or lack of fusion
- Excessive intermetallic compound formation (σ, μ, or Laves phases)
- Carbon enrichment bands that could serve as preferential paths for hydrogen diffusion
Quality Control Requirements
| Inspection Method | Acceptance Criteria | Standard Reference |
|---|---|---|
| Visual inspection | No undercut > 1 mm, no surface cracks | NB/T 47014 |
| Magnetic particle testing (MT) | No linear indications > 1.5 mm | JB/T 4730.4 |
| Ultrasonic testing (UT) | No lack of fusion, no porosity clusters | JB/T 4730.3 |
| Hardness testing | Within specified range ± 3 HRC | ASTM E18 |
| Dilution analysis | ≤ 15% (for Ni-based overlays) | API 934 |
| HTHA testing (if applicable) | No cracking after 1000 h at service conditions | API 934 |
Engineering Practice and Standards Compliance
The Lanzhou Lanchi Heavy Equipment application is directly relevant to the fabrication of large-diameter hydrogenation reactors and other petrochemical pressure vessels. These vessels typically consist of a carbon steel or low-alloy steel shell with a nickel-based alloy overlay or cladding layer to resist HTHA and corrosion. The domestic development of single-layer strip ESW overlay technology addresses a critical supply chain vulnerability—reliance on imported overlay welding consumables and proprietary welding procedures.
From a standards perspective, the overlay weld procedure must be qualified in accordance with NB/T 47014 (Qualification Test for Welding Procedure of Pressure Vessel) and ASME Section IX (for export applications). Key qualification parameters include:
- Base metal P-No. and F-No. grouping compatibility
- Filler metal classification and chemistry verification
- Thermal cycle control (preheat, interpass, and post-weld heat treatment)
- Non-destructive examination coverage and acceptance criteria
The economic benefits of single-layer ESW overlay are substantial for large-diameter vessels. For a typical 6-meter diameter hydrogenation reactor shell with a 6 mm overlay requirement, the single-layer ESW approach can reduce welding time by 40–50% compared to multi-pass SAW overlay, while simultaneously reducing the number of weld joints and potential defect sites.
Key Reflections
The development of domestic single-layer strip ESW overlay technology represents a meaningful advancement in China's pressure vessel manufacturing capability. The technology's success depends on the integration of several critical factors: consumable development (strip electrodes with appropriate chemistry and mechanical properties), process equipment (stable power sources with precise current control), and procedure qualification (rigorous welding procedure specification development and validation). The study demonstrates that Chinese manufacturers can achieve overlay weld quality comparable to international standards when proper process control and quality assurance systems are in place. The broader implication is that domestication of specialized welding technologies reduces project costs and delivery schedules for China's growing petrochemical and energy sectors.