CLADDING TECHNOLOGY SHANXI CO., LTDDomestic Single-Layer Strip Electrode Electroslag Welding Overlay Technology for Pressure Vessels
Literature Overview
The 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 of Special Materials Welding for Pressure Vessels addresses a critical gap in domestic cladding technology. Published in 2016 in the journal "China Chemical Equipment," this work focuses on developing and validating a single-layer strip electrode electroslag welding (ESW) overlay process tailored for the fabrication of clad-plate pressure vessels in the petrochemical and chemical equipment industries. The research is particularly significant because it targets the domestication of overlay welding technologies that were previously reliant on imported consumables and process specifications, directly contributing to the self-sufficiency goals of China's heavy equipment manufacturing sector.
Core Technical Content and Process Parameters
The single-layer strip electrode ESW overlay process differs fundamentally from multi-layer SAW or multi-pass overlay approaches. In this configuration, a continuous strip electrode (typically 25–50 mm wide, 1.0–2.0 mm thick) serves as both the melting consumable and the heat source carrier, with the electroslag pool acting as the primary thermal source for dilution control and heat input management. The strip electrode is fed continuously at a controlled rate, and the electroslag pool is maintained by a consumable flux or self-fluxing mechanism.
| Parameter | Typical Range | Engineering Significance |
|---|---|---|
| Electrode current | 800–1500 A | Controls heat input and dilution rate |
| Travel speed | 200–500 mm/min | Balances deposition rate and dilution |
| Electrode feed rate | 0.5–1.5 m/min | Must synchronize with travel speed |
| Flux coverage | 25–50 mm slag pool | Ensures stable electroslag formation |
| Base material preheat | 100–250 °C | Reduces cracking susceptibility |
| Interpass temperature | 150–350 °C | Controls microstructure refinement |
| Dilution rate target | 10–25% | Critical for corrosion resistance retention |
The key innovation in this research lies in achieving acceptable overlay properties with a single pass, which dramatically reduces production time and cost compared to traditional multi-layer ESW overlay. The authors systematically investigated the relationship between welding parameters and the resulting dilution rate, microstructure, and mechanical properties of the overlay layer.
Microstructure and Dilution Analysis
The dilution rate is the single most critical quality parameter in single-layer ESW overlay. For corrosion-resistant overlay applications, such as those involving 304L, 316L, or 321 stainless steel strip electrodes on carbon steel or low-alloy steel substrates, the dilution rate must typically be maintained below 25% to ensure adequate chromium and nickel content in the overlay layer. The authors demonstrated that the dilution rate in single-layer ESW is governed by the heat input per unit length, which is a function of current, voltage, and travel speed.
The microstructure of the single-layer overlay typically exhibits a columnar dendritic morphology growing from the fusion boundary upward. At high dilution rates, carbide precipitation along grain boundaries becomes pronounced, particularly in Fe-Cr-Ni systems where M23C6 and Cr7C3 carbides form preferentially at temperatures between 550–800 °C. The authors reported that by optimizing the travel speed and electrode feed rate synchronization, they achieved dilution rates in the range of 15–22% for 316L strip electrode on Q345R base plates, with overlay hardness in the range of 180–220 HV.
Standards Compliance and Quality Assurance
The research addresses compliance with GB/T 150 (Pressure Vessels), NB/T 47014 (Qualification Testing of Welding Procedures for Pressure Vessels and Pressure Parts), and JB/T 4730 (Non-Destructive Testing of Welded Joints in Steel Pressure Vessels). For single-layer ESW overlay, the qualification procedure must demonstrate:
- Bond strength verification through bend testing or shear testing per NB/T 47014
- Dilution rate measurement via spectrographic analysis at multiple positions across the overlay
- Radiographic or ultrasonic testing of the overlay for porosity, lack of fusion, and slag inclusions
- Mechanical property testing including tensile strength, elongation, and hardness
The authors emphasized that single-layer ESW overlay presents unique NDE challenges because the overlay thickness is typically 3–6 mm, which falls within the transitional range where both RT and UT have limitations. They recommended a combined approach using PAUT (Phased Array Ultrasonic Testing) for dilution measurement and MT (Magnetic Particle Testing) for surface defect detection.
Engineering Practice Integration
From a practical standpoint, the single-layer strip electrode ESW overlay process is most applicable to large-diameter vessels, heat exchanger shells, and storage tanks where the overlay area is extensive and production efficiency is paramount. The process is particularly well-suited for:
- Hydrogenation reactor shells requiring 304L or 316L overlay on 16MnR or 15CrMoR base plates
- Ammonia synthesis loop heat exchangers with corrosion-resistant overlay requirements
- Large spherical storage tanks with atmospheric carbon steel shells requiring selective overlay
The economic analysis presented by the authors indicates that single-layer ESW overlay can reduce production time by 40–60% compared to multi-layer SAW overlay, with corresponding reductions in labor cost and consumable usage. However, the process requires specialized equipment including a strip electrode feeder, flux delivery system, and precise travel speed control, which represents a significant capital investment.
Key Technical Challenges and Countermeasures
| Challenge | Root Cause | Countermeasure |
|---|---|---|
| Excessive dilution (>25%) | High heat input, low travel speed | Increase travel speed, reduce current |
| Incomplete fusion at fusion boundary | Low heat input, high travel speed | Increase current, preheat base material |
| Cracking in overlay layer | High carbon equivalent, hydrogen pickup | Preheat base material, post-weld heat treatment |
| Slag inclusion in overlay | Inadequate flux coverage, turbulence | Maintain stable flux pool, control welding current |
| Non-uniform dilution across width | Electrode misalignment, travel speed variation | Implement automatic tracking, synchronize feed and travel |
Study Insights and Implications
This research represents a meaningful contribution to the domestication of pressure vessel cladding technology in China. The single-layer ESW overlay approach offers a compelling alternative to multi-layer processes for large-scale applications, provided that the dilution rate can be reliably controlled. The key lesson for practicing engineers is that process qualification is non-negotiable: each combination of base material, overlay material, and welding parameters must be individually qualified per NB/T 47014, and the dilution rate must be verified at multiple locations across the overlay width. Furthermore, the economic advantages of single-layer ESW overlay should be weighed against the equipment investment required, particularly for smaller fabrication shops. The research also highlights the importance of process parameter synchronization—specifically the coordination between electrode feed rate and travel speed—as the primary mechanism for dilution control in single-layer ESW overlay.