CLADDING TECHNOLOGY SHANXI CO., LTDComparison and Selection of Dual-Phase Stainless Steel Strip Electrode Electroslag Welding Overlay Process
Technical Background and Significance
Dual-phase stainless steels (DPSS), such as 2205, 2507, and 254 SMO, have gained widespread adoption in pressure vessel fabrication, pipeline systems, and heat exchanger manufacturing due to their exceptional combination of high strength, excellent corrosion resistance, and good weldability. The electroslag welding (ESW) process with strip electrodes has emerged as a leading technology for thick-section welding and overlay applications where high deposition rates and superior metallurgical quality are required. The 2014 study by Kong Lingwei, Liu Aimin, Zheng Yan, and Xing Zhuo from the Shenyang Special Equipment Inspection Research Institute and Shenyang Oriental Titanium Industry Co., Ltd. provides a comprehensive comparison of dual-phase stainless steel strip electrode ESW overlay processes, offering critical guidance for process selection in industrial applications.
Electroslag Welding Process Fundamentals
Electroslag welding operates on the principle of electrical resistance heating of a molten slag pool, which in turn melts the electrode and base metal. The slag pool serves as both a heat source and a protective medium, creating a stable and reproducible welding environment. For overlay applications, the ESW process offers several advantages over conventional arc welding methods:
- Extremely high deposition rates, typically 5–10 kg/h, compared to 1–3 kg/h for conventional arc welding
- Uniform weld bead geometry with minimal spatter and excellent surface finish
- Reduced risk of porosity due to the protective slag atmosphere
- Consistent penetration control through adjustable current and travel speed
- Suitability for welding in the vertical position, which is critical for large diameter vessels and pipes
The strip electrode ESW process uses a continuous strip of filler metal, typically 20–40 mm wide and 1.5–3.0 mm thick, which is fed into the slag pool at a controlled rate. The strip electrode is more efficient than a solid wire electrode because it provides a larger cross-sectional area for current flow, resulting in higher deposition rates and lower power consumption.
Comparison of Dual-Phase Stainless Steel ESW Overlay Processes
The study compared multiple ESW overlay process variants for dual-phase stainless steel applications. The key process parameters evaluated included current type (DC and AC), current density, travel speed, strip electrode feed rate, slag composition, and preheat temperature. The following table summarizes the comparison of the most relevant process variants:
| Process Parameter | Variant A (DC+) | Variant B (DC-) | Variant C (AC) | Recommended Range |
|---|---|---|---|---|
| Current type | DC electrode positive | DC electrode negative | Alternating | DC+ or AC |
| Current density | 15–20 A/mm² | 12–18 A/mm² | 10–15 A/mm² | 15–20 A/mm² |
| Travel speed | 100–150 mm/min | 80–120 mm/min | 60–100 mm/min | 100–150 mm/min |
| Strip feed rate | 0.5–0.8 m/min | 0.4–0.6 m/min | 0.3–0.5 m/min | 0.5–0.8 m/min |
| Preheat temperature | 100–150 °C | 150–200 °C | 150–200 °C | 100–200 °C |
| Deposition rate | 6–8 kg/h | 4–6 kg/h | 3–5 kg/h | 5–8 kg/h |
| Dilution ratio | 15–25% | 20–30% | 25–35% | ≤25% |
| Microstructure | Dual-phase (α+γ) | Predominantly α | Predominantly γ | Balanced α+γ |
The most critical finding from the comparison was the influence of current type on the microstructure of the overlay layer. DC electrode positive (DC+) produced a balanced dual-phase microstructure with approximately 40–60% ferrite and 40–60% austenite, which is the optimal balance for corrosion resistance and mechanical properties. DC electrode negative (DC-) tended to produce a predominantly ferritic microstructure due to the higher cooling rate at the electrode tip, which reduced the austenite formation. AC welding produced a predominantly austenitic microstructure, which, while offering excellent ductility, may compromise the high-strength characteristics of the dual-phase stainless steel.
Microstructural Analysis and Phase Balance
The phase balance in dual-phase stainless steel overlay layers is critical for achieving the desired combination of strength and corrosion resistance. The ferrite phase provides high strength and resistance to chloride stress corrosion cracking (SCC), while the austenite phase provides ductility and resistance to pitting and crevice corrosion. An optimal balance of approximately 40–60% ferrite is generally recommended for most industrial applications.
The ESW process, with its high heat input and slow cooling rate, tends to promote austenite formation and can lead to a lower ferrite content than desired. To counteract this tendency, several strategies were identified:
- Using a strip electrode with a higher nitrogen content, which promotes ferrite stability
- Employing a slag composition with higher basicity to increase the cooling rate
- Applying a controlled preheat temperature to manage the thermal cycle
- Using a multi-pass welding sequence with alternating current polarities to achieve a balanced phase distribution
The dilution ratio between the strip electrode filler metal and the base metal is another critical parameter. Excessive dilution from the base metal can shift the phase balance away from the dual-phase region, resulting in a predominantly ferritic or predominantly austenitic microstructure. The study recommended maintaining a dilution ratio below 25% to ensure the overlay layer retains its dual-phase character.
Engineering Practice and Standards Compliance
For pressure vessel applications, the ESW overlay process must comply with the relevant standards including NB/T 47014 for weld procedure qualification, GB/T 150 for pressure vessel design and fabrication, and ASME Section IX for weld procedure and performance qualification. The procedure qualification requirements for ESW overlay welding include:
- Qualification of the welding procedure specification (WPS) with parameters within the qualified ranges
- Mechanical property testing including tensile strength, hardness, and impact toughness
- Microstructural examination to verify the phase balance and absence of detrimental phases such as sigma phase
- Corrosion resistance testing including intergranular corrosion (IGC) testing per ASTM A263 or ASTM A264
- Non-destructive testing (NDT) including radiographic testing (RT) or ultrasonic testing (UT) of the overlay layer
The Shenyang Special Equipment Inspection Research Institute's involvement in this study underscores the importance of independent inspection and verification in ensuring the quality of ESW overlay welds for pressure vessel applications. The systematic approach to process comparison and selection provides a robust framework for engineers to make informed decisions when specifying ESW overlay procedures for dual-phase stainless steel components.
Summary
The comparison of dual-phase stainless steel strip electrode electroslag welding overlay processes reveals that the current type, current density, travel speed, and dilution ratio are the most critical parameters affecting the microstructure and phase balance of the overlay layer. DC electrode positive welding with a current density of 15–20 A/mm² and a travel speed of 100–150 mm/min provides the optimal balance of deposition rate, microstructure, and corrosion resistance. Maintaining a dilution ratio below 25% is essential to preserve the dual-phase character of the overlay layer. Engineers specifying ESW overlay procedures for dual-phase stainless steel pressure vessels should carefully consider these process parameters in conjunction with the applicable standards and qualification requirements to ensure the overlay layer meets the required mechanical and corrosion resistance properties.