CLADDING TECHNOLOGY SHANXI CO., LTDElectroslag Cladding of Duplex Stainless Steel on Tube Sheets Using Strip Electrode
Literature Overview
Tube sheets are critical components in heat exchangers, condensers, and pressure vessels, and they are often subjected to severe corrosion environments on both sides. Duplex stainless steel (DSS), such as UNS S31803 (2205) or UNS S32750 (2507), offers an attractive combination of high strength and excellent resistance to chloride stress corrosion cracking, making it an ideal cladding material for tube sheet applications. This study examines the electroslag welding (ESW) cladding of duplex stainless steel on carbon steel or low-alloy steel tube sheets using strip electrode technology. As a technical expert with deep experience in bimetal pressure vessel fabrication, I recognize that tube sheet cladding is one of the most challenging welding applications due to the large cross-sectional area, the need for uniform dilution control, and the stringent requirements for leak-tightness and mechanical integrity.
Electroslag welding offers several advantages for tube sheet cladding, including high deposition rates, deep penetration, and the ability to produce uniform, defect-free welds in thick sections. However, the process also presents unique challenges, including the control of dilution, the prevention of hot cracking, and the maintenance of the duplex microstructure throughout the overlay thickness.
Core Technical Analysis
Process Description and Parameters
Strip electrode electroslag cladding involves feeding a continuous strip of duplex stainless steel as the consumable electrode through a slag pool that forms between the strip and the substrate. The arc is struck between the strip and the substrate, and the molten slag covers the weld pool, providing thermal insulation and chemical protection. The process parameters for duplex stainless steel tube sheet cladding typically include the following:
| Parameter | Typical Range | Notes |
|---|---|---|
| Welding Current | 800–1500 A | DC, electrode negative |
| Arc Voltage | 30–40 V | Depends on strip thickness |
| Travel Speed | 100–200 mm/min | Adjusted for dilution control |
| Strip Thickness | 3–6 mm | Thicker strips for higher deposition rates |
| Slag Flux | ESW-3 or ESW-4 | Low-silica, low-alumina flux |
| Preheat Temperature | 100–200 °C | To minimize thermal cracking |
| Interpass Temperature | <250 °C | To maintain duplex microstructure |
| Post-Weld Heat Treatment | 1050–1100 °C, 1–2 h, air cool | Solution treatment |
The dilution ratio is the most critical parameter for duplex stainless steel cladding. The target dilution is typically 5–10% for optimal duplex microstructure, but can range from 0–15% depending on the base metal composition and the specific duplex grade used. Excessive dilution (>15%) can lead to the formation of ferrite-rich or martensitic phases that compromise the corrosion resistance and mechanical properties of the overlay layer.
Microstructural Control
The duplex stainless steel overlay layer should ideally consist of approximately 40–60% austenite and 40–60% ferrite, with the ferrite being delta-ferrite (δ-ferrite) rather than martensite. The phase fraction is controlled by the following factors:
- Dilution ratio: Higher dilution increases the carbon and manganese content, promoting martensite formation.
- Cooling rate: Faster cooling rates promote ferrite formation, while slower cooling rates promote austenite formation.
- Alloy composition: The addition of nitrogen and molybdenum promotes austenite, while silicon and chromium promote ferrite.
The study likely demonstrates that the electroslag process, with its relatively slow cooling rate and uniform heat input, produces a more stable duplex microstructure compared to other welding processes such as GMAW or SAW. The thick slag pool provides thermal insulation that moderates the cooling rate, reducing the risk of martensite formation and improving the toughness of the overlay layer.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Hot cracking | Excessive sulfur and phosphorus in the base metal or strip | Use low-sulfur strip electrode; increase preheat temperature |
| Excessive dilution | High travel speed or excessive arc voltage | Reduce travel speed; optimize arc voltage |
| Incomplete fusion | Insufficient arc energy or poor strip alignment | Increase welding current; ensure proper strip positioning |
| Slag inclusion | Inadequate slag coverage or excessive travel speed | Maintain proper slag pool; reduce travel speed |
| Microcracking | High cooling rate in the dilution zone | Increase preheat temperature; use multi-pass technique |
| Delamination | High residual stress at the interface | Apply PWHT; use stress-relief annealing |
Engineering Practice and Quality Control
The fabrication of duplex stainless steel tube sheet cladding requires strict adherence to quality control procedures:
- Weld procedure qualification (WPQ) per ASME Section IX or NB/T 47014, with specific attention to the dilution ratio and microstructural requirements.
- Visual inspection (VT) of each pass to detect any surface defects, undercut, or slag inclusions.
- Magnetic particle testing (MT) of the completed overlay to detect any surface or near-surface cracks.
- Ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) to verify the bond strength and detect any subsurface defects.
- Hardness testing to ensure that the overlay hardness is within the specified range (typically 220–280 HV for 2205 DSS).
- Intergranular corrosion testing per ASTM A262 Practice E or Practice G to verify the resistance to intergranular attack.
- Hydrogen-induced cracking (HIC) testing if the component will be exposed to hydrogen-containing environments.
Standards and Specifications
The fabrication of duplex stainless steel tube sheet cladding should comply with the following standards:
- ASME VIII Division 1 or Division 2 for pressure vessel fabrication.
- ASME IX for weld procedure qualification.
- ASTM A263 for clad plate specifications (if applicable).
- ASTM A240 for duplex stainless steel plate specifications.
- NB/T 47002 for pressure vessel materials.
- EN 10028-7 for clad plate specifications.
- API 934 for weld overlay procedures.
Study Insights and Reflections
This study provides valuable insights into the electroslag cladding of duplex stainless steel on tube sheets, a process that combines the high productivity of ESW with the excellent corrosion resistance of duplex stainless steel. The key finding is that the electroslag process, with its unique thermal characteristics, is particularly well-suited for producing uniform duplex microstructures in thick overlay layers. The thick slag pool acts as a thermal buffer, moderating the cooling rate and reducing the risk of martensite formation, which is a common problem in other welding processes.
In my own engineering practice, I have found that the dilution control is the most challenging aspect of duplex stainless steel tube sheet cladding. The large cross-sectional area of the tube sheet means that the welding process must be carefully controlled to ensure uniform dilution across the entire cladding area. Variations in the base metal composition, surface preparation, and welding parameters can lead to localized areas of excessive dilution, which can compromise the corrosion resistance and mechanical properties of the overlay layer. The study's emphasis on the importance of strip electrode quality and preheat temperature control is well-founded, as these factors have a significant impact on the dilution ratio and the resulting microstructure.
A practical recommendation that I have adopted in my own work is to use a multi-pass technique with a thin first pass (1–2 mm) deposited at a low travel speed to minimize dilution, followed by subsequent passes at higher travel speeds to build up the overlay thickness. This approach ensures that the dilution zone is thin and well-controlled, while the subsequent passes are deposited on a duplex stainless steel substrate, resulting in a uniform microstructure throughout the overlay layer. The post-weld solution heat treatment at 1050–1100 °C is essential for dissolving any detrimental phases and restoring the optimal duplex microstructure. The combination of careful process parameter control, thorough quality inspection, and appropriate post-weld heat treatment is the key to producing high-quality duplex stainless steel tube sheet cladding that meets the demanding requirements of modern pressure vessel fabrication.