CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Strip Cladding of Duplex Stainless Steel
Material Characteristics and Cladding Challenges
Duplex stainless steels, characterized by a balanced microstructure of austenite and ferrite (typically 40–60% each), offer superior mechanical properties and corrosion resistance compared to conventional austenitic stainless steels. Grades such as 2205 (UNS S31803), 2507 (UNS S32750), and 199 (UNS S31806) provide yield strengths of 450–700 MPa, excellent resistance to chloride stress corrosion cracking, and good pitting corrosion resistance. These properties make duplex steels ideal for aggressive environments in the oil and gas, chemical, and marine industries.
Strip cladding of duplex stainless steel presents unique challenges due to the material's susceptibility to intermetallic phase formation (σ, χ, and Laves phases) during welding. These phases precipitate in the ferrite phase during prolonged exposure to the 600–1000 °C temperature range, leading to embrittlement and loss of corrosion resistance. Additionally, the high thermal conductivity of duplex steel requires careful heat input control to avoid excessive dilution and thermal distortion.
Strip Cladding Process Overview
Strip cladding, also known as roll-bonded cladding, involves the cold or hot rolling of a strip of cladding material onto a base steel substrate under high pressure. The process creates a metallurgical bond through plastic deformation and diffusion at the interface. For duplex stainless steel cladding, the strip is typically 0.5–3 mm thick and is rolled onto a carbon steel or low-alloy steel base plate.
| Parameter | Typical Range | Notes |
|---|---|---|
| Strip thickness | 0.5–3.0 mm | Thinner strips for precision applications |
| Base plate thickness | 6–100 mm | Depends on pressure vessel design |
| Rolling temperature | 900–1100 °C (hot) or room temp (cold) | Hot rolling for thicker strips |
| Rolling pressure | 1000–3000 MPa | Ensures metallurgical bond |
| Bond strength | ≥ 150 MPa (shear) | Per ASTM A263/A264 |
| Clad ratio | 1:10 to 1:4 | Cladding thickness to total thickness |
The process typically involves heating the strip and base plate to the rolling temperature, aligning them, and passing them through a rolling mill under controlled pressure. The high pressure causes plastic deformation at the interface, breaking oxide layers and creating fresh metal-to-metal contact. Subsequent cooling allows diffusion bonding to strengthen the interface.
Heat Treatment and Phase Control
Post-rolling heat treatment is critical for duplex stainless steel cladding. The solution annealing treatment dissolves intermetallic phases and restores the balanced austenite-ferrite microstructure. Typical solution annealing parameters include:
- Temperature: 1050–1100 °C for 2205; 1100–1150 °C for 2507
- Cooling rate: Water quench or rapid air cooling
- Hold time: 1–2 hours per 25 mm thickness
- Target microstructure: 40–60% ferrite (measured by magnetic ferrite content)
The ferrite content is monitored using magnetic permeability measurements, with the acceptable range typically between 35 and 65% ferrite. Excessive ferrite increases the risk of intergranular corrosion, while excessive austenite reduces the yield strength advantage of the duplex structure.
Defect Analysis and Quality Control
Common defects in duplex stainless steel strip cladding include delamination, insufficient bond strength, excessive intermetallic phases, and surface defects. Each defect has specific root causes and countermeasures that must be addressed through process optimization.
| Defect | Root Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Delamination | Inadequate rolling pressure, surface contamination | Visual, UT | Increase pressure, clean surfaces |
| Low bond strength | Low rolling temperature, oxide scale | Shear test, UT | Optimize temperature, remove scale |
| Intermetallic phases | Excessive heat input, slow cooling | Metallography, SEM | Reduce heat input, rapid cooling |
| Surface pitting | Roll wear, strip defects | Visual, PT | Maintain rolls, inspect strips |
| Excessive ferrite | Inadequate solution annealing | Magnetic test | Adjust annealing parameters |
Non-destructive testing of clad plates includes ultrasonic testing (UT) for bond quality, magnetic particle testing (MT) for surface cracks, and eddy current testing (ET) for surface defects. Destructive testing includes shear strength testing per ASTM A263, intergranular corrosion testing per ASTM A263 Appendix A, and metallographic examination of the interface.
Engineering Practice and Standards Compliance
Strip cladding of duplex stainless steel must comply with relevant standards including ASTM A263 (clad plate for pressure vessels), ASTM A264 (clad plate for general use), EN 10028-7, and GB/T 22692. These standards specify requirements for material composition, mechanical properties, bond strength, and testing methods.
In pressure vessel fabrication, the clad plate must be inspected according to NB/T 4730 (ultrasonic testing) and JB/T 4730 (non-destructive testing). The welding of clad plates requires qualified welding procedures that address the heat-affected zone properties and avoid excessive dilution of the cladding layer. Welding processes such as GTAW for root passes and SAW or FCAW for fill passes are commonly used, with careful control of heat input to prevent intermetallic phase formation.
The fabrication of pressure vessels from duplex clad plates requires additional attention to welding procedure qualification. The welding procedure specification (WPS) must be qualified per NB/T 47014 or ASME IX, with specific attention to the cladding layer properties after welding. The heat input during welding should be limited to 1.0–2.5 kJ/mm to minimize the risk of intermetallic phase precipitation in the heat-affected zone.
Study Insights and Material Selection Guidance
The study of duplex stainless steel strip cladding reveals that the material's superior properties come with specific processing requirements that must be rigorously followed. The balance between mechanical performance and corrosion resistance is achieved through precise control of the austenite-ferrite ratio, which is sensitive to heat treatment parameters and welding conditions.
From a design perspective, duplex clad plates offer an excellent combination of strength and corrosion resistance, allowing for thinner base plates and reduced weight compared to austenitic alternatives. However, the higher cost of duplex materials and the need for careful fabrication control must be weighed against the performance benefits. For applications where chloride stress corrosion cracking is a concern, duplex cladding provides a significant advantage over 304 or 316 cladding.
The key takeaway from this study is that successful duplex cladding requires a holistic approach encompassing material selection, process optimization, quality control, and fabrication expertise. Each step in the process—from strip production to final welding—must be carefully controlled to maintain the material's unique properties. Engineers should pay particular attention to the heat input during welding and the solution annealing treatment, as these factors have the most significant impact on the long-term performance of the cladded component.