Post-Weld Heat Treatment Effects on SAF2507 Strip Cladding Layer Microstructure and Properties

Literature Overview and Research Context

This 2014 study by Yang Li, Shi Wei, Zhao Jiangtao, and Li Jinmei from the Testing Center of Lanzhou Lanchi Heavy Equipment Co., Ltd. investigates the influence of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of SAF2507 super-duplex stainless steel strip cladding layers. Super-duplex stainless steel, with its high equivalent chromium number (Cr_eq approximately 28) and nitrogen content (approximately 0.2–0.4 wt%), offers exceptional resistance to chloride-induced stress corrosion cracking, pitting, and crevice corrosion. However, the weld overlay process inevitably alters the original microstructure, potentially degrading the corrosion resistance and mechanical properties of the cladding layer.

SAF2507 is widely used in offshore platforms, desalination plants, chemical processing, and nuclear facilities where aggressive chloride environments prevail. Strip cladding, as defined in ASTM A264 and EN 10028-7, involves welding a strip of alloy onto a carbon steel or low-alloy steel backing plate. The resulting bimetallic product combines the strength and economy of carbon steel with the corrosion resistance of the super-duplex overlay.

Microstructural Evolution During Cladding and PWHT

The as-welded microstructure of SAF2507 strip cladding typically consists of a mixed ferrite-austenite matrix with possible formation of intermetallic phases, particularly the sigma (σ) phase and chi (χ) phase, depending on the thermal history. The sigma phase, an iron-chromium intermetallic compound with a tetragonal crystal structure (FeCr), forms preferentially at grain boundaries and is detrimental to both toughness and corrosion resistance.

As-Welded Condition

After PWHT at 1050 °C (Solution Treatment)

PWHT Parameters and Their Effects

The study examines the effect of solution heat treatment at elevated temperatures on the cladding layer. For super-duplex stainless steels, the recommended solution treatment temperature is typically 1020–1100 °C, followed by rapid water quenching. The exact parameters depend on the specific grade, thickness, and intended service conditions.

Temperature Effects

• Below 1000 °C: Insufficient dissolution of intermetallic phases; sigma phase persists and may even grow if the temperature falls within the precipitation range (900–1100 °C for prolonged exposure).
• 1020–1100 °C: Optimal range for complete dissolution of sigma and chi phases, with full homogenization of the ferrite-austenite matrix.
• Above 1150 °C: Risk of excessive grain growth, which may reduce toughness and increase susceptibility to intergranular corrosion.

Cooling Rate Effects

The cooling rate after solution treatment is critical. Slow cooling through the 900–1100 °C range promotes re-precipitation of intermetallic phases. Water quenching or rapid air cooling is necessary to suppress sigma phase formation. For thick sections, the cooling rate through the critical temperature range may be insufficient, leading to residual intermetallic phases near the weld root.

Mechanical Property Assessment

PWHT significantly influences the mechanical properties of the SAF2507 cladding layer. The solution-treated condition typically shows:

• Tensile strength: 550–650 MPa (as-welded) vs. 500–600 MPa (solution-treated)
• Yield strength: 450–550 MPa (as-welded) vs. 400–500 MPa (solution-treated)
• Elongation: 25–35% (as-welded) vs. 30–40% (solution-treated)
• Impact energy: Significantly improved after PWHT due to elimination of brittle intermetallic phases

The reduction in hardness after PWHT is generally acceptable and is accompanied by improved ductility and fracture toughness, which are more critical for pressure vessel applications.

Corrosion Resistance and Intergranular Corrosion

The primary motivation for PWHT of SAF2507 cladding layers is the restoration of corrosion resistance. Intermetallic phases, particularly sigma phase, are chromium-depleted regions that act as initiation sites for intergranular corrosion and stress corrosion cracking. After solution treatment, the PREN (Pitting Resistance Equivalent Number) of the cladding layer should be restored to the base metal value of approximately 38–40.

Intergranular corrosion testing according to ASTM G110 or GB/T 4334 should be performed on solution-treated samples to verify the effectiveness of the heat treatment. The critical chromium depletion zone near grain boundaries should be eliminated or reduced to a depth of less than 1 μm.

Engineering Practice and Standards Compliance

For pressure vessel applications governed by GB/T 150 or ASME VIII Div. 2, the PWHT of SAF2507 clad plate requires careful consideration. The heat treatment temperature and duration must be specified in the fabrication documentation and verified through microstructural examination and corrosion testing. The weld overlay procedure qualification per NB/T 47014 or ASME IX should include the PWHT condition in the qualification parameters.

A key practical challenge is the thermal distortion of the clad plate during solution treatment. The differential thermal expansion between the carbon steel backing and the SAF2507 overlay can induce delamination or cracking at the bond line. Controlled heating and cooling rates, typically not exceeding 150 °C/h, are recommended to minimize thermal stresses.

Key Reflections and Study Insights

This study underscores a fundamental principle in super-duplex stainless steel welding: the as-welded condition is rarely the optimal condition for service. The thermal cycle of welding inevitably drives the microstructure away from the solution-treated state, and PWHT is essential to restore the intended balance of phases and properties. For engineers involved in the design and fabrication of SAF2507-clad pressure vessels, the following practices should be adopted:

1. Always specify PWHT as a mandatory post-weld operation for SAF2507 overlay welds.
2. Verify the effectiveness of PWHT through microstructural examination, hardness testing, and intergranular corrosion testing.
3. Control the heating and cooling rates during PWHT to prevent delamination of the bond line.
4. Document the PWHT cycle in the fabrication records and include it in the procedure qualification.

The integration of microstructural analysis with mechanical and corrosion property testing provides a comprehensive quality assurance framework for SAF2507 clad products. This approach ensures that the corrosion resistance advantages of super-duplex stainless steel are fully realized in the final product, rather than being compromised by welding-induced microstructural degradation.