Effect of Sigma Phase on Microstructure and Properties of Duplex Stainless Steel Overlay Layer

Background on Duplex Stainless Steel Overlay

Duplex stainless steels (DSS), characterized by a microstructure consisting of approximately equal amounts of austenite and ferrite, offer an excellent combination of strength, corrosion resistance, and toughness. They are widely used in overlay welding applications for chemical processing equipment, oil and gas platforms, and marine applications. However, the stability of the duplex microstructure is highly sensitive to welding thermal cycles, and the formation of intermetallic phases such as sigma (σ) phase can severely degrade the mechanical and corrosion properties. This literature review examines the formation mechanisms, effects, and prevention strategies for sigma phase in duplex stainless steel overlay layers.

Sigma Phase Formation Mechanism

The sigma phase (CrFe) is a brittle intermetallic compound with a tetragonal crystal structure that forms in duplex stainless steels during prolonged exposure to temperatures in the range of 600-900°C. The formation of sigma phase is governed by the following factors:

• Temperature: Sigma phase formation is most rapid in the range of 700-850°C. Below 600°C, the kinetics are too slow to be significant. Above 900°C, the phase dissolves back into the matrix.
• Time: The formation kinetics follow a parabolic relationship with time. Even short exposures (1-2 hours) at 800°C can lead to significant sigma phase formation.
• Composition: High chromium and molybdenum content promotes sigma phase formation. The chromium equivalent (Creq) and nickel equivalent (Nieq) ratio is a key predictor, with Creq/Nieq ratios above 2.5 indicating high susceptibility.
• Welding thermal cycle: The number of thermal cycles and the peak temperature of each cycle affect the cumulative sigma phase formation.

The following table summarizes the sigma phase formation kinetics for a typical 2205 duplex stainless steel overlay:

Effects on Mechanical and Corrosion Properties

The formation of sigma phase has profound effects on the properties of duplex stainless steel overlay layers:

Mechanical properties:

• Hardness increases by 100-200 HV due to the hard, brittle sigma phase particles
• Tensile strength initially increases but then decreases as the sigma phase volume fraction exceeds 10%
• Elongation decreases significantly, from typical values of 20-30% to less than 5% when sigma phase exceeds 15%
• Impact toughness drops dramatically, making the overlay layer susceptible to brittle fracture

Corrosion properties:

• Pitting corrosion resistance decreases due to chromium depletion in the ferrite phase adjacent to sigma phase particles
• The critical pitting temperature (CPT) can decrease by 20-40°C in heavily sigma-phase-contaminated overlays
• Intergranular corrosion susceptibility increases due to chromium depletion along grain boundaries
• Stress corrosion cracking resistance is also affected, particularly in chloride-containing environments

Prevention and Mitigation Strategies

The literature review identifies several strategies to prevent or minimize sigma phase formation in duplex stainless steel overlay layers:

1. Process control: Use low-heat-input welding processes (GTAW, laser cladding) to minimize the time spent in the critical temperature range. Keep the interpass temperature below 200°C for multipass welding.
2. Thermal cycle management: Limit the number of thermal cycles in the overlay layer. Each additional pass increases the cumulative time at elevated temperature. Plan the welding sequence to minimize the number of passes.
3. Post-weld heat treatment: A solution treatment at 1050-1100°C followed by rapid quenching can dissolve sigma phase and restore the duplex microstructure. However, this must be performed carefully to avoid other microstructural changes.
4. Composition optimization: Select overlay materials with balanced Creq and Nieq ratios. Materials such as UNS S32750 (25% Cr, 7% Ni, 3% Mo) have lower sigma phase susceptibility than UNS S31803 (22% Cr, 5% Ni, 3% Mo).
5. Microstructural monitoring: Use metallographic examination with specialized etchants (such as Massol's reagent) to detect sigma phase. A volume fraction below 5% is generally acceptable for most applications.

Engineering Practice Recommendations

Based on the study, the following recommendations are provided for practical applications:

• For pressure vessel overlay welding, the welding procedure should be qualified with a thermal cycle that simulates the actual number of passes and interpass temperatures
• The overlay layer should be inspected for sigma phase using optical microscopy after the final welding pass
• For applications where the overlay will be exposed to temperatures above 600°C in service, the sigma phase formation should be evaluated as part of the design basis
• The welding procedure specification (WPS) should include a maximum allowable interpass temperature and a maximum number of passes
• For repair welding, the thermal input of the repair weld must be carefully controlled to avoid introducing sigma phase into the existing overlay layer

The study concludes that sigma phase formation is the primary degradation mechanism for duplex stainless steel overlay layers, and its prevention requires a combination of careful process design, composition selection, and post-weld inspection. Engineers should treat sigma phase control as a critical aspect of the overlay welding qualification process, not as an afterthought.