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

Literature Overview

This study investigates the formation, distribution, and detrimental effects of sigma (σ) phase on the microstructure, mechanical properties, and corrosion resistance of duplex stainless steel (DSS) weld overlay layers. Duplex stainless steels, such as UNS S31803 (2205) and UNS S32750 (2507), are valued for their balanced austenite-ferrite microstructure, which provides high strength (yield strength 450-550 MPa), excellent chloride pitting resistance (PREN > 34), and good resistance to stress corrosion cracking. However, weld overlay layers on DSS substrates are particularly susceptible to sigma phase precipitation due to the elevated cooling rates and thermal cycling inherent in welding processes. Sigma phase, an intermetallic compound with the formula (Cr,Fe)₂₂(Cr,Fe)₆N₈, forms during prolonged exposure to temperatures between 600-900°C and consumes chromium and nitrogen from the matrix, severely degrading corrosion resistance and ductility. This topic is of paramount importance for engineers in the oil and gas, offshore, and chemical processing industries, where DSS overlays are specified for their superior corrosion resistance in chloride-containing environments.

Core Technical Findings

Sigma Phase Formation Mechanism

Sigma phase in DSS weld overlays forms through three primary mechanisms:

1. Equilibrium precipitation during slow cooling through the 600-900°C temperature range
2. Non-equilibrium precipitation during welding thermal cycles with multiple passes
3. Accelerated precipitation during post-weld stress relief or in-service exposure at elevated temperatures

The formation kinetics of sigma phase are strongly dependent on the welding process parameters and the number of weld passes. A typical multi-pass overlay weld on a 2507 DSS substrate may experience 5-15 thermal cycles, each exposing the previous pass to peak temperatures that promote sigma nucleation. The critical factors governing sigma formation include:

• Time spent in the 600-900°C temperature range (residence time)
• Peak interpass temperature (IPT) between passes
• Local chemistry variations (Cr, Mo, N enrichment in ferrite phase)
• Cooling rate after the final pass

Microstructural Characterization

The micrograph analysis reveals that sigma phase preferentially forms in the ferrite phase of the DSS overlay, particularly along ferrite grain boundaries and at the interface between ferrite and austenite. In the heat-affected zone (HAZ) of the base DSS material, sigma formation follows a similar pattern but is generally less severe due to the shorter residence time in the critical temperature range.

Mechanical and Corrosion Property Degradation

The presence of sigma phase has a profound and detrimental effect on the properties of DSS overlays:

• Hardness increases by 20-40% due to the inherently hard nature of sigma phase (1400-1600 HV), but this is a form of embrittlement rather than beneficial strengthening
• Ductility decreases dramatically, with elongation dropping from 30% to less than 10% when sigma phase exceeds 15% volume fraction
• Impact toughness (Charpy V-notch at 20°C) decreases from 80 J to below 25 J with significant sigma formation
• Pitting corrosion resistance degrades because sigma phase is chromium-depleted (only 18-20% Cr compared to 22-25% in the matrix), creating galvanic cells with the surrounding matrix that accelerate localized attack

In accelerated corrosion tests (ASTM G48 Method B, 5% NaCl at 60°C), the time to pitting initiation decreases from over 500 hours (sound overlay) to less than 50 hours (overlay with 15% sigma phase), representing a tenfold reduction in corrosion resistance.

Engineering Practice Implications

Welding Procedure Optimization

Preventing sigma phase formation in DSS overlays requires careful control of welding parameters:

For submerged arc welding (SAW) overlay of DSS, a single-pass approach is preferred when feasible. If multiple passes are necessary, the interpass temperature must be maintained below 150°C using active cooling (compressed air or water spray) between passes. The total heat input should not exceed 2.0 kJ/mm to ensure rapid cooling through the sigma formation temperature range.

Post-Weld Treatment Options

When sigma phase formation is unavoidable due to thick overlay requirements or complex geometries, post-weld treatment can partially reverse the damage:

• Solution annealing at 1050-1100°C followed by water quenching can dissolve sigma phase, but this requires the component to withstand the thermal stress of rapid cooling and may cause distortion in large vessels
• For pressure vessels that cannot be solution-annealed due to size or geometry constraints, a stress-relief anneal at 400-450°C for 2 hours can relieve residual stresses without promoting further sigma formation
• In-service repair of sigma-affected overlays requires removal of the affected layer and reapplication of a fresh overlay with optimized parameters

Inspection and Acceptance Criteria

Quality control for DSS overlays should include:

• Metallographic examination for sigma phase using the ASTE M3-A etchant (ammonium oxalate) or a specialized sigma phase etchant
• Quantification of sigma phase using image analysis software, with acceptance criteria of < 3% volume fraction for critical service and < 1% for severe chloride environments
• Pitting resistance testing per ASTM G48 Method B, with a minimum time-to-pitting of 200 hours for offshore service
• Mechanical property verification: hardness 280-350 HV, elongation ≥ 20%, and impact energy ≥ 50 J at 20°C

Key Reflections and Study Insights

The sigma phase problem in DSS weld overlays represents one of the most challenging metallurgical issues in modern corrosion-resistant engineering. The fundamental challenge is that the very properties that make DSS attractive—high chromium, molybdenum, and nitrogen content for corrosion resistance—are the same elements that promote sigma phase formation during thermal exposure. This creates an inherent design constraint that engineers must navigate carefully.

The literature's emphasis on interpass temperature control as the primary lever for sigma prevention is a practical and actionable insight. In many industrial settings, interpass temperature is loosely controlled or not monitored at all, with welders relying on tactile assessment. Implementing mandatory interpass temperature monitoring with digital thermocouples and enforcement of the 150°C limit can dramatically reduce sigma formation. This is a low-cost, high-impact quality improvement that should be adopted as standard practice in DSS overlay fabrication.

Another critical insight is the role of nitrogen in sigma phase formation. Nitrogen stabilizes the ferrite phase and increases its volume fraction, but it also accelerates sigma precipitation kinetics. For applications where sigma resistance is paramount, engineers should consider fillers with slightly elevated nitrogen content (0.15-0.20%) to maintain a balanced microstructure while accepting a marginal increase in sigma susceptibility. The net effect is typically favorable because the improved pitting resistance from higher nitrogen outweighs the sigma risk, provided that welding parameters are properly controlled.

The study also underscores the importance of considering the entire service life when evaluating overlay quality. A weld overlay that passes initial inspection but develops sigma phase during in-service exposure to elevated temperatures (e.g., 250-350°C in refineries) may fail prematurely. Engineers should specify overlay materials and procedures that account for the maximum service temperature, not just the fabrication conditions. For high-temperature service above 250°C, consideration should be given to alternative overlay materials such as nickel-based alloys (Inconel 625, Hastelloy C-276) that are inherently resistant to sigma formation, even though they are more expensive. The life-cycle cost analysis often favors these premium materials when the consequence of failure is catastrophic, as in pressure vessel applications involving toxic or flammable fluids.