Heat Input Control in Duplex Pipe Welding: Preventing Sigma Phase Embrittlement in the HAZ

If you weld duplex or super duplex stainless steels (DSS/SDSS), you walk a tightrope. Go too cold, and you get excessive ferrite and loss of toughness. Go too hot, and you invite the silent killer of duplex welds: Sigma (σ) Phase embrittlement in the Heat Affected Zone (HAZ).

Sigma phase is an intermetallic compound that makes welds brittle and destroys corrosion resistance. While it forms in the base metal during improper heat treatment, in welding, it is a direct consequence of poor heat input control.

This article provides a technical deep-dive on how to tailor your heat input to prevent sigma formation in duplex pipe welding, ensuring mechanical integrity and pitting resistance.

The Problem: Why Sigma Forms in the HAZ

Sigma phase precipitation occurs when the weld HAZ is exposed to temperatures roughly between 600°C and 950°C for too long . In a duplex microstructure, sigma nucleates primarily at ferrite/austenite interfaces. It grows by consuming ferrite (δ), which is rich in chromium and molybdenum—the very elements that give duplex its corrosion resistance .

The mechanism is eutectoid: δ → σ + γ₂ (secondary austenite) . As sigma forms, it leaches Cr and Mo from the surrounding matrix. Once precipitated, removing sigma requires a full solution annealing treatment (1050-1100°C), which is impractical for large pipe spools . In the as-welded condition, sigma is a hard, brittle phase that drastically reduces impact toughness and pitting resistance.

The Primary Variable: Heat Input and Cooling Rate (t12/8)

While chemistry is critical, the thermal cycle is the trigger. The industry standard for measuring this is the cooling time from 1200°C to 800°C (t12/8) . This range covers the critical window for both austenite formation and sigma precipitation.

To avoid sigma while maintaining phase balance, you generally need a t12/8 of 8 to 15 seconds. If t12/8 exceeds approximately 15 seconds (which occurs with high heat input), you risk sigma formation .

For a 22% Cr duplex (like 2205), the recommended heat input for pipe welding typically ranges from 0.5 kJ/mm to 2.5 kJ/mm for 10-15mm wall thickness . However, recent studies suggest these limits are not static; they change based on wall thickness and joint geometry.

New Perspectives: Plate Thickness Changes the Rules

Recent research challenges the assumption that "high heat input is always bad." A 2025 study on thick-plate duplex welding demonstrated that for plates exceeding 10mm in thickness, the upper limit of heat input increases significantly .

• For thin-wall pipe (<10mm): Slow heat dissipation keeps the HAZ hot. Using standard heat input calculations is critical to avoid burning through or overheating.

• For thick-wall pipe (>14mm): The mass of the pipe acts as a massive heat sink, drawing heat away quickly. In these cases, a higher heat input is required to achieve proper fusion and austenite formation. The study confirmed that with 20mm thick plates, heat inputs of 5 kJ/mm and 7 kJ/mm were used without any sigma precipitation in the HAZ .

Takeaway: You must calculate heat input relative to material thickness. The thicker the pipe, the higher the "safe" heat input can be, because the cooling rate remains fast enough to bypass the sigma nose of the TTT curve.

The Super Duplex Challenge (25Cr Grades)

When welding super duplex grades (e.g., 2507, Zeron 100), the margin for error shrinks. These higher-alloyed materials have a higher driving force for sigma precipitation.

• Root Pass Sensitivity: In pipe welding, the root pass is particularly vulnerable. Research suggests that root passes should be made with a higher heat input (over 1.0 kJ/mm) compared to the first few subsequent passes. Why? Low heat input in the root can cause chromium nitrides to precipitate on the inside surface due to lack of reheating .

• Fitness-for-Purpose: While specifications often demand zero sigma, industrial reality (especially in UK North Sea projects) shows that limited sigma in the low-temperature HAZ (2-5mm from the fusion line) can sometimes be tolerated. Studies have accepted up to 2.5% volume fraction of sigma in HAZ, provided CTOD (Crack Tip Opening Displacement) fracture toughness and ASTM G48 corrosion tests are passed . However, this is a qualification limit, not a production target.

Practical Guidelines for the Welding Engineer

To prevent sigma phase in duplex pipe welds, move beyond generic "low heat" rules. Implement these controls:

1. Manage Interpass Temperature

Sigma is cumulative. As you add passes, the HAZ from previous passes gets reheated into the danger zone (700-900°C). Strictly limit interpass temperature. For super duplex (25% Cr), the maximum interpass temperature is generally 150°C . For standard 22% Cr duplex, 200°C is usually the cap .

2. Avoid the "Many Small Passes" Trap

It seems counter-intuitive, but using a large number of low-heat-input passes can be more dangerous than fewer, larger passes. Each small pass reheats the previous HAZ, effectively aging the material and precipitating sigma and chi (χ) phases .

• Strategy: Use a welding sequence that minimizes the number of thermal cycles on any specific area of the HAZ.

3. Balance Ferrite Numbers

Sigma forms more readily in ferrite. If your HAZ ferrite content exceeds 70-75%, the risk of sigma precipitation skyrockets because you have more unstable ferrite available to transform.

• Ensure your shielding gas and filler metals promote adequate austenite reformation. Nickel-based fillers help, but be cautious: higher Ni widens the sigma formation temperature range .

4. Calculate Effective Heat Input

Don't rely on "feel." Use the formula:

$$\text{Heat Input (kJ/mm)}=\frac{\text{Voltage (V)}\times \text{Current (A)}\times 60}{\text{Travel Speed (mm/min)}\times 1000}$$

For GTAW (TIG) processes, remember to account for the thermal efficiency factor (η), which is typically around 0.6 for TIG .

Conclusion

Sigma phase embrittlement is a preventable failure mode. It is a sign that the weld saw too much heat for too long—either globally (high heat input in thin pipe) or locally (multiple reheats from small passes).

Modern welding of duplex pipes requires a shift from "following the WPS" to understanding the thermal history. By controlling t12/8 cooling times, respecting interpass limits, and adjusting heat input for actual pipe wall thickness, you can produce HAZ microstructures that are free of sigma, tough, and corrosion-resistant.