Heat-Affected Zone Sensitization in Duplex Grades

🎯 Learning Objectives

✓ Explain the metallurgical mechanism of HAZ sensitization in duplex grades using phase diagrams and time-temperature-transformation behavior
✓ Analyze weld thermal cycles to identify critical temperature ranges and dwell times that promote deleterious phase formation
✓ Apply ASTM A923 test methods to evaluate HAZ corrosion resistance and interpret pass/fail criteria
✓ Design a welding procedure specification (WPS) that limits interpass temperature and cooling rate to suppress σ-phase formation in UNS S32205/S32750

📖 Why This Matters

In offshore oil & gas platforms, subsea piping, and desalination plants, duplex stainless steels like UNS S32205 and S32750 are chosen for their strength and corrosion resistance—but if improperly welded, the heat-affected zone can lose up to 80% of its pitting resistance. Real-world failures have led to leaks in seawater injection lines and unplanned shutdowns costing millions. Understanding and preventing HAZ sensitization isn’t academic—it’s a frontline integrity requirement.

📘 Core Principles

Duplex stainless steels rely on a near-equal balance (~40–60% each) of austenite (γ) and ferrite (δ) phases for optimal mechanical and corrosion properties. During welding, the HAZ experiences rapid heating and cooling. In the critical range of ~400–900 °C, especially 600–900 °C, ferrite transforms partially into brittle intermetallic phases: sigma (σ), chi (χ), and sometimes secondary austenite (γ₂). Sigma phase forms most rapidly at ~750 °C with >10 s dwell time—and consumes chromium and molybdenum, creating Cr-depleted zones adjacent to precipitates. This micro-galvanic effect accelerates localized corrosion. The risk escalates with higher alloy content (e.g., superduplex S32750), elevated interpass temperatures (>100 °C), and slow cooling rates through the critical range.

📐 Critical Cooling Rate Estimation

The minimum required cooling rate from 1000 °C to 700 °C (CR₇₀₀) determines whether sigma phase nucleation is avoided. Below ~10 °C/s, sigma formation becomes probable; above ~50 °C/s, it’s suppressed. CR₇₀₀ is estimated from heat input and joint geometry.

💡 Worked Example

Problem: A weld on UNS S32750 is made with heat input H = 1.2 kJ/mm, plate thickness t = 16 mm, and ambient temperature T₀ = 25 °C. Estimate CR₇₀₀ using Kirkaldy’s empirical correlation.

1. Step 1: Convert heat input to kJ/cm: H = 1.2 kJ/mm × 10 = 12 kJ/cm

2. Step 2: Apply Kirkaldy formula: CR₇₀₀ ≈ 5400 / (H × t⁰·⁵) [°C/s], where t = 1.6 cm → t⁰·⁵ ≈ 1.265

3. Step 3: CR₇₀₀ ≈ 5400 / (12 × 1.265) ≈ 5400 / 15.18 ≈ 355 °C/s — well above 50 °C/s threshold

Answer: The estimated cooling rate is ~355 °C/s, which strongly suppresses sigma phase formation. However, this assumes no preheat and forced convection — real interpass conditions may reduce effective CR₇₀₀ by >90%.

🏗️ Real-World Application

In a 2019 North Sea subsea tie-in project, UNS S32750 spool pieces failed ASTM G48 Method A (ferric chloride) testing after field welding. Metallurgical analysis revealed 8–12 vol% sigma phase in the HAZ due to interpass temperatures exceeding 150 °C and inadequate purge gas flow causing localized reheating. Remediation included revising WPS to enforce interpass ≤100 °C, using automated GTAW with pulsed current, and adding post-weld rapid water quenching — verified by ferritoscope readings and SEM/EDS phase mapping.