11.6 Duplex Stainless Metallurgy & Dissimilar-Metal Ferrite Prediction

Duplex Stainless Steel Welding Metallurgy

Duplex stainless steels (e.g., 2205: 22% Cr, 5% Ni, 3% Mo, 0.17% N) feature a balanced, two-phase microstructure of approximately 40% to 60% austenite (gamma) and 40% to 60% delta ferrite (delta). They provide roughly double the yield strength of austenitic grades, coupled with superior resistance to chloride stress corrosion cracking (CSCC) and localized pitting (Pitting Resistance Equivalent Number, PRE_N = Cr + 3.3*Mo + 16*N >= 35).

Solidification & Phase Balance Kinetics

Unlike austenitic stainless steels, duplex alloys solidify 100% as delta ferrite (L -> delta). Austenite only forms as a solid-state transformation product upon cooling between 1300°C and 1000°C:

• Rapid Cooling (Low Heat Input, < 0.5 kJ/mm): Insufficient time for nitrogen and nickel to diffuse and nucleate austenite. The HAZ freezes with excessive ferrite (> 75% to 85% delta). Excess nitrogen cannot remain dissolved in ferrite, precipitating as brittle chromium nitrides (Cr2N) within ferrite grains, destroying impact toughness and pitting corrosion resistance.
• Slow Cooling (Excessive Heat Input, > 2.5 kJ/mm): Prolonged residence time in the 600°C to 950°C range promotes precipitation of brittle intermetallic phases:
  • Sigma phase (sigma, Fe-Cr-Mo intermetallic): Highly embrittling; as little as 1% to 2% sigma phase drastically degrades room-temperature toughness and corrosion resistance.
  • Chi phase (chi) and secondary austenite (gamma_2).

Welding Engineering Protocols for Duplex Steels

1. Heat Input Constraints: Maintain heat input strictly within 0.5 to 2.5 kJ/mm for standard 2205 duplex, and 0.2 to 1.5 kJ/mm for 2507 super-duplex.
2. Maximum Interpass Temperature: Constrain interpass temperatures to <= 150°C (300°F) for 2205, and <= 100°C (212°F) for 2507.
3. Over-Alloyed Filler Metals: Autogenous welding (no filler) produces an unacceptably ferritic weld metal (> 80% delta). Weld procedures mandate nickel-overalloyed filler metals (e.g., AWS ER2209 with 8.5% to 10.0% Ni to join 5% Ni base plate) to drive austenite reformation in the fusion zone.
4. Nitrogen-Bearing Shielding Gas: In GTAW and GMAW, high arc temperatures drive nitrogen out of the molten pool. Adding 1% to 2% N2 to argon shielding gas (and 99.999% N2 for root purging) maintains nitrogen levels, promoting prompt austenite reformation and preserving corrosion resistance.

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Worked Numerical Example: Dissimilar Metal Weld & WRC-1992 Ferrite Calculation

Problem Statement

A welding engineer must qualify a procedure for joining a carbon steel pressure vessel nozzle (ASTM A106 Grade B) to an austenitic stainless steel pipe (ASTM A312 TP304L) using Gas Tungsten Arc Welding (GTAW) with AWS ER309L filler metal. The joint design is a single-V butt weld. Metallurgical cross-sectioning verifies an overall weld dilution of 30% (15% from A106B and 15% from 304L), with 70% contributed by the ER309L filler metal.

The certified compositions are as follows (weight percent):

• Base Metal 1 (A106B): 0.22% C, 0.80% Mn, 0.25% Si, 0.05% Cr, 0.02% Mo, 0.08% Ni, 0.15% Cu, 0.008% N, 0.00% Nb
• Base Metal 2 (304L): 0.025% C, 1.40% Mn, 0.45% Si, 18.30% Cr, 0.20% Mo, 8.20% Ni, 0.30% Cu, 0.060% N, 0.00% Nb
• Filler Metal (ER309L): 0.020% C, 1.70% Mn, 0.40% Si, 23.50% Cr, 0.10% Mo, 13.20% Ni, 0.10% Cu, 0.045% N, 0.00% Nb

Engineering Tasks:

1. Calculate the Chromium Equivalent (Cr_eq) and Nickel Equivalent (Ni_eq) for all three materials using the WRC-1992 formulas:

   Cr_eq = Cr + Mo + 0.7*Nb
   Ni_eq = Ni + 35*C + 20*N + 0.25*Cu
   

2. Calculate the resulting mixed weld deposit Cr_eq and Ni_eq based on the specified dilution fractions (15% BM1 + 15% BM2 + 70% FM).
3. Using the linearized WRC-1992 iso-ferrite relationship for the 2 to 15 FN regime:

   FN approx -14.4 + 3.96 * (Cr_eq - 0.93*Ni_eq)
   

   estimate the Ferrite Number (FN) of the deposit.
4. Metallurgically evaluate whether the deposit successfully prevents solidification cracking, avoids excessive delta ferrite, and avoids martensite formation.

Step-by-Step Solution

Step 1: Calculate WRC-1992 Equivalents for Components

• Base Metal 1 (A106B):

  Cr_eq, BM1 = 0.05 + 0.02 + 0.7*(0) = 0.07%
  Ni_eq, BM1 = 0.08 + 35*(0.22) + 20*(0.008) + 0.25*(0.15)
  Ni_eq, BM1 = 0.08 + 7.70 + 0.16 + 0.0375 = 7.9775% approx 7.98%
  

• Base Metal 2 (304L):

  Cr_eq, BM2 = 18.30 + 0.20 + 0.7*(0) = 18.50%
  Ni_eq, BM2 = 8.20 + 35*(0.025) + 20*(0.060) + 0.25*(0.30)
  Ni_eq, BM2 = 8.20 + 0.875 + 1.20 + 0.075 = 10.35%
  

• Filler Metal (ER309L):

  Cr_eq, FM = 23.50 + 0.10 + 0.7*(0) = 23.60%
  Ni_eq, FM = 13.20 + 35*(0.020) + 20*(0.045) + 0.25*(0.10)
  Ni_eq, FM = 13.20 + 0.70 + 0.90 + 0.025 = 14.825% approx 14.83%
  

Step 2: Calculate Mixed Weld Metal Equivalents Applying the dilution mass balance:

Cr_eq, deposit = 0.15*(Cr_eq, BM1) + 0.15*(Cr_eq, BM2) + 0.70*(Cr_eq, FM)
Cr_eq, deposit = 0.15*(0.07) + 0.15*(18.50) + 0.70*(23.60)
Cr_eq, deposit = 0.0105 + 2.775 + 16.52 = 19.3055% approx 19.31%

Ni_eq, deposit = 0.15*(Ni_eq, BM1) + 0.15*(Ni_eq, BM2) + 0.70*(Ni_eq, FM)
Ni_eq, deposit = 0.15*(7.978) + 0.15*(10.35) + 0.70*(14.825)
Ni_eq, deposit = 1.1967 + 1.5525 + 10.3775 = 13.1267% approx 13.13%

Step 3: Estimate Ferrite Number (FN)

Effective Parameter = Cr_eq - 0.93*Ni_eq = 19.31 - 0.93*(13.13) = 19.31 - 12.21 = 7.10
FN approx -14.4 + 3.96*(7.10) = -14.4 + 28.12 = 13.72 approx 13.7 FN

Note: Graphical reading on the official WRC-1992 coordinate grid at (19.31, 13.13) yields 9.0 to 12.0 FN.

Step 4: Metallurgical Evaluation

• Solidification Cracking: With Cr_eq / Ni_eq = 19.31 / 13.13 = 1.47, the weld pool solidifies in the primary ferritic FA mode. The presence of approx 9 to 12 FN provides complete immunity against hot cracking.
• Martensite Avoidance: The deposit coordinates (19.31, 13.13) fall completely outside the martensite boundary on the Schaeffler/WRC diagrams, ensuring a tough, ductile austenitic-ferritic matrix.
• Conclusion: ER309L provides the necessary buffering margin against carbon steel dilution, preventing both brittle martensite and hot cracking.

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Real-World Engineering Scenarios & Exam Pitfalls

Industrial Scenario: Intergranular Stress Corrosion Failure in Brewery Piping

A craft brewing facility installed an uninsulated AISI 304 schedule 10 process piping system carrying warm wash water (65°C) with trace chlorides. Piping spools were field-welded using manual GTAW with standard AWS ER308 filler wire. After 14 months of operation, multiple circumferential hairline leaks opened in the Heat-Affected Zone approximately 2 mm away from the weld toe. Metallographic sectioning revealed severe grain-boundary ditching conforming to ASTM A262 Practice A, with deep intergranular stress corrosion cracks propagating through chromium-depleted boundaries.

Root Cause Analysis:

1. The pipe base material was dual-certified standard 304 with carbon measured at 0.065%.
2. Welds were made using high heat input (> 2.2 kJ/mm) with no interpass temperature restrictions, leaving the HAZ in the 500°C to 850°C sensitization window for minutes.
3. The fabricator failed to specify low-carbon "L" grades.

Corrective Action: The entire piping network was replaced with AISI 304L (C = 0.022%) welded with ER308L, heat input was constrained to <= 1.0 kJ/mm, and maximum interpass temperature was capped at 100°C, completely eliminating sensitization.

Common Exam Traps

Exam Trap 1: Assuming 100% Austenite Is Best for Corrosion Resistance While fully austenitic welds (0 FN) are required for specialized urea or concentrated nitric acid equipment, selecting 0 FN for general structural fabrication is disastrous. A fully austenitic weld pool solidifies in Mode A, possessing practically zero tolerance for sulfur and phosphorus and resulting in severe centerline hot cracking. You must maintain 3 to 8 FN for structural austenitic welds.

Exam Trap 2: Neglecting Nitrogen in the Schaeffler Diagram Exam questions often ask why the Schaeffler diagram should not be used for modern stainless steel calculations. Schaeffler completely omits nitrogen. Nitrogen is an exceptionally potent austenite stabilizer (20 to 30 times more powerful than nickel). Neglecting nitrogen causes Schaeffler to grossly overestimate delta ferrite, leading to inaccurate filler metal selections. Always cite the WRC-1992 diagram.

Exam Trap 3: Cooling Rate Strategy for Duplex Stainless Steels Do not apply the carbon steel rule ("slower cooling is always safer") or the austenitic rule ("quench as fast as possible") to duplex stainless steel. Extremely rapid cooling leaves duplex HAZs with > 80% brittle ferrite and chromium nitride precipitates. Extremely slow cooling causes catastrophic embrittling sigma phase. Duplex welding requires a strictly controlled moderate cooling window governed by heat inputs of 0.5 to 2.5 kJ/mm.