11.5 Stainless Families, Schaeffler and WRC-1992 Diagrams, Ferrite Number & Sensitization

11.3 Stainless Steels: Schaeffler/WRC-1992 Diagrams, Sensitization & Duplex

Quick Answer: Welding stainless steels requires precise thermodynamic control of phase balance, solid-state precipitation, and solidification modes. Austenitic grades (e.g., 304L, 316L) require a controlled delta ferrite content of 3 to 8 Ferrite Number (FN) to guarantee primary ferrite (FA) solidification, preventing solidification cracking while avoiding brittle sigma phase formation at high temperatures or loss of cryogenic impact toughness. Modern constitution prediction relies on the WRC-1992 diagram, which accurately incorporates nitrogen and copper equivalents. Sensitization—intergranular corrosion caused by Cr23C6 carbide precipitation and adjacent chromium depletion (< 12% Cr) in the 500°C to 850°C window—is eliminated using low-carbon "L" grades (C <= 0.030%) or stabilized grades (321 with Ti, 347 with Nb). Duplex stainless steels (2205, 2507) require strict heat input management (0.5 to 2.5 kJ/mm), nickel-overalloyed filler metals, and nitrogen-supplemented shielding to maintain a 40% to 60% austenite-ferrite balance.

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Stainless Steel Alloy Families & Welding Metallurgy

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Constitutional Diagrams: Schaeffler, DeLong & WRC-1992

To predict the as-solidified weld metal microstructure resulting from base metal dilution and filler metal alloying, welding engineers rely on constitutional diagrams that plot Chromium Equivalent (ferrite stabilizers) versus Nickel Equivalent (austenite stabilizers).

                 EVOLUTION OF CONSTITUTIONAL DIAGRAMS

    SCHAEFFLER (1949)              DeLONG (1973)               WRC-1992 (Modern)
    - Historic baseline            - Added Nitrogen (30*N)     - AWS A5.4 / ASME IX Standard
    - Cr_eq: Cr+Mo+1.5Si+0.5Nb     - Up to 18 FN               - Cr_eq: Cr + Mo + 0.7Nb
    - Ni_eq: Ni+30C+0.5Mn          - Underestimates high FN    - Ni_eq: Ni + 35C + 20N + 0.25Cu
    - Ignores Nitrogen             - Predicts Ferrite Number   - Eliminates Si and Mn errors

Mathematical Formulations

1. Schaeffler Diagram (1949):

   Cr_eq, Schaeffler = Cr + Mo + 1.5*Si + 0.5*Nb
   Ni_eq, Schaeffler = Ni + 30*C + 0.5*Mn
   

   Limitations: Developed before the significant role of nitrogen in stainless steel was understood. Completely ignores nitrogen, leading to large errors in modern nitrogen-strengthened or argon-shielded welds.

2. DeLong Diagram (1973):

   Cr_eq, DeLong = Cr + Mo + 1.5*Si + 0.5*Nb
   Ni_eq, DeLong = Ni + 30*C + 30*N + 0.5*Mn
   

   Advancement: Introduced nitrogen weighting (30 x N) and introduced the standardized magnetic Ferrite Number (FN) scale up to 18 FN.

3. WRC-1992 Diagram (Kotecki & Siewert - Current Industry Standard): Adopted by AWS A5.4, ASME Section III, and ASME Section IX as the authoritative reference diagram for weld metal constitution:

   Cr_eq, WRC-1992 = Cr + Mo + 0.7*Nb
   Ni_eq, WRC-1992 = Ni + 35*C + 20*N + 0.25*Cu
   

   Key Engineering Improvements:

   • Eliminates silicon (Si) and manganese (Mn) terms because extensive experimental data demonstrated their effects are negligible within normal commercial ranges (< 1% Si, < 2.5% Mn).
   • Corrects carbon potency from 30 to 35, and nitrogen potency from 30 to 20.
   • Incorporates copper (0.25 x Cu) as an austenite stabilizer.
   • Extends FN prediction reliably up to 100 FN, fully covering duplex and super-duplex stainless steels.

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Ferrite Number (FN) & Solidification Modes

Ferrite Number vs. Percent Ferrite

Historically, ferrite was measured by metallographic point counting, which produced extreme inter-laboratory scatter (5% to 15% discrepancy on identical samples). To resolve this, AWS A4.2 and ISO 8249 established the Ferrite Number (FN), an internationally calibrated magnetic measurement scale using instruments such as the Magne-Gage or Ferritescope. For standard austenitic steels below 10 FN, FN is roughly equal to volume percent ferrite (1 FN approx 1 vol%). Above 10 FN, the relationship becomes non-linear.

The Four Solidification Modes of Austenitic Welds

Solidification behavior is governed by the ratio of equivalents (Cr_eq / Ni_eq):

                  SOLIDIFICATION MODES IN AUSTENITIC WELDS

  Mode A (Cr/Ni < 1.25)        : L -> L + A -> A  [HIGH CRACK RISK!]
  Mode AF (1.25 < Cr/Ni < 1.48): L -> L + A -> L + A + F -> A + F
  Mode FA (1.48 < Cr/Ni < 1.95): L -> L + F -> L + F + A -> F + A  [CRACK-RESISTANT!]
  Mode F (Cr/Ni > 1.95)        : L -> L + F -> F  [HIGH FERRITE]

1. Mode A (Fully Austenitic): Liquid solidifies directly to primary austenite (L -> gamma). Low-melting impurities (S, P) are rejected into liquid films wetting the smooth, planar austenite boundaries. Severe risk of solidification cracking.
2. Mode AF (Primary Austenite with Interdendritic Ferrite): Austenite dendrites solidify first, with delta ferrite forming from terminal liquid at dendrite boundaries.
3. Mode FA (Primary Ferrite with Interdendritic Austenite - TARGET REGIME): Delta ferrite solidifies first as skeletal dendrites (L -> delta), followed by a peritectic-eutectic reaction where austenite nucleates and consumes liquid and ferrite (L + delta -> gamma). Solidification in FA mode provides two vital cracking defenses:
   • Impurity Solubility: The solubility of sulfur and phosphorus in delta ferrite is roughly 10 times higher than in austenite, sequestering S and P within solid solution.
   • Tortuous Grain Boundaries: The migrating phase transformation creates complex, jagged interphase boundaries that physically block crack propagation.
4. Mode F (Fully Ferritic): Primary ferrite solidifies (L -> delta) without peritectic transformation; cools with high residual ferrite.

Engineering Targets for Ferrite Number

• General Fabrication (Crack Prevention): 3 to 8 FN (or 4 to 10 FN). Guarantees FA mode solidification without compromising mechanical properties.
• Cryogenic Service (LNG, -196°C): Restrict to <= 3 to 5 FN. Body-centered cubic delta ferrite undergoes a ductile-to-brittle transition, degrading Charpy V-notch energy and lateral expansion at liquid nitrogen temperatures.
• High-Temperature Service (> 550°C): Restrict to <= 8 to 10 FN. Prolonged exposure to elevated temperatures transforms delta ferrite into brittle sigma phase.
• Urea / Highly Oxidizing Nitric Acid Service: Mandates 0 FN (Fully Austenitic). Delta ferrite suffers preferential selective dissolution in concentrated nitric acid; requires specialized ultra-low S and P grades (e.g., 310L, 316L Modified).

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Sensitization & Intergranular Corrosion (IGC)

Thermodynamics of Chromium Depletion

When austenitic stainless steels are heated into or slowly cooled through the critical temperature range of 500°C to 850°C (950°F to 1550°F)—such as in the weld Heat-Affected Zone—sensitization occurs:

                       SENSITIZATION MICROSTRUCTURE

            [ Austenite Grain ]             [ Austenite Grain ]
                   (18% Cr)                        (18% Cr)
                      \                               /
                       \   Chromium-Depleted Zone    /
                        \       (< 12% Cr)          /
                         v                         v
                      |=====|                   |=====|
                      | Cr  |  Cr23C6 Carbide   | Cr  |
                      | Dep |  Precipitates at  | Dep |
                      | Zone|  Grain Boundary   | Zone|
                      |=====|                   |=====|
                              Grain Boundary Interface

1. Kinetics: Carbon is a small interstitial atom with a high diffusion coefficient, while chromium is a large substitutional atom that diffuses much slower. Carbon diffuses rapidly from grain interiors to prior-austenite grain boundaries.
2. Carbide Precipitation: Carbon combines with chromium along the grain boundary to precipitate complex chromium carbides, predominantly Cr23C6 (or M23C6):

   23 Cr + 6 C -> Cr23C6
   

3. Chromium-Depleted Zone: Because chromium diffusion is slow, the chromium extracted to form the carbide is drawn almost entirely from a thin layer (< 100 nm) immediately adjacent to the grain boundary. Matrix chromium concentration in this zone drops precipitously from the baseline 18% down to < 12%—falling below the critical threshold required to form a protective, passive chromium oxide (Cr2O3) film.
4. Galvanic Dissolution: When exposed to electrolyte environments (marine, chemical process, acid), the narrow chromium-depleted boundary behaves as a tiny active anode driven by the massive surrounding cathode of passive grain interiors, resulting in catastrophic intergranular corrosion (IGC) or intergranular stress corrosion cracking (IGSCC).

Industrial Mitigation Strategies

                    THE THREE SENSITIZATION DEFENSES
                                   |
        +--------------------------+--------------------------+
        |                          |                          |
        v                          v                          v
  [ Low-Carbon Grades ]     [ Stabilized Grades ]     [ Solution Annealing ]
  - AISI 304L / 316L        - AISI 321 (Ti >= 5*C)    - 1050°C to 1150°C Soak
  - C <= 0.030 wt%          - AISI 347 (Nb >= 10*C)   - Rapid Water Quench
  - Starves carbide growth  - Ties up carbon at >1000°C - Redissolves carbides

1. Low-Carbon "L" Grades (AISI 304L, 316L): Specifying maximum carbon <= 0.030% starves the carbide precipitation reaction. The "nose" of the Time-Temperature-Sensitization (TTS) curve is shifted from seconds to hours, allowing standard multi-pass welding without sensitization.
2. Stabilized Grades (AISI 321, 347): Incorporate strong carbide-forming elements:
   • Titanium (AISI 321): Requires Ti >= 5 x C. Forms titanium carbide (TiC) at temperatures > 1000°C, consuming carbon before chromium carbides can precipitate.
   • Niobium (AISI 347): Requires Nb >= 10 x C. Forms niobium carbide (NbC).
   • Knife-Line Attack (KLA): In narrow HAZ regions heated to > 1200°C, TiC and NbC dissolve. If subsequent passes reheat this zone to 500°C to 850°C, chromium carbides precipitate, creating a razor-thin band of attack. Mitigated via a post-weld stabilization heat treatment at 870°C to 900°C (1600°F to 1650°F) to redissolve Cr23C6 and re-precipitate stable TiC/NbC.
3. Full Solution Annealing: Heating the completed weldment to 1050°C to 1150°C, soaking to fully dissolve all carbides back into solid solution, followed by a rapid water quench past 500°C in seconds.

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