17.3 Stainless Filler Specifications, Solidification Modes & Dissimilar-Metal Joints

17.2 Stainless Steel, Nickel & Aluminum Filler Metal Selection & Dilution

Quick Answer: Stainless steel consumables are governed by AWS A5.4 (SMAW: E308L-16, E309L-16, E316L-16, E2209-16), AWS A5.9 (bare wire: ER308L, ER309L, ER316L, ER2209), and AWS A5.22 (flux-cored: E308LT0-1). Austenitic deposits require 3 to 10 Ferrite Number (FN) to eliminate solidification hot cracking (microfissuring). When joining carbon steel to austenitic stainless steel, ER309L provides over-alloyed 23% Cr – 13% Ni chemistry that accommodates up to 25–30% base metal dilution without entering the brittle martensitic phase. However, for service temperatures exceeding 425°C (800°F), nickel-base consumables (AWS A5.14 ERNiCr-3 / Inconel 82 or ERNiCrMo-3 / Inconel 625) are compulsory to block carbon migration from the ferritic steel into the stainless weld and to bridge the coefficient of thermal expansion (CTE) mismatch. For aluminum (AWS A5.10), ER4043 (Al-5% Si) resists cracking and flows smoothly, while ER5356 (Al-5% Mg) offers superior shear strength and post-anodizing color matching, but 5xxx fillers with > 3% Mg are restricted to operating temperatures < 65°C (150°F) to prevent catastrophic stress corrosion cracking.

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Stainless Steel Filler Metal Specifications: AWS A5.4, A5.9 & A5.22

Welding of stainless steels requires precise thermodynamic and metallurgical balancing to maintain passive chromium oxide (Cr2O3) corrosion resistance, prevent intergranular sensitization, and suppress solidification cracking.

+-----------------------------------------------------------------------------------------+
|                        STAINLESS STEEL CONSUMABLE SPECIFICATION MATRIX                  |
+------------------------------------+--------------------------+-------------------------+
| Product Form                       | AWS Specification        | Representative Grades   |
+------------------------------------+--------------------------+-------------------------+
| Covered Electrodes (SMAW)          | AWS A5.4 / A5.4M         | E308L-16, E309L-15,     |
|                                    |                          | E316L-17, E2209-16      |
| Solid Bare Wire / Rod (GMAW/GTAW)  | AWS A5.9 / A5.9M         | ER308L, ER309L,         |
|                                    |                          | ER316L, ER2209          |
| Flux Cored & Metal Cored (FCAW/MCAW)| AWS A5.22 / A5.22M      | E308LT0-1, E309LT1-4,   |
|                                    |                          | EC316L                  |
+------------------------------------+--------------------------+-------------------------+

SMAW Coating Suffix Designators (AWS A5.4)

The suffix following the alloy designation indicates coating chemistry, electrical characteristics, and usability:

• -15 (Lime-Basic Covering): Contains calcium carbonate (CaCO3) and calcium fluoride (CaF2). Operates strictly on DCEP. Produces a slightly convex bead with fast-freezing slag. It generates the lowest weld metal oxygen content (< 350 ppm) and lowest inclusion density, delivering superior cryogenic Charpy notch toughness at -196°C (-320°F). Preferred for pipe welding in 5G/6G positions.
• -16 (Titania-Rutile Covering): Contains high titanium dioxide (TiO2) with minor basic minerals. Operates on AC or DCEP. Produces a smooth spray-type arc, concave-to-flat bead profile, fine ripple pattern, and self-releasing slag. Ideal for general sheet metal and flat/horizontal fabrication.
• -17 (Silica-Titania Covering): A modified rutile coating with high silicon oxide content. Operates on AC or DCEP. Delivers exceptional molten puddle fluidity and wetting at the weld toes, virtually eliminating undercut. Best suited for horizontal fillet welds and cosmetic surface passes.

The "L" Grade Imperative: Sensitization Prevention

In grades like ER308L, ER309L, and ER316L, the "L" designates low carbon (C <= 0.03%). During slow cooling through or service within the sensitization temperature window (450°C to 850°C / 840°F to 1560°F), carbon precipitates at austenite grain boundaries as chromium-rich carbides:

$$23[Cr] + 6[C] \longrightarrow Cr_{23}C_6$$

Because chromium diffuses slowly in austenite compared to carbon, the regions immediately adjacent to the grain boundaries become depleted in chromium below the critical 12% passivation limit. This triggers catastrophic intergranular corrosion (IGC). Restricting carbon to <= 0.03% prevents Cr23C6 formation, ensuring the corrosion resistance of the heat-affected zone and weld metal in the as-welded condition.

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Solidification Modes & Ferrite Number (FN) Prediction

Austenitic stainless steel weld metals solidify in one of four distinct modes depending on the ratio of Chromium Equivalent (Cr_eq) to Nickel Equivalent (Ni_eq):

                               SOLIDIFICATION MODES IN STAINLESS WELDS

        Mode A                Mode AF                 Mode FA                 Mode F
    (Austenitic)        (Austenite-Ferrite)     (Ferrite-Austenite)        (Ferritic)
     Cr_eq / Ni_eq           Cr_eq / Ni_eq           Cr_eq / Ni_eq        Cr_eq / Ni_eq
        < 1.25                1.25 - 1.48             1.48 - 1.95            > 1.95
    L -> L + gamma        L -> L + gamma          L -> L + delta          L -> L + delta
        -> gamma               -> gamma + delta        -> delta + gamma        -> delta
  [Severe Hot Cracking] [Moderate Crack Risk]  [CRACK RESISTANT: 3-10 FN] [Embrittlement]

Hot Cracking (Microfissuring) Mechanics

Fully austenitic weld metals (solidification Modes A and AF) are notoriously prone to solidification hot cracking. Low-melting-point tramp elements—specifically sulfur (S) and phosphorus (P)—segregate to liquid films along interdendritic grain boundaries during the final stages of freezing. Iron and nickel sulfides/phosphides (FeS, Ni3S2) form eutectics that remain liquid down to 988°C.

When primary ferrite solidifies first (Mode FA, typical of 308L and 316L with Cr_eq / Ni_eq ≈ 1.5 - 1.8):

1. Delta ferrite (delta) possesses a high solubility for sulfur (up to 0.18%) and phosphorus, effectively sequestering tramp elements inside the solid lattice.
2. The convoluted, tortuous delta / gamma interphase boundary prevents the formation of continuous, planar liquid films.
3. The design target for structural austenitic stainless welds is 3 to 10 Ferrite Number (FN) (per AWS A4.2 / WRC-1992). Less than 3 FN risks hot cracking; greater than 10 FN risks low-temperature embrittlement and rapid transformation to brittle sigma phase during elevated-temperature exposure (550°C–900°C).

Constitution Diagrams: WRC-1992 Diagram

The WRC-1992 diagram (Welding Research Council) is the current ASME Section IX and AWS standard for predicting Ferrite Number in stainless steel welds:

$$Cr_{eq} = \%Cr + \%Mo + 0.7 \%Nb$$ $$Ni_{eq} = \%Ni + 35 \%C + 20 \%N + 0.25 \%Cu$$

(Note: Schaeffler and DeLong diagrams also incorporate manganese and silicon, but WRC-1992 eliminates silicon and reduces manganese weighting to achieve the highest predictive accuracy for modern arc weldments).

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Dissimilar Metal Welding: Carbon Steel to Stainless Steel

Joining a ferritic carbon steel (e.g., ASTM A36 or ASTM A516 Gr 70) to an austenitic stainless steel (e.g., AISI 304L) represents one of the most critical engineering challenges in petrochemical plants and power boilers.

                          DISSIMILAR METAL WELD JOINT SCHEMATIC

          ASTM A516 Gr 70                                      AISI 304L
          (Carbon Steel)                                       (Austenitic Stainless)
        +-----------------+                                  +-----------------+
        | C = 0.20%       |        ER309L or Inconel 82      | Cr = 18.5%      |
        | Cr = 0.05%      |        Weld Deposit Deposit      | Ni = 8.5%       |
        | Ni = 0.05%      +                  +               | C = 0.02%       |
        +-----------------+---+              +---+-----------+-----------------+
                              |<--- Fusion ->|   |
                              |     Boundary     |

The ER309L Dilution Strategy

If an engineer erroneously specifies an ER308L filler metal (19% Cr – 9% Ni) to join A516 Gr 70 to 304L with a typical 25% total dilution (12.5% from carbon steel, 12.5% from stainless steel):

• The severe influx of carbon and unalloyed iron from the carbon steel base metal dilutes the chromium and nickel concentrations.
• On the Schaeffler diagram, the resulting weld puddle composition drops into the martensitic transformation zone (Cr_eq ≈ 14%, Ni_eq ≈ 9%).
• Upon cooling, hard, untempered martensite forms in the root pass, triggering immediate transverse cracking under shrinkage strains.

To prevent this, AWS A5.9 ER309L (23% Cr – 13% Ni) is specified. The excess chromium and nickel compensate for iron dilution, guaranteeing that the mixed weld pool lands safely in the austenite + ferrite region with 4 to 10 FN, completely avoiding martensite.

The High-Temperature Boundary Problem: Carbon Migration

While ER309L performs flawlessly at ambient and moderate service temperatures, it suffers a fatal metallurgical failure mechanism at service temperatures above 425°C (800°F):

                               CARBON MIGRATION MECHANISM (> 425°C)

          Carbon Steel (Ferritic)               Austenitic Stainless Weld (309L)
       +----------------------------|-----------------------------------------+
       | High Carbon Activity       | High Chromium Affinity for Carbon       |
       |                            |                                         |
       |      [C] ---------->       |   [C] + [Cr] -> Cr23C6 Carbides         |
       |                            |                                         |
       | Decarburized Ferrite Zone  | Hyper-Carburized Carbide Precipitation  |
       | - Extremely Low Strength   | - Highly Brittle Interface              |
       | - Coarse Ferrite Grains    | - High Creep-Fatigue Notch Sensitivity  |
       +----------------------------|-----------------------------------------+
                                Fusion Line

1. Thermodynamic Driving Force: Carbon possesses a vastly higher chemical affinity for chromium than for iron. Across the fusion line, a massive gradient in carbon chemical activity exists.
2. Diffusion Kinetics: At service temperatures above 425°C, interstitial carbon atoms diffuse rapidly out of the carbon steel and into the stainless steel weld metal.
3. Decarburized Soft Zone: A band of pure, coarse-grained ferrite forms on the carbon steel side. Its tensile and creep strength drop by over 50%.
4. Carbide Precipitation Hard Zone: Immediately across the fusion line in the stainless weld, carbon reacts with chromium to precipitate a continuous, dense band of Cr23C6 carbides, creating a brittle notch.
5. Under cyclic thermal and mechanical loading, premature creep-rupture cracking initiates along this weakened decarburized line.

The Nickel-Base Consumable Solution

To eliminate carbon migration and mitigate thermal stress in high-temperature dissimilar joints, the welding engineer must specify nickel-chromium filler metals:

• AWS A5.14 ERNiCr-3 (Inconel 82 bare wire) / AWS A5.11 ENiCrFe-3 (Inconel 182 electrode): 67% Ni min, 18–22% Cr, 2.5–3.5% Nb+Ta, 2.5–3.5% Mn.
• AWS A5.14 ERNiCrMo-3 (Inconel 625 bare wire) / AWS A5.11 ENiCrMo-3 (Inconel 625 electrode): 58% Ni min, 20–23% Cr, 8–10% Mo, 3.15–4.15% Nb+Ta.

Why Nickel Alloys Succeed:

1. Carbon Diffusion Barrier: The solubility and diffusion coefficient of carbon in a high-nickel face-centered cubic matrix are orders of magnitude lower than in iron. Carbon migration is effectively stopped.
2. Coefficient of Thermal Expansion (CTE) Buffer:
   • Carbon Steel CTE: ≈ 12.0 × 10^-6 / °C
   • Austenitic Stainless (304L) CTE: ≈ 17.5 × 10^-6 / °C
   • Nickel Alloy (Inconel 82/625) CTE: ≈ 13.5 × 10^-6 / °C The nickel weld metal acts as a compliant thermal buffer, splitting the thermal expansion differential in half and dramatically extending thermal fatigue life.

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