11.3 Solidification Cracking, Bead Aspect Ratio, Liquation & Lamellar Tearing

11.2 Hot Cracking (Solidification/Liquation) & Lamellar Tearing Mechanisms

Quick Answer: Hot cracking encompasses high-temperature fractures occurring during the solidification of weld metal or localized melting within the subsolidus Heat-Affected Zone (HAZ). Solidification cracking occurs along the fusion zone centerline within the Brittle Temperature Range (BTR) when low-melting liquid eutectic films (primarily iron sulfides Fe-FeS and phosphides Fe-Fe3P) are pulled apart by transverse thermal contraction strains; it is suppressed by enforcing an Mn/S ratio > 25 to 30, limiting P + S <= 0.015%, and maintaining a depth-to-width (D/W) ratio < 0.8. Liquation cracking occurs in the partially melted zone (PMZ) via constitutional liquation of boundary precipitates. In contrast, lamellar tearing is a mechanical, through-thickness step-like fracture in heavy rolled plates (thickness > 25 mm) driven by high Z-direction shrinkage stresses acting across rolled-out, planar Type II MnS inclusion stringers, requiring "Z-grade" steels (ASTM A770), buffer layers, or joint redesign.

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Solidification Cracking: Thermodynamics & Metallurgy

Solidification cracking (also termed hot tearing or centerline cracking) occurs in the weld metal fusion zone during the terminal stages of dendritic crystallization, specifically within the Brittle Temperature Range (BTR):

BTR = T_liquidus - T_coherence/solidus

where T_coherence is the coherence temperature at which growing solid dendrites touch and interlock (fraction solid f_s approx 0.70 to 0.90), preventing free liquid feeding to relieve thermal contraction.

                  DENDRITIC SOLIDIFICATION & LIQUID FILMS

      Advancing Solid Dendrites             Interdendritic Liquid Films
      +-----------------------+              (Segregated FeS, Fe3P, BTR)
      |   Solid Martensite /  |                  |
      |   Ferrite Dendrites   |                  v
      |      \\\\\\\\\\\\\   |             |~~~~~~~~~~~~|
      +-----------------------+             | Liquid     |  <-- Contraction
            -->   <--                       | Interlayer |      Strain (epsilon)
       Thermal Contraction                  |~~~~~~~~~~~~|      Pulls Film
         Tensile Stresses                   +------------+      Apart!
      +-----------------------+                  |
      |   Solid Martensite /  |                  v
      |   Ferrite Dendrites   |             CENTERLINE HOT CRACK
      |      /////////////   |             FORMS AT SOLIDUS!
      +-----------------------+             

Solute Redistribution & Equilibrium Partition Coefficient

As solid grains solidify, solute elements are rejected from the advancing solid front into the remaining interstitial liquid in accordance with the equilibrium partition coefficient (k_0):

k_0 = C_s / C_L

where C_s and C_L are the solute concentrations in the solid and liquid phases at equilibrium. Elements with extremely low partition coefficients (k_0 << 1.0) segregate heavily into the last remaining liquid at dendrite boundaries and along the weld centerline:

• Sulfur (k_0 approx 0.05 in austenite): Forms an iron-iron sulfide eutectic (Fe-FeS) that solidifies at a remarkably low temperature of 988°C (1810°F)—more than 500°C below the solidus of unalloyed steel (1538°C).
• Phosphorus (k_0 approx 0.06 in austenite): Forms an iron-iron phosphide eutectic (Fe-Fe3P) melting at 1050°C (1922°F).
• Carbon, Boron, and Niobium: Further widen the BTR and depress terminal liquid freezing points.

Because these thin liquid interlayers wet the solid dendritic boundaries over a broad temperature range, the cohesive strength of the mushy zone is practically zero. When transverse thermal contraction strains exceed the critical fracture strain of the mushy boundary (epsilon_thermal > epsilon_fracture), the liquid film tears open, creating an intergranular/interdendritic solidification crack.

The Critical Role of the Manganese-to-Sulfur Ratio (Mn/S)

To suppress the low-melting Fe-FeS eutectic, sufficient manganese must be present in the weld deposit to drive the high-temperature chemical reaction:

FeS + Mn -> MnS + Fe

Manganese sulfide (MnS) possesses a high melting point of 1610°C (2930°F), well above the liquidus of structural steel. Consequently, MnS precipitates early during solidification as discrete, harmless, spherical/globular inclusions floating within dendrite cores, consuming sulfur before low-melting boundary films can form.

• Engineering Rule: To guarantee complete suppression of FeS wetting films, the manganese-to-sulfur ratio must satisfy:

  Mn / S >= 25 to 30
  

• For high-restraint joints, carbon-manganese steel weld metals should maintain Mn/S > 35 with total phosphorus and sulfur constrained to [P] + [S] <= 0.015% to 0.020%.

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Bead Geometry & Weld Pool Aspect Ratio

Solidification cracking is intimately coupled with the macroscopic shape of the weld pool and the resulting bead geometry.

                 BEAD ASPECT RATIO & GRAIN IMPINGEMENT

    DEEP / NARROW (D/W > 1.0)              WIDE / CONVEX (D/W < 0.8)
      [ High Hot Crack Risk ]                [ Crack-Resistant ]
          Bead Width (W)                         Bead Width (W)
       |<--------------->|                    |<------------------->|
       +                 +                    +                     +
       | \             / |                    \                     /
       |  \     |     /  |                     \                   /
       |   \    |    /   |                      \       ^         /
Depth  |    \   v   /    |               Depth   \     / \       /
 (D)   |     \  |  /     |                (D)     \   /   \     /
       |      \ | /      |                         +---+---+---+
       +-------v-+-------+                               v
          Centerline Crack                      Curved, interlocking
          traps low-melting                     dendrites bridge pool
          liquid eutectics!                     without planar seam!

Depth-to-Width Ratio (D/W)

1. Deep, Narrow Beads (D/W > 1.0): Typical of high-current Submerged Arc Welding (SAW), keyhole plasma/laser welding, or narrow-gap root passes. The thermal gradient drives columnar dendrites to grow perpendicular to the retreating pool boundaries. In deep, narrow grooves, opposing dendrites from opposite sidewalls meet abruptly along a single, planar vertical line at the joint center. All rejected low-melting solutes (S, P) are swept into this centerline plane of impingement, producing severe centerline solidification cracks.
2. Wide, Convex Beads (D/W = 0.5 to 0.8, or W/D >= 1.25): Columnar dendrites grow upward and inward with curved trajectories, creating an interlocking, staggered cellular pattern that allows continuous liquid feeding to backfill shrinkage voids without forming a continuous planar weak seam.

Bead Surface Contour (Convex vs. Concave)

• Concave Bead Caps: Thermal shrinkage places the outer surface of a concave fillet or groove weld under extreme transverse tension during terminal solidification, pulling surface liquid films apart.
• Convex Bead Caps: Thermal shrinkage exerts internal compressive forces across the cap center, suppressing hot crack opening.

Travel Speed & Pool Geometry (Elliptical vs. Tear-Drop)

• High Travel Speed: Stretches the molten pool into an elongated tear-drop shape with a sharp, pointed trailing boundary. Dendrites grow perpendicular to the pool boundary and meet at a sharp 90° impingement angle along the centerline, promoting cracking.
• Moderate Travel Speed: Maintains an elliptical pool contour where dendrites curve gradually toward the welding axis, interlocking smoothly along the weld axis.

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Liquation Cracking in the Heat-Affected Zone (HAZ)

Liquation cracking (also termed hot micro-fissuring) occurs in the solid Heat-Affected Zone immediately adjacent to the fusion boundary—a microstructural subzone known as the Partially Melted Zone (PMZ). Unlike solidification cracking, which occurs in the molten pool, liquation cracking occurs in the base metal matrix that remained solid during the welding cycle.

                    CONSTITUTIONAL LIQUATION IN HAZ (PMZ)

     Molten Pool  | Fusion Boundary |  Partially Melted Zone (PMZ)
                  |                 |
       Liquid     |   Solid Coarse  |   Secondary Phase Precipitate (e.g., FeS, NbC)
       Weld Metal |   Austenite     |   reacts with surrounding matrix at T_eutectic < T_solidus!
                  |   Grains        |        
                  |                 |             [ Solid Matrix ]
                  |                 |            /                \
                  |                 |           (  Liquated Film   )
                  |                 |            \  along Grain   /
                  |                 |             [  Boundary    ]
                  |                 |                    |
                  |                 |                    v
                  |                 |           PULLS APART UNDER
                  |                 |           CONTRACTION STRESS!

Mechanism of Constitutional Liquation

Constitutional liquation occurs when secondary-phase precipitate particles (e.g., niobium carbides NbC, titanium carbonitrides Ti(C,N), iron sulfides FeS, or chromium borides) react eutectically with the surrounding solid iron or nickel matrix during rapid heating:

1. During rapid welding thermal cycles, solid-state dissolution of precipitates cannot proceed to completion before reaching the eutectic temperature T_eutectic.
2. At T_peak >= T_eutectic (which is lower than the equilibrium solidus of the alloy), a localized liquid film forms at the precipitate-matrix interface.
3. This liquid film rapidly wets along prior-austenite grain boundaries via capillary action, forming continuous grain boundary liquid films.
4. As the adjacent weld metal solidifies and contracts, transverse tensile stresses pull these liquated grain boundaries apart, generating microscopic intergranular fissures (0.1 to 1.0 mm long).

Liquation cracking is especially prevalent in precipitation-strengthened nickel superalloys (Inconel 718, Waspaloy), fully austenitic stainless steels, and high-strength microalloyed steels with high niobium ([Nb] > 0.04%) or boron contents.

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Lamellar Tearing: Mechanics, Inclusions & Joint Restraint

Lamellar tearing is a catastrophic, step-like subsurface cracking phenomenon occurring parallel to the rolling plane of heavy structural rolled steel plates (typically thicknesses t >= 25 to 50 mm). It occurs primarily beneath welded T-joints, corner joints, and cruciform connections where full-penetration groove welds impose high through-thickness (Z-direction / short-transverse) tensile contraction stresses.

                       LAMELLAR TEARING MORPHOLOGY

                 Attached Flange / Stiffener
                            |   |
                            |   |
                            |   |
                       +----+---+----+
                       |    |   |    |
                  Weld |   /     \   | Weld
                 Metal |  /       \  | Metal
                       +-+---------+-+
       ==================+=========+================== Main Plate Surface
       ----------------------------------------------- 
       --- [Planar Decollimations along MnS Stringers] (Terraces)
          |                                         |
          v (Vertical Shear Tears)                  v
       -----------------------------------------------
       =============================================== Plate Bottom Surface

The Three Necessary Conditions for Lamellar Tearing

1. Susceptible Plate Material: Rolled plate containing high volume fractions of non-metallic, planar inclusions oriented parallel to the rolling surface.
2. High Through-Thickness Tensile Stress: Weld design and joint restraint that orient weld shrinkage vectors perpendicular to the rolling plane (the Z-axis / short-transverse direction).
3. Unfavorable Joint Configuration: Rigid connections (e.g., thick diaphragm-to-box columns, heavy nozzle necks) that concentrate shrinkage strains within a narrow through-thickness band.

Role of Non-Metallic Inclusions

During ingoting and continuous casting, non-metallic inclusions solidify. Subsequent heavy hot-rolling flattens these inclusions into broad, planar platelets or "stringers":

• Type II Manganese Sulfides (MnS): Highly plastic at hot-rolling temperatures (1000°C to 1200°C); roll into elongated, paper-thin ribbons or flattened sheets.
• Silicates (SiO2 complexes): Deform into thin, brittle planar disks.
• Alumina Clusters (Al2O3): Line up in stringers along rolling flow lines.

Under through-thickness tensile stress, decohesion occurs along the weak inclusion-matrix interfaces, generating horizontal planar microcracks ("terraces"). As strain accumulates, the intervening vertical ligaments between adjoining terraces fail by shear ductile overload, linking the terraces into a macroscopic, step-like "terrace-and-wall" fracture.

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