11.4 Lamellar Tearing Mitigation & Hot-Cracking Susceptibility Evaluation

Engineering Mitigation of Lamellar Tearing

1. Material Specification: "Z-Grade" Steels (ASTM A770 / EN 10164)

Standard tensile tests measure mechanical properties in the longitudinal (rolling) or transverse directions. Due to inclusion flattening, short-transverse (through-thickness) ductility is dramatically inferior. To guarantee resistance to lamellar tearing, engineers specify Z-direction quality steels evaluated via ASTM A770 / A770M (Through-Thickness Tension Testing):

• Z-Grade Designations:
  • Z15: Minimum average through-thickness reduction of area (ZRA) >= 15%.
  • Z25: Minimum average ZRA >= 25% (Standard for critical offshore and seismic joints).
  • Z35: Minimum average ZRA >= 35% (Severe restraint, extreme heavy plate applications).
• Clean Steel Metallurgy: Achieving Z25/Z35 properties requires:
  • Vacuum degassing to eliminate dissolved gases.
  • Ultra-low sulfur refining ([S] <= 0.005%, frequently <= 0.002%).
  • Calcium Treatment (Inclusion Shape Control): Injecting calcium-silicon (Ca-Si) wire into the ladle converts elongated MnS into hard, non-deformable spherical calcium sulfides/calcium aluminates (CaS * Al2O3). These spherical inclusions retain their shape during rolling, eliminating planar stringers and raising Z-direction reduction of area above 40%.

2. Buttering (Buffer Layer Technique)

When Z-grade plate is unavailable, a ductile buffer layer (buttering) can be deposited directly on the surface of the susceptible plate before joint assembly:

• Deposit 2 to 3 layers (6 to 10 mm thickness) of a low-strength, high-ductility weld metal (e.g., ER70S-3, E7018-1, or austenitic nickel alloy).
• Grind the buttering flat, then fit up the mating member and weld the full joint entirely to the buttered layer.
• The buttering layer spreads transverse contraction strains across a broader volume and isolates the susceptible inclusion-rich plate core from high peak welding stresses.

3. Joint Redesign to Avoid Z-Direction Stresses

                  JOINT REDESIGN TO PREVENT LAMELLAR TEARING

      SUSCEPTIBLE DESIGN (POOR)                 MITIGATED DESIGN (EXCELLENT)
      (Tensile Shrinkage Acts Across            (Shrinkage Vector Transferred
       Plate Through-Thickness)                  to In-Plane Shear or Forging)
               |   |                                     |   |
               |   |                                     |   |
               |   |                                     |   |
         +-----+---+-----+                         +-----+---+-----+
         |     |   |     |                         |     |   |     |
         |    /     \    |                         |    /     \    |
         |   /       \   |                         |   /       \   |
       ==+==+=========+==+===                    ==+==+=========+==+===
       ||   | Tear!   |    ||                    ||  | Machined |  | ||
       ||   +---------+    ||                    ||  | Corner / |  | ||
       ||  Rolled Plate    ||                    ||  | Forging  |  | ||
       ======================                    ======================

• Move bevel preparations from the attached through-plate to the cross-member so that weld fusion crosses the plate surface at an oblique angle.
• Replace welded T-joints with rolled or forged transition sections (e.g., cast steel nodes in offshore jackets) that place weld seams outside the Z-direction stress zone.
• Use recessed corner joints where the vertical plate extends past the corner, converting through-thickness tension into in-plane longitudinal shear.

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Worked Numerical Example: Hot Cracking Susceptibility & Aspect Ratio Evaluation

Problem Statement

A mechanized Submerged Arc Welding (SAW) procedure is being developed to weld a longitudinal seam in a heavy structural box girder fabricated from carbon steel plate. Metallurgical analysis of the test weld reveals the following fusion zone chemical composition (weight percent):

• Carbon (C): 0.16%
• Manganese (Mn): 0.75%
• Silicon (Si): 0.35%
• Sulfur (S): 0.030%
• Phosphorus (P): 0.025%
• Niobium (Nb): 0.035%

The initial welding procedure operates at 32 V, 750 A, and 450 mm/min travel speed (7.5 mm/s), producing a macro-etched cross-section with:

• Bead Width (W): 14.0 mm
• Bead Depth (D): 17.5 mm

Engineering Tasks:

1. Calculate the Manganese-to-Sulfur ratio (Mn/S) and evaluate compliance with crack-prevention criteria.
2. Calculate the Unit of Cracking Susceptibility (UCS) for carbon-manganese weld metals using the established UK Welding Institute formulation:

   UCS = 230*C + 190*S + 75*P + 45*Nb - 12.3*Si - 5.4*Mn - 1.0
   

   where values > 20 indicate severe solidification cracking susceptibility, 10 to 20 indicates borderline risk, and < 10 indicates high resistance.
3. Calculate the depth-to-width aspect ratio (D/W) of the initial weld bead and evaluate crack risk.
4. Recommend optimized welding parameter adjustments (voltage, current, travel speed) to achieve a crack-resistant bead profile (D/W <= 0.70) without sacrificing required weld throat penetration.

Step-by-Step Solution

Step 1: Calculate Mn/S Ratio

Mn / S = 0.75% / 0.030% = 25.0

Evaluation: The ratio equals 25.0, which is at the absolute lower threshold of the acceptable 25 to 30 boundary. Given the high sulfur concentration (0.030%) and presence of phosphorus (0.025%), the alloy lacks an adequate safety margin to suppress low-melting Fe-FeS films under rigid restraint.

Step 2: Calculate Unit of Cracking Susceptibility (UCS)

UCS = 230*(0.16) + 190*(0.030) + 75*(0.025) + 45*(0.035) - 12.3*(0.35) - 5.4*(0.75) - 1.0
UCS = 36.8 + 5.70 + 1.875 + 1.575 - 4.305 - 4.05 - 1.0
UCS = 45.95 - 9.355 = 36.595 approx 36.6

Evaluation: Because UCS = 36.6 significantly exceeds the critical threshold of 20.0, this weld metal chemistry is classified as exceptionally prone to solidification cracking.

Step 3: Calculate Bead Aspect Ratio (D/W)

D / W = 17.5 mm / 14.0 mm = 1.25

Evaluation: The depth-to-width ratio D/W = 1.25 > 1.0 produces a deep, narrow weld pool. Solidifying columnar dendrites meet at an abrupt 90° angle at the joint centerline. Because UCS = 36.6 and D/W = 1.25, centerline hot cracking is virtually guaranteed under transverse restraint.

Step 4: Process Parameter Optimization To eliminate centerline cracking:

1. Increase Arc Voltage (from 32 V to 36 V): Arc length expands, widening the arc cone and broadening the bead width (W) from 14 mm to 22 mm.
2. Reduce Welding Current (from 750 A to 600 A): Decreases arc force and plasma jet momentum, reducing penetration depth (D) from 17.5 mm to 12.0 mm.
3. Resulting Aspect Ratio:

   D_new / W_new = 12.0 mm / 22.0 mm = 0.545 <= 0.70
   

4. Consumable Substitution: Switch to a higher-manganese, basic-flux combination (e.g., Lincoln 880M flux with Lincolnweld L-61 wire) to elevate weld metal manganese to 1.20% and reduce sulfur to <= 0.010%, driving Mn/S > 120 and reducing UCS < 10.

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Real-World Engineering Scenarios & Exam Pitfalls

Industrial Scenario: Lamellar Tearing in an Offshore Platform Jacket Node

During the fabrication of a critical node connection for an offshore oil platform, 75 mm thick ASTM A572 Grade 50 rolled plates were welded into a heavy cruciform joint using full-penetration submerged arc welds with heavy joint restraint. Radiographic testing (RT) showed no fusion defects. However, subsequent straight-beam ultrasonic testing (UT) revealed intermittent planar discontinuities 5 to 8 mm below the plate surface, extending several hundred millimeters parallel to the weld fusion line.

Failure Analysis:

• Core trepan samples exhibited classic step-like "terrace-and-wall" lamellar tearing.
• Mill test records revealed the plate was standard structural grade with sulfur at 0.028% and no through-thickness reduction of area guarantee. Heavy planar Type II MnS inclusions were verified via scanning electron microscopy.

Remediation:

1. The defective plates were gouged out completely.
2. Replacement plates were ordered to ASTM A770 Z35 specifications ([S] <= 0.002%, calcium treated for inclusion globularity, through-thickness reduction of area > 35%).
3. Before making the CJP welds, an 8 mm three-pass buttering layer of ductile E7018-1 weld metal was deposited across the plate face to absorb residual transverse shrinkage strain.

Common Exam Traps

Exam Trap 1: Confusing Solidification Cracking with Lamellar Tearing Solidification cracking occurs in the weld metal along the centerline or between dendrites at temperatures near the alloy solidus (> 1000°C). Lamellar tearing occurs in the base metal plate beneath the weld along inclusion stringers parallel to the rolling plane at temperatures below 200°C.

Exam Trap 2: Increasing Heat Input to "Soften" Hot Cracks A dangerous misconception is that increasing heat input prevents hot cracking by retarding the cooling rate (as it does for cold cracking). For hot cracking, increasing heat input worsens cracking susceptibility! Higher heat input expands the volume of the molten pool, broadens the Brittle Temperature Range (BTR), coarsens dendrites, increases microsegregation, and increases total contraction strain.

Exam Trap 3: The Directionality of Lamellar Tearing Stresses Lamellar tearing is driven solely by tensile strains acting in the Z-direction (through-thickness / short-transverse) perpendicular to the plate surface. It cannot be caused by longitudinal stresses running parallel to the plate rolling direction.