10.6 HAZ Anatomy, Microstructural Sub-Zones & Multi-Pass Reheated Regions

10.3 The Heat-Affected Zone (HAZ): Grain Coarsening, Refinement & Properties

Quick Answer: The Heat-Affected Zone (HAZ) represents the portion of base metal whose microstructure and mechanical properties are altered by welding thermal cycles without undergoing macroscopic melting. In a single-pass weld on C-Mn steel, the HAZ subdivides into four metallurgical zones corresponding to peak temperatures ($T_p$): the Coarse-Grained HAZ (CGHAZ, $T_p > 1100^\circ\text{C}$), Fine-Grained HAZ (FGHAZ, $900^\circ\text{C} < T_p < 1100^\circ\text{C}$), Intercritical HAZ (ICHAZ, $727^\circ\text{C} < T_p < 900^\circ\text{C}$), and Subcritical HAZ (SCHAZ, $T_p < 727^\circ\text{C}$). The CGHAZ dissolves grain-pinning precipitates, producing giant prior austenite grains (ASTM 1–3) that transform into hard, low-toughness lath martensite or upper bainite. In multi-pass welds, secondary thermal cycles produce the Intercritically Reheated CGHAZ (ICCGHAZ), where continuous necklaces of high-carbon Martensite-Austenite (M-A) microphases precipitate along prior austenite grain boundaries, acting as the primary Local Brittle Zone (LBZ) that triggers catastrophic CTOD toughness failure. Peak HAZ hardness must be restricted (e.g., $250\text{ HV}$ for NACE sour service; $350\text{ HV}$ for structural fabrication) using preheat and heat-input controls.

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Anatomy of the Heat-Affected Zone & Thermal Gradients

Fusion welding establishes an intense, localized heat source that creates steep spatial temperature gradients across the base metal. The metallurgical transformations occurring at any point in the solid metal are governed primarily by the peak temperature attained ($T_p$) and the subsequent cooling rate ($\Delta t_{8/5}$).

                     CROSS-SECTIONAL ANATOMY OF A SINGLE-PASS WELD HAZ

     Peak Temp (Tp)
        1538°C +=========================+  FUSION ZONE (FZ) [Complete Melting]
               | ~~~~~~~~~~~~~~~~~~~~~~~ |
     Liquidus  |-------------------------|  FUSION BOUNDARY / UNMIXED ZONE (UMZ)
               |                         |  
      > 1100°C |   COARSE-GRAINED HAZ    |  ASTM 1-3 Prior γ Grains; Dissolution of Pinners;
               |        (CGHAZ)          |  Lath Martensite / Upper Bainite; Primary LBZ
               |                         |  
    Ac3-1100°C |    FINE-GRAINED HAZ     |  ASTM 8-10 Equiaxed Grains; Recrystallized;
               |        (FGHAZ)          |  High Toughness & High Ductility
               |                         |  
       Ac1-Ac3 |    INTERCRITICAL HAZ    |  Partial Reaustenitization; Carbon Partitioning;
               |        (ICHAZ)          |  Brittle Martensite-Austenite (M-A) Islands
               |                         |  
        < Ac1  |     SUBCRITICAL HAZ     |  Tempering of Base Metal; Carbide Spheroidization;
               |        (SCHAZ)          |  Localized Softening or Strain Aging
               |                         |  
      Ambient  +=========================+  UNAFFECTED BASE METAL

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Detailed Microstructural Sub-Zones of a Single-Pass Weld

1. The Fusion Boundary & Unmixed Zone (UMZ)

Immediately adjacent to the fusion line lies a narrow stagnant boundary layer of liquid metal known as the unmixed zone (UMZ). Because liquid metal flow velocity drops to zero at the solid boundary (no-slip hydrodynamic condition), this liquid experiences no convective stirring with the filler metal. The UMZ solidifies with a chemical composition identical to the base metal, but with a cast dendritic microstructure. In dissimilar metal welds (e.g., stainless steel filler on carbon steel base metal), the UMZ produces localized pockets of unalloyed martensite prone to underbead cracking.

2. Coarse-Grained HAZ (CGHAZ)

• Peak Temperature: $1100^\circ\text{C} < T_p < T_{\text{solidus}}$ ($1450^\circ\text{C}$ to $1500^\circ\text{C}$).
• Grain Coarsening Kinetics: In structural microalloyed steels, grain boundaries are pinned against migration at normal temperatures by fine, sub-micron carbonitride precipitates (Zener pinning pressure: $P_Z = \frac{3 F_v \gamma_{gb}}{2 r}$, where $F_v$ is volume fraction and $r$ is precipitate radius). Above $1100^\circ\text{C}$, the thermodynamic solubility products of precipitates (such as $\text{AlN}$, $\text{Nb}(\text{C},\text{N})$, and $\text{V}(\text{C},\text{N})$) are exceeded: $$\log_{10} [\text{Nb}][\text{C}] = 2.26 - \frac{6770}{T\ (\text{K})}$$ These pinning precipitates dissolve into solid solution or coarsen rapidly per Lifshitz-Slyozov-Wagner (LSW) Ostwald ripening kinetics. Without Zener pinning, austenite grain boundaries migrate freely, driven by the reduction in interfacial grain boundary area. Prior austenite grain size surges to ASTM 1 to 3 ($>100\text{ to }250\ \mu\text{m}$), compared to ASTM 8 to 10 ($<20\ \mu\text{m}$) in the base metal.
• Hardenability Surge: Coarse austenite grains have drastically reduced grain-boundary surface area per unit volume. Because ferrite and pearlite nucleate heterogeneously at grain boundaries, their nucleation is severely suppressed. The effective continuous-cooling transformation curves are pushed to longer times, dramatically increasing local hardenability.
• Transformation Products: Upon cooling, coarse austenite grains transform into lath martensite, Widmanstätten ferrite sideplates, and coarse upper bainite. Dissolved microalloying elements (Nb, V) remain trapped in solid solution, providing intense solid-solution and subsequent precipitation hardening that embrittles the matrix.
• Mechanical Impact: The CGHAZ forms the primary Local Brittle Zone (LBZ) in single-pass welds, exhibiting the lowest Charpy V-notch energy and the highest susceptibility to Hydrogen-Induced Cold Cracking (HICC).

3. Fine-Grained HAZ (FGHAZ)

• Peak Temperature: $Ac_3 < T_p \le 1100^\circ\text{C}$ (typically $900^\circ\text{C} \text{ to } 1100^\circ\text{C}$).
• Transformation Mechanism: Peak temperatures exceed $Ac_3$, allowing complete reaustenitization of the base metal. However, temperatures are insufficient to dissolve refractory carbonitride precipitates (especially $\text{TiN}$ and fine $\text{Nb}(\text{C},\text{N})$). Undissolved precipitates exert strong Zener drag, restricting austenite grain growth.
• Microstructure on Cooling: The fine austenite grains (ASTM 9–11) provide high grain-boundary nucleation site density, promoting transformation into very fine, equiaxed polygonal ferrite and fine pearlite.
• Mechanical Impact: The FGHAZ exhibits the highest Charpy V-notch impact toughness and ductility in the entire welded joint, often surpassing the properties of the normalized base plate.

4. Intercritical HAZ (ICHAZ)

• Peak Temperature: $Ac_1 < T_p < Ac_3$ (typically $727^\circ\text{C} \text{ to } 900^\circ\text{C}$).
• Partial Austenitization & Carbon Partitioning: Only regions of the base metal with favorable local carbon availability (pearlite colonies and ferrite grain boundaries) transform into austenite. Proeutectoid ferrite remains solid and untransformed.
  • Because carbon is roughly $50\times$ more soluble in FCC austenite than in BCC ferrite, carbon rapidly diffuses from the surrounding ferrite into these newly formed, small austenite islands.
  • An alloy with a bulk carbon content of $0.12\text{ wt}%$ produces austenite pools enriched to $0.50\text{ to }0.80\text{ wt}%$ carbon!
• Formation of M-A Constituent: Due to their high carbon concentration, these small austenite islands possess extreme hardenability. Upon continuous cooling, they bypass ferrite/pearlite transformation and transform into hard, high-carbon lath/twinned martensite and untransformed retained austenite, known collectively as Martensite-Austenite (M-A) constituent.
• Mechanical Impact: The microstructure consists of hard, brittle M-A constituent islands embedded in a soft, yielding ferrite matrix. Under tensile or impact loading, severe strain incompatibility develops at the M-A/ferrite interfaces, initiating micro-cleavage cracks that degrade fracture toughness.

5. Subcritical HAZ (SCHAZ)

• Peak Temperature: $T_p < Ac_1$ ($<727^\circ\text{C}$).
• Transformation Mechanism: No phase transformation to austenite occurs. The thermal cycle acts as a localized tempering treatment.
• Microstructural Changes: In quenched-and-tempered (QT) or thermomechanically controlled processed (TMCP) steels, the SCHAZ can experience over-tempering: carbide coarsening, dislocation recovery, and slight subgrain growth. In cold-worked steels, dynamic strain-aging embrittlement can occur if interstitial nitrogen and carbon pin dislocation arrays.
• Mechanical Impact: QT and TMCP steels frequently exhibit a localized soft zone (dip in Vickers hardness) in the SCHAZ. Although yield strength may drop slightly, fracture toughness remains high.

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Multi-Pass Thermal Cycles & Complex Reheated HAZ Zones

In thick structural sections, welds are deposited in multiple passes. The thermal cycle of each subsequent pass reheats and alters the microstructural zones established by preceding passes. The most critical microstructural region in structural engineering is the reheated coarse-grained HAZ:

                         REHEATED CGHAZ REGIONS IN MULTI-PASS WELDS

             Pass 2 Arc / Weld Pool
                 \          /
                  \~~~~~~~~/
      =============+======+======================================
      Pass 1 FZ    | FZ 2 |
      -------------+------+--------------------------------------
      Prior CGHAZ  |      |   GG-CGHAZ  (Reheated Tp > 1100°C: Grain coarsened again)
      from Pass 1  |      |   FG-CGHAZ  (Reheated Ac3 - 1100°C: Refined into tough, fine grains)
                   |      |   ICCGHAZ   (Reheated Ac1 - Ac3: M-A Necklaces along prior γ GBs) <-- CRITICAL LBZ!
                   |      |   SCRCGHAZ  (Reheated < Ac1: Tempered; toughness restored)
                   |      |   UCCGHAZ   (Unaltered CGHAZ: Not reheated by subsequent pass)
      =============+======+======================================

The Four Reheated CGHAZ Classifications

When the coarse-grained HAZ produced by Pass 1 is thermally cycled by Pass 2:

1. Unaltered CGHAZ (UCCGHAZ): Portions of the original CGHAZ that remain untouched by subsequent thermal cycles (e.g., near the weld cap). Retains the low toughness of single-pass CGHAZ.
2. Grain-Coarsened Reheated CGHAZ (GC-CGHAZ): Reheated to $T_p > 1100^\circ\text{C}$. The microstructure remains coarse-grained, preserving high hardenability.
3. Fine-Grained Reheated CGHAZ (FG-CGHAZ): Reheated into the normalizing range ($Ac_3 < T_p < 1100^\circ\text{C}$). The giant prior austenite grains are completely reaustenitized and recrystallize into exceptionally fine, equiaxed grains, fully restoring high impact toughness.
4. Subcritically Reheated CGHAZ (SCRCGHAZ): Reheated below $Ac_1$ ($T_p < 727^\circ\text{C}$). The thermal cycle acts as a localized post-weld heat treatment, tempering hard martensitic and bainitic laths, reducing dislocation density, and improving toughness.
5. Intercritically Reheated CGHAZ (ICCGHAZ) — The Critical Local Brittle Zone:
   • Thermal Condition: Reheated precisely into the intercritical window ($Ac_1 < T_p < Ac_3$, $\sim 750^\circ\text{C} \text{ to } 850^\circ\text{C}$).
   • Mechanism: Austenite nucleates preferentially at the prior austenite grain boundaries of the original coarse grains. Carbon partitions rapidly into these grain-boundary austenite films.
   • Microstructure: Upon cooling, these carbon-enriched boundary films transform into continuous, interconnected "necklaces" of brittle Martensite-Austenite (M-A) constituent along the prior austenite grain boundaries.
   • Fracture Mechanics Impact: The ICCGHAZ represents the most hazardous Local Brittle Zone (LBZ) in welded structural steel. In Crack Tip Opening Displacement (CTOD) testing of offshore structures and pressure vessels, if a fatigue pre-crack front samples as little as $5%\text{ to }15%$ of ICCGHAZ, the specimen exhibits catastrophic brittle cleavage pop-ins, dropping CTOD values from $>0.80\text{ mm}$ down to $<0.05\text{ mm}$.

Temper Bead Welding (TBW) Technique

To eliminate hazardous ICCGHAZ and UCCGHAZ regions without performing furnace Post-Weld Heat Treatment (PWHT), welding engineers employ Temper Bead Welding (governed by ASME Section IX QW-290 and NBIC Part 3):

• Layer 1 beads are deposited with controlled, small heat input to produce a shallow CGHAZ.
• Layer 2 beads are deposited with higher heat input ($1.2\text{ to }1.5\times$ Layer 1) and precise bead overlap ($50%\text{ to }60%$).
• The thermal field of Layer 2 penetrates precisely through Layer 1's CGHAZ, reheating it into the FG-CGHAZ ($Ac_3 - 1100^\circ\text{C}$) or SCRCGHAZ ($<Ac_1$) regime, refining coarse grains and tempering martensite while ensuring Layer 2's own CGHAZ resides entirely within the deposited weld metal of Layer 1.

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Special Attributes of the Base Metal: As-Cast Structure, Deformation Texture and Flame-Cut Oxide

AWS B5.16 Clause 8.3.4 requires knowledge of the special attributes of base metal — as-cast structure, deformation texture, and oxide on flame-cut surfaces. The same nominal grade behaves very differently depending on how the plate or part was produced and prepared.

As-Cast Structure

A casting solidifies into coarse columnar and equiaxed grains with interdendritic segregation of sulphur, phosphorus and carbon, and it has never been hot-worked to break that structure up. Compared with a rolled plate of the same composition, an as-cast section has:

• Lower toughness, because coarse prior grains give a low cleavage resistance.
• Higher hot-cracking susceptibility, because segregated low-melting films already sit on the grain boundaries.
• Greater section-to-section variation, since cooling rate varied with local thickness in the mould.

Welding repairs on castings therefore use higher preheat, lower heat input per pass, and in grey and ductile cast iron either a nickel-iron filler with peening and slow cooling, or a deliberate cold-welding technique with short stringers.

Deformation Texture

Hot and cold rolling elongate grains and flatten inclusions into the rolling plane. Two consequences follow:

• Mechanical anisotropy. Longitudinal properties exceed transverse properties, and both exceed through-thickness (Z-direction) properties. Charpy results differ substantially between L-T and T-L orientations from the same plate.
• Planar inclusion arrays. Manganese sulphide inclusions flatten into stringers parallel to the surface. When a restrained joint imposes through-thickness tensile strain, those stringers decohere and link up — which is exactly the lamellar tearing mechanism.

Banded ferrite-pearlite microstructure is the visible signature of this texture, and it is why Z-quality plate is specified by guaranteed through-thickness reduction of area rather than by composition.

Oxide on Flame-Cut Surfaces

An oxyfuel-cut edge is not clean metal. The cutting reaction deliberately produces iron oxide, and the kerf face is left with:

• A tightly adherent magnetite scale that does not flux away readily and causes lack of fusion, slag entrapment and porosity if welded over.
• A thin hardened and carbon-enriched rim, produced by rapid self-quenching of the cut face and, in hydrocarbon-fuel cutting, slight carbon pickup, which raises local hardness and cracking risk.

The engineering requirement is unambiguous: thermally cut edges intended for welding are ground or wire-brushed to bright metal, and structural codes additionally limit cut-surface roughness and notch depth. Plasma-cut stainless and duplex edges carry the analogous problem of a nitrogen-enriched layer when nitrogen or air is used as the plasma gas, and that layer is also removed before welding.