10.7 HAZ Hardness Traverses, Toughness Gradients & Peak-Temperature Control

Hardness Traverses & Fracture Toughness Gradients

Microhardness testing across a weld cross-section provides a direct mechanical fingerprint of the microstructural transformations.

                         VICKERS HARDNESS PROFILE ACROSS WELDMENT

      Hardness (HV10) ^
                      |                  CGHAZ Peak Hardness
                      |                       /\
                      |                      /  \
             350 HV --+---------------------/----+----------------- AWS D1.1 HICC Limit
                      |                    /|    |\
             250 HV --+-------------------/-+----+-\--------------- NACE MR0175 Sour Limit
                      |                  /  |    |  \
             Base HV -+-----------------'   |    |   '------------ Base Metal Baseline
                      |       FZ      |CGHAZ|ICHAZ| FGHAZ| SCHAZ |
                      +------------------------------------------->
                                  Distance from Weld Centerline

Industrial Hardness Ceilings

Industry fabrication codes mandate strict upper limits on HAZ peak hardness:

• NACE MR0175 / ISO 15156 (Sour Oil & Gas Service): Maximum allowable HAZ hardness is $250\text{ HV}$ ($22\text{ HRC}$). In wet sour environments containing hydrogen sulfide ($\text{H}_2\text{S}$), atomic hydrogen entry triggers catastrophic Sulfide Stress Cracking (SSC) along hard martensitic boundaries if hardness exceeds $250\text{ HV}$.
• AWS D1.1 Structural Welding Code - Steel: Recommends limiting peak HAZ hardness to $350\text{ HV}$ (or $380\text{ HV}$ for high-strength grades) to prevent fabrication hydrogen-induced cold cracking (delayed underbead cracking).

Carbon Equivalent Formulations

Hardenability and cracking susceptibility are estimated via empirical carbon equivalent equations:

1. IIW Carbon Equivalent ($CE_{\text{IIW}}$) — for conventional steels ($C > 0.18%$): $$CE_{\text{IIW}} = \text{C} + \frac{\text{Mn}}{6} + \frac{\text{Cr} + \text{Mo} + \text{V}}{5} + \frac{\text{Ni} + \text{Cu}}{15}$$ Steels with $CE_{\text{IIW}} > 0.40%$ require preheating; steels with $CE_{\text{IIW}} > 0.45%$ are highly prone to HAZ martensite and require strict preheat and interpass controls.
2. Ito-Bessyo Cracking Parameter ($P_{cm}$) — for modern low-carbon TMCP steels ($C < 0.16%$): $$P_{cm} = \text{C} + \frac{\text{Si}}{30} + \frac{\text{Mn} + \text{Cu} + \text{Cr}}{20} + \frac{\text{Ni}}{60} + \frac{\text{Mo}}{15} + \frac{\text{V}}{10} + 5\text{B}$$

Modeling HAZ Peak Temperature: The Rosenthal Equation

For two-dimensional heat conduction in a plate of thickness $h$ (thin plate or full-penetration weld), Rosenthal's formulation relates peak temperature ($T_p$) at lateral distance $y$ from the weld fusion line to welding heat input:

$$\frac{1}{T_p - T_0} = \frac{4.13 \cdot \rho c_p \cdot h \cdot y}{H_{\text{net}}} + \frac{1}{T_m - T_0}$$

where:

• $T_0$: Workpiece initial preheat temperature ($^\circ\text{C}$).
• $T_m$: Melting temperature of the steel (typically $1500^\circ\text{C}$).
• $\rho c_p$: Volumetric heat capacity of steel ($\approx 0.0044\text{ J/mm}^3\cdot^\circ\text{C}$).
• $h$: Plate thickness ($\text{mm}$).
• $y$: Perpendicular distance from the fusion boundary ($\text{mm}$).
• $H_{\text{net}} = \eta \cdot \frac{V \cdot I}{v}$: Net linear heat input ($\text{J/mm}$), with arc thermal efficiency $\eta$.

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Comprehensive Worked Numerical Example: HAZ Peak Temperature & Hardness Control

Problem Statement

A welding engineer is preparing a Welding Procedure Specification (WPS) for joining $12.0\text{ mm}$ thick structural steel plate for a sour service pipeline manifold. The project is governed by NACE MR0175, mandating that HAZ hardness must not exceed $250\text{ HV}$.

Welding Parameters & Material Data:

• Steel Composition: $0.12%\text{ C}$, $1.30%\text{ Mn}$, $0.25%\text{ Si}$, $0.015%\text{ P}$, $0.005%\text{ S}$, $0.20%\text{ Cr}$, $0.15%\text{ Mo}$, $0.30%\text{ Ni}$, $0.03%\text{ V}$.
• Melting Temperature: $T_m = 1500^\circ\text{C}$.
• Transformation Temperatures: $Ac_3 = 880^\circ\text{C}$, $Ac_1 = 727^\circ\text{C}$. Coarse-grained boundary threshold: $T_{\text{coarsen}} = 1150^\circ\text{C}$.
• Volumetric Heat Capacity: $\rho c_p = 0.0044\text{ J/mm}^3\cdot^\circ\text{C}$.
• Initial Preheat Temperature: $T_0 = 20^\circ\text{C}$.
• Welding Process: Mechanized GMAW, $V = 28\text{ V}$, $I = 280\text{ A}$, travel speed $v = 5.0\text{ mm/s}$, arc efficiency $\eta = 0.80$.

Calculate:

1. The net linear heat input $H_{\text{net}}$ in $\text{J/mm}$ and $\text{kJ/mm}$.
2. The peak temperatures ($T_p$) at lateral distances of $y = 1.2\text{ mm}$ and $y = 3.5\text{ mm}$ from the fusion line, and identify which HAZ sub-zone each location represents.
3. The IIW Carbon Equivalent ($CE_{\text{IIW}}$) of the base steel.
4. Evaluate compliance with NACE MR0175. If the fast cooling rate produces a CGHAZ hardness of $310\text{ HV}$ under $T_0 = 20^\circ\text{C}$, determine the engineering remediation required to achieve $\le 250\text{ HV}$.

Step-by-Step Solution

Step 1: Calculate Net Linear Heat Input ($H_{\text{net}}$)

$$P_{\text{arc}} = V \cdot I = 28\text{ V} \times 280\text{ A} = 7840\text{ W} = 7840\text{ J/s}$$ $$H_{\text{gross}} = \frac{P_{\text{arc}}}{v} = \frac{7840\text{ J/s}}{5.0\text{ mm/s}} = 1568\text{ J/mm} = 1.568\text{ kJ/mm}$$ $$H_{\text{net}} = \eta \cdot H_{\text{gross}} = 0.80 \times 1568\text{ J/mm} = 1254.4\text{ J/mm} = 1.254\text{ kJ/mm}$$

Step 2: Calculate Peak Temperatures ($T_p$) via Rosenthal 2D Equation The 2D Rosenthal peak temperature relation is:

$$\frac{1}{T_p - T_0} = \frac{4.13 \cdot \rho c_p \cdot h \cdot y}{H_{\text{net}}} + \frac{1}{T_m - T_0}$$

Calculate common thermal constant terms ($T_0 = 20^\circ\text{C}$, $T_m = 1500^\circ\text{C}$):

$$\frac{1}{T_m - T_0} = \frac{1}{1500 - 20} = \frac{1}{1480} = 6.7568 \times 10^{-4}\ ^\circ\text{C}^{-1}$$ $$\text{Slope Factor } S = \frac{4.13 \cdot \rho c_p \cdot h}{H_{\text{net}}} = \frac{4.13 \times 0.0044\text{ J/mm}^3\cdot^\circ\text{C} \times 12.0\text{ mm}}{1254.4\text{ J/mm}} = \frac{0.218064}{1254.4} = 1.7384 \times 10^{-4}\ ^\circ\text{C}^{-1}\text{mm}^{-1}$$

• At Distance $y = 1.2\text{ mm}$:

  $$\frac{1}{T_p - 20} = (1.7384 \times 10^{-4} \times 1.2) + 6.7568 \times 10^{-4} = 2.0861 \times 10^{-4} + 6.7568 \times 10^{-4} = 8.8429 \times 10^{-4}\ ^\circ\text{C}^{-1}$$ $$T_p - 20 = \frac{1}{8.8429 \times 10^{-4}} = 1130.85^\circ\text{C} \implies T_p = 1150.85^\circ\text{C} \approx 1151^\circ\text{C}$$

  Sub-Zone Classification: Because $T_p \ge 1150^\circ\text{C}$, this location lies at the boundary of the Coarse-Grained HAZ (CGHAZ), where grain-boundary precipitates dissolve and rapid austenite grain coarsening occurs.

• At Distance $y = 3.5\text{ mm}$:

  $$\frac{1}{T_p - 20} = (1.7384 \times 10^{-4} \times 3.5) + 6.7568 \times 10^{-4} = 6.0844 \times 10^{-4} + 6.7568 \times 10^{-4} = 1.2841 \times 10^{-3}\ ^\circ\text{C}^{-1}$$ $$T_p - 20 = \frac{1}{1.2841 \times 10^{-3}} = 778.74^\circ\text{C} \implies T_p = 798.74^\circ\text{C} \approx 799^\circ\text{C}$$

  Sub-Zone Classification: Because $Ac_1 (727^\circ\text{C}) < T_p < Ac_3 (880^\circ\text{C})$, this location lies directly within the Intercritical HAZ (ICHAZ), undergoing partial austenitization with intense carbon partitioning into isolated austenite pools that form brittle M-A constituent upon cooling.

Step 3: Calculate Carbon Equivalent ($CE_{\text{IIW}}$)

$$CE_{\text{IIW}} = \text{C} + \frac{\text{Mn}}{6} + \frac{\text{Cr} + \text{Mo} + \text{V}}{5} + \frac{\text{Ni} + \text{Cu}}{15}$$

Substitute alloy percentages:

$$CE_{\text{IIW}} = 0.12 + \frac{1.30}{6} + \frac{0.20 + 0.15 + 0.03}{5} + \frac{0.30 + 0}{15}$$ $$CE_{\text{IIW}} = 0.12 + 0.2167 + 0.0760 + 0.0200 = 0.4327 \implies 0.43\%$$

Step 4: Evaluate Compliance and Formulate Engineering Remediation

• Compliance Assessment: With $CE_{\text{IIW}} = 0.43%$ and an unheated plate ($T_0 = 20^\circ\text{C}$), the fast cooling rate yields a CGHAZ hardness of $310\text{ HV}$, substantially violating the NACE MR0175 ceiling of $250\text{ HV}$ (non-conformance for sour service).
• Engineering Remediation: To retard the cooling rate (extend $\Delta t_{8/5}$) and allow austenite to transform into softer ferrite-pearlite/bainite rather than hard lath martensite:
  1. Apply Preheat: Introduce a minimum preheat of $T_0 = 150^\circ\text{C}$ to $175^\circ\text{C}$ per EN 1011-2 / AWS D1.1 Annex H. This reduces thermal conductivity drain, extending $\Delta t_{8/5}$ above $15-20\text{ s}$ and dropping CGHAZ peak hardness below $250\text{ HV}$.
  2. Post-Weld Heat Treatment (PWHT): If preheat alone is insufficient, specify furnace PWHT at $600^\circ\text{C} \pm 15^\circ\text{C}$ for $1\text{ hour}$ per inch of thickness to temper martensite and reduce residual stresses.

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Real-World Engineering Scenarios & Exam Pitfalls

Industrial Case: Brittle Fracture in Offshore Riser Flange Welds (ICCGHAZ Failure)

During fabrication qualification testing of heavy-wall ($50\text{ mm}$) forged high-strength steel riser flanges for an offshore platform, the welding procedure underwent full-scale Crack Tip Opening Displacement (CTOD) testing at $-10^\circ\text{C}$ per BS 7448. While weld metal and fine-grained HAZ specimens achieved outstanding CTOD values ($>0.95\text{ mm}$), three consecutive specimens notched precisely into the HAZ failed catastrophically via brittle pop-ins at CTOD values of $\delta = 0.032\text{ mm}$, failing the project threshold of $\delta_{\text{crit}} \ge 0.25\text{ mm}$.

Metallographic Sectioning: Metallographic examination of the fracture surfaces revealed that the fatigue pre-crack had intersected the Intercritically Reheated Coarse-Grained HAZ (ICCGHAZ) over $18%$ of the specimen thickness. High-resolution field-emission SEM revealed continuous necklaces of high-carbon Martensite-Austenite (M-A) constituent along the prior austenite grain boundaries ($>150\ \mu\text{m}$ grain diameter). Micro-cleavage cracks had initiated at the brittle M-A/ferrite boundary interfaces under high triaxial crack-tip constraint, propagating dynamically across the coarse grain facets without plastic deformation.

Engineering Remediation:

1. Implemented a strict Temper Bead Welding (TBW) protocol: layer 2 heat input was increased by $30%$ relative to layer 1, and bead overlap was fixed at $55%$, ensuring that the second pass thermally cycled the prior CGHAZ into the normalizing range ($950-1050^\circ\text{C}$ FG-CGHAZ), completely grain-refining the coarse prior structure.
2. Reduced maximum interpass temperature from $250^\circ\text{C}$ to $175^\circ\text{C}$ to accelerate cooling through the intercritical transformation window, minimizing the volume fraction and necklace connectivity of M-A islands.
3. Re-qualification CTOD tests exceeded $0.65\text{ mm}$ across all valid test specimens.

Common Exam Traps

Exam Trap 1: Confusing Single-Pass ICHAZ with Multi-Pass ICCGHAZ Certification exam questions often probe candidate understanding of Local Brittle Zones (LBZs). While the single-pass ICHAZ contains brittle M-A constituent, its base metal grain size is small (fine ferrite), limiting the cleavage crack facet size. The ICCGHAZ (Intercritically Reheated CGHAZ) is vastly more dangerous because M-A constituent necklaces precipitate along giant prior austenite grain boundaries (ASTM 1–3). This provides huge, continuous cleavage planes, creating the lowest fracture toughness in any steel weldment.

Exam Trap 2: Assuming the Fine-Grained HAZ (FGHAZ) Has Low Toughness Questions frequently ask which HAZ region has the lowest toughness, presenting FGHAZ, CGHAZ, ICHAZ, and SCHAZ as options. Candidates often incorrectly guess the FGHAZ because it is adjacent to the coarse zone. The FGHAZ actually possesses the highest toughness and ductility of all HAZ regions because the peak temperature ($Ac_3$ to $1100^\circ\text{C}$) achieves complete grain refinement into ultra-fine equiaxed grains.

Exam Trap 3: Believing Post-Weld Heat Treatment (PWHT) Always Restores Toughness to ICCGHAZ In conventional C-Mn steels, PWHT tempers martensite and improves toughness. However, in steels containing microalloying additions of vanadium, niobium, or titanium, subcritical PWHT at $580-620^\circ\text{C}$ can trigger secondary precipitation of ultra-fine coherent carbonitrides (e.g., $\text{V}_4\text{C}_3$) in the coarse grains, causing reheat embrittlement and further degrading CTOD toughness rather than improving it.