Introduction to the Heat-Affected Zone
The heat-affected zone (HAZ) is the region of base metal adjacent to the fusion zone of a weld that has undergone microstructural changes due to the welding thermal cycle without actually melting. These changes — which may include grain coarsening, phase transformations, carbide precipitation, or tempering of existing microstructures — profoundly affect the mechanical properties and service performance of the weld joint. The HAZ is the region where weld-related failures most commonly initiate.
In structural steel fabrication, understanding and controlling HAZ microstructure and properties is the central challenge of welding metallurgy. This article provides a detailed, technical account of HAZ formation, sub-zone characteristics, hardness gradients, cold cracking susceptibility, and prevention strategies.
The Weld Thermal Cycle
During welding, each point in the HAZ experiences a unique thermal cycle characterised by a peak temperature (Tpeak), a heating rate, and a cooling rate. The cooling rate at 800–500°C (the t₈/₅ cooling time, or Δt₈/₅) is the parameter most used to characterise HAZ thermal severity, because most relevant microstructural transformations in structural steel occur in this temperature range.
Δt₈/₅ increases with:
- Higher heat input (J/mm = I × V × 60 / (1000 × v))
- Higher preheat and interpass temperature
- Larger section thickness (3D heat flow to 2D as thickness increases)
- Joint geometry (lower t₈/₅ for thick butt joints vs thin fillet welds)
HAZ Sub-Zones: Microstructure and Properties
The HAZ is not uniform — it consists of distinct sub-zones, each experiencing different peak temperatures and therefore different microstructures. Moving from the fusion boundary outward:
1. Coarse-Grain HAZ (CGHAZ) — Tpeak: 1,100–1,500°C
Immediately adjacent to the fusion boundary, the steel is heated to temperatures well above Ac3. At these temperatures, austenite grains grow rapidly — the Zener pinning from Nb/Ti/V carbides and nitrides dissolves, removing all grain growth inhibition. Prior austenite grain sizes of 50–200 µm are typical in the CGHAZ, compared with 10–30 µm in the base metal.
On cooling, the coarse austenite grains transform with fewer grain boundary nucleation sites per unit area, producing coarse transformation products. Depending on cooling rate:
- Slow cooling (high heat input): Upper bainite or polygonal ferrite + upper bainite. Relatively low hardness but reduced toughness due to coarse structure and ferrite-carbide aggregate.
- Moderate cooling: Lower bainite with some lath martensite. Hardness 300–400 HV.
- Fast cooling (low heat input, thin plate): Lath martensite or mixed martensite-bainite. Hardness 350–500+ HV depending on carbon equivalent.
The CGHAZ consistently has the lowest toughness in the HAZ — its large grain size, coarse transformation products, and susceptibility to local brittle zones (LBZ) make it the most critical region for structural integrity assessment.
2. Fine-Grain HAZ (FGHAZ) — Tpeak: 900–1,100°C
Slightly removed from the fusion boundary, peak temperature is above Ac3 but not high enough for significant grain growth (grain boundary pinning particles retain some effect). Reaustenitisation produces fine austenite grains that transform on cooling to fine-grained polygonal ferrite + pearlite or fine bainite. FGHAZ properties often exceed the base metal, particularly for microalloyed HSLA steels.
3. Intercritical HAZ (ICHAZ) — Tpeak: Ac1–Ac3 (730–900°C)
Partial reaustenitisation occurs. Only the microstructure adjacent to carbide dissolution sites transforms to austenite; the rest remains ferrite. On cooling, the transformed austenite regions — enriched in carbon — may form martensite-austenite (M-A) constituents, small islands of hard martensite surrounded by retained austenite. These M-A islands significantly degrade toughness and are a major concern in offshore structural steel welding specifications (CTOD testing of ICHAZ is often required).
4. Subcritical HAZ (SCHAZ) — Tpeak:
No phase transformation occurs. The effect is tempering of any existing martensite (in Q&T steels, this is the HAZ softening zone) and carbide coarsening in normalised or thermomechanically processed steels. For high-strength Q&T steels (e.g. S690QL, HY-100), SCHAZ softening reduces base metal strength to below the specified minimum — a critical design consideration.
HAZ Hardness: Measurement and Acceptance Criteria
HAZ hardness is measured by Vickers hardness traverses per ISO 9015-1 (or ASTM E92 for Vickers). Standard traverse patterns:
- 1 mm from the fusion boundary in the HAZ
- Traverse continuing across the weld cap HAZ at 1 mm and 2 mm below the cap surface
- Root HAZ traverse at 1 mm from the root fusion line
Key acceptance criteria:
Standard / Application
Max HAZ Hardness
Rationale
NACE MR0175 / ISO 15156 (sour service)
250 HV10
Prevent sulphide stress cracking (SSC) in H₂S environments
AWS D1.1 (structural)
325 HV (implied)
Cold cracking threshold for prequalified joints
EN ISO 15614-1 (PQR)
380 HV10 (C/low alloy)
General structural applications without special requirement
DNV GL (offshore structural)
325 HV10 (HAZ)
Fatigue and fracture mechanics requirements for safety-critical welds
Cold Cracking (Hydrogen-Induced Cracking)
Mechanism
Cold cracking (also called hydrogen-induced cracking, HIC, or delayed cracking) occurs in the CGHAZ of hardenable steels, typically at temperatures below 150°C — hours to days after welding. It requires three conditions:
- Susceptible microstructure: Martensite or hard bainite (>350 HV) in the CGHAZ
- Sufficient diffusible hydrogen: From moisture in electrodes, flux, or base metal surface
- Tensile residual stress: Sufficient restraint to prevent stress relaxation
Hydrogen diffuses rapidly in BCC martensite, concentrating at stress-raising features (crack tips, weld toes, fusion boundary) and grain boundaries. At sufficient concentration, hydrogen reduces the cohesive strength of grain boundaries and crack surfaces, initiating intergranular or transgranular fracture under the applied and residual stress field.
Carbon Equivalent and Cold Cracking Susceptibility
The International Institute of Welding (IIW) carbon equivalent (CE) predicts cold cracking susceptibility:
CE_IIW = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
The CEN (Pcm) formula is preferred for modern microalloyed steels with CE < 0.35%:
Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B
Interpretation of CE_IIW:
- CE < 0.35%: Generally weldable without preheat (thin sections)
- CE 0.35–0.45%: Low risk; preheat may be required for thick sections or high restraint
- CE 0.45–0.55%: Moderate risk; preheat required for most applications
- CE >0.55%: High risk; significant preheat; low-hydrogen consumables essential
Preheat Calculation — CEN Method (EN 1011-2 Annex C)
T_p = 700 × CEN + 160 × tanh(d/35) + 62 × HD^0.35 + (53 × CEN − 32) × Q − 328
where T_p is minimum preheat (°C), d is section thickness (mm), HD is diffusible hydrogen level (ml/100g), and Q is heat input (kJ/mm). This formula accounts for the four critical variables: hardenability (CEN), section size, hydrogen level, and heat input.
Prevention of Cold Cracking
Measure
Effect
Notes
Low-hydrogen consumables (H4, H5)
Reduces HD source
ISO 3690 testing; bake electrodes before use
Preheat (100–250°C)
Slows cooling → softer HAZ; aids H diffusion
Maintain throughout welding and immediate PWHT
Controlled heat input
Optimise Δt₈/₅ to produce bainite rather than martensite
Too high = grain coarsen; too low = martensite. Optimum window exists.
Buttering
Deposits compatible intermediate layer, reducing CE in HAZ
Used for dissimilar metal welds
Post-heat (250–350°C, 1 hr)
Drives out diffusible H before cracking initiates
Applied immediately after welding before cooling
PWHT (600–650°C)
Tempers martensite; eliminates residual stress; removes H
Required for pressure vessel welds above certain thickness
Multi-Pass Weld HAZ: The ICCGHAZ Problem
In multi-pass welds, each subsequent pass re-heats the HAZ of previous passes. When the CGHAZ of an early pass is reheated to the intercritical range (Ac1–Ac3) by a subsequent pass, the resulting Intercritically Reheated CGHAZ (ICCGHAZ) — sometimes called the “local brittle zone” — can exhibit dramatically reduced Charpy toughness (as low as 5–15 J at -40°C) compared to adjacent regions.
The ICCGHAZ mechanism: The coarse martensite-bainite of the CGHAZ is partially re-austenitised at high-carbon regions (carbide-ferrite boundaries). These carbon-enriched austenite pools transform to martensite-austenite (M-A) constituents on the second cooling cycle, producing brittle islands within an otherwise tough microstructure. Thermal cycle simulation (Gleeble testing) and CTOD fracture toughness testing are used to characterise and qualify welding procedures for safety-critical offshore applications.
Frequently Asked Questions
Q: Why does NACE limit HAZ hardness to 250 HV for sour service?
A: Sulphide stress cracking (SSC) in H₂S environments is a form of hydrogen embrittlement driven by cathodic hydrogen absorption from the H₂S corrosion reaction. Susceptibility increases sharply above 22 HRC (≈250 HV). The 250 HV limit in NACE MR0175/ISO 15156 is a conservative threshold below which SSC risk is considered negligible for most service conditions.
Q: How do you measure HAZ hardness in a completed structure?
A: Portable hardness testers (Equotip/Leeb, portable Vickers) are used for in-situ measurement. For weld procedure qualification (PQR), laboratory Vickers hardness traverses on sectioned, mounted, and polished weld macro-sections are performed per ISO 9015-1.
Conclusion
The HAZ is the region of a weld joint where microstructural changes imposed by the welding thermal cycle most directly affect structural integrity. The CGHAZ is the critical sub-zone — its combination of coarse grain, hard transformation products, and susceptibility to cold cracking and low-temperature brittle fracture makes it the focus of qualification testing and process control. Preheat, controlled heat input, low-hydrogen consumables, and PWHT are the engineer’s tools for managing HAZ risk. See also: Welding Austenitic Stainless Steel and Quenching and Martensite Formation.
References
- Blodgett, O.W. and Miller, D.K., Fabricators’ and Erectors’ Guide to Welded Steel Construction. Lincoln Electric, 1999.
- TWI (The Welding Institute), Guidelines for the Avoidance of Hydrogen Cracking in Structural Steel Welding. TWI, 2004.
- EN ISO 15614-1: Specification and Qualification of Welding Procedures for Metallic Materials. ISO, 2017.
- NACE MR0175 / ISO 15156: Petroleum and Natural Gas Industries — Materials for Use in H₂S-Containing Environments.
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No phase transformation occurs. The effect is tempering of any existing martensite (in Q&T steels, this is the HAZ softening zone) and carbide coarsening in normalised or thermomechanically processed steels. For high-strength Q&T steels (e.g. S690QL, HY-100), SCHAZ softening reduces base metal strength to below the specified minimum — a critical design consideration.
HAZ Hardness: Measurement and Acceptance Criteria
HAZ hardness is measured by Vickers hardness traverses per ISO 9015-1 (or ASTM E92 for Vickers). Standard traverse patterns:
- 1 mm from the fusion boundary in the HAZ
- Traverse continuing across the weld cap HAZ at 1 mm and 2 mm below the cap surface
- Root HAZ traverse at 1 mm from the root fusion line
Key acceptance criteria:
| Standard / Application | Max HAZ Hardness | Rationale |
|---|---|---|
| NACE MR0175 / ISO 15156 (sour service) | 250 HV10 | Prevent sulphide stress cracking (SSC) in H₂S environments |
| AWS D1.1 (structural) | 325 HV (implied) | Cold cracking threshold for prequalified joints |
| EN ISO 15614-1 (PQR) | 380 HV10 (C/low alloy) | General structural applications without special requirement |
| DNV GL (offshore structural) | 325 HV10 (HAZ) | Fatigue and fracture mechanics requirements for safety-critical welds |
Cold Cracking (Hydrogen-Induced Cracking)
Mechanism
Cold cracking (also called hydrogen-induced cracking, HIC, or delayed cracking) occurs in the CGHAZ of hardenable steels, typically at temperatures below 150°C — hours to days after welding. It requires three conditions:
- Susceptible microstructure: Martensite or hard bainite (>350 HV) in the CGHAZ
- Sufficient diffusible hydrogen: From moisture in electrodes, flux, or base metal surface
- Tensile residual stress: Sufficient restraint to prevent stress relaxation
Hydrogen diffuses rapidly in BCC martensite, concentrating at stress-raising features (crack tips, weld toes, fusion boundary) and grain boundaries. At sufficient concentration, hydrogen reduces the cohesive strength of grain boundaries and crack surfaces, initiating intergranular or transgranular fracture under the applied and residual stress field.
Carbon Equivalent and Cold Cracking Susceptibility
The International Institute of Welding (IIW) carbon equivalent (CE) predicts cold cracking susceptibility:
CE_IIW = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
The CEN (Pcm) formula is preferred for modern microalloyed steels with CE < 0.35%:
Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B
Interpretation of CE_IIW:
- CE < 0.35%: Generally weldable without preheat (thin sections)
- CE 0.35–0.45%: Low risk; preheat may be required for thick sections or high restraint
- CE 0.45–0.55%: Moderate risk; preheat required for most applications
- CE >0.55%: High risk; significant preheat; low-hydrogen consumables essential
Preheat Calculation — CEN Method (EN 1011-2 Annex C)
T_p = 700 × CEN + 160 × tanh(d/35) + 62 × HD^0.35 + (53 × CEN − 32) × Q − 328
where T_p is minimum preheat (°C), d is section thickness (mm), HD is diffusible hydrogen level (ml/100g), and Q is heat input (kJ/mm). This formula accounts for the four critical variables: hardenability (CEN), section size, hydrogen level, and heat input.
Prevention of Cold Cracking
| Measure | Effect | Notes |
|---|---|---|
| Low-hydrogen consumables (H4, H5) | Reduces HD source | ISO 3690 testing; bake electrodes before use |
| Preheat (100–250°C) | Slows cooling → softer HAZ; aids H diffusion | Maintain throughout welding and immediate PWHT |
| Controlled heat input | Optimise Δt₈/₅ to produce bainite rather than martensite | Too high = grain coarsen; too low = martensite. Optimum window exists. |
| Buttering | Deposits compatible intermediate layer, reducing CE in HAZ | Used for dissimilar metal welds |
| Post-heat (250–350°C, 1 hr) | Drives out diffusible H before cracking initiates | Applied immediately after welding before cooling |
| PWHT (600–650°C) | Tempers martensite; eliminates residual stress; removes H | Required for pressure vessel welds above certain thickness |
Multi-Pass Weld HAZ: The ICCGHAZ Problem
In multi-pass welds, each subsequent pass re-heats the HAZ of previous passes. When the CGHAZ of an early pass is reheated to the intercritical range (Ac1–Ac3) by a subsequent pass, the resulting Intercritically Reheated CGHAZ (ICCGHAZ) — sometimes called the “local brittle zone” — can exhibit dramatically reduced Charpy toughness (as low as 5–15 J at -40°C) compared to adjacent regions.
The ICCGHAZ mechanism: The coarse martensite-bainite of the CGHAZ is partially re-austenitised at high-carbon regions (carbide-ferrite boundaries). These carbon-enriched austenite pools transform to martensite-austenite (M-A) constituents on the second cooling cycle, producing brittle islands within an otherwise tough microstructure. Thermal cycle simulation (Gleeble testing) and CTOD fracture toughness testing are used to characterise and qualify welding procedures for safety-critical offshore applications.
Frequently Asked Questions
Q: Why does NACE limit HAZ hardness to 250 HV for sour service?
A: Sulphide stress cracking (SSC) in H₂S environments is a form of hydrogen embrittlement driven by cathodic hydrogen absorption from the H₂S corrosion reaction. Susceptibility increases sharply above 22 HRC (≈250 HV). The 250 HV limit in NACE MR0175/ISO 15156 is a conservative threshold below which SSC risk is considered negligible for most service conditions.
Q: How do you measure HAZ hardness in a completed structure?
A: Portable hardness testers (Equotip/Leeb, portable Vickers) are used for in-situ measurement. For weld procedure qualification (PQR), laboratory Vickers hardness traverses on sectioned, mounted, and polished weld macro-sections are performed per ISO 9015-1.
Conclusion
The HAZ is the region of a weld joint where microstructural changes imposed by the welding thermal cycle most directly affect structural integrity. The CGHAZ is the critical sub-zone — its combination of coarse grain, hard transformation products, and susceptibility to cold cracking and low-temperature brittle fracture makes it the focus of qualification testing and process control. Preheat, controlled heat input, low-hydrogen consumables, and PWHT are the engineer’s tools for managing HAZ risk. See also: Welding Austenitic Stainless Steel and Quenching and Martensite Formation.
References
- Blodgett, O.W. and Miller, D.K., Fabricators’ and Erectors’ Guide to Welded Steel Construction. Lincoln Electric, 1999.
- TWI (The Welding Institute), Guidelines for the Avoidance of Hydrogen Cracking in Structural Steel Welding. TWI, 2004.
- EN ISO 15614-1: Specification and Qualification of Welding Procedures for Metallic Materials. ISO, 2017.
- NACE MR0175 / ISO 15156: Petroleum and Natural Gas Industries — Materials for Use in H₂S-Containing Environments.
📚 RELATED ARTICLES & TOOLS
🛒 RECOMMENDED BOOKS & TOOLS
As an Amazon Associate, MetallurgyZone earns from qualifying purchases. This helps us keep the content free.
📗AWS Welding Handbook Vol. 1 – Welding Science & Technology (10th Ed.)View on Amazon ↗🔧Bridge Cam Weld Gauge – AWS/EN Weld InspectionView on Amazon ↗🔧Tempilstik Temperature Indicating Sticks – Welding Preheat VerificationView on Amazon ↗🛡️Lincoln Electric VIKING 3350 Auto-Darkening Welding HelmetView on Amazon ↗