Heat-Affected Zone in Steel Welds: Microstructure, Hardness, and Cold Cracking

The heat-affected zone (HAZ) is the region of base metal adjacent to the weld fusion boundary that has undergone microstructural changes due to the welding thermal cycle without actually melting. These changes — grain coarsening, phase transformations, carbide precipitation, M-A constituent formation, or tempering of existing martensite — profoundly affect the mechanical properties and service integrity of the weld joint. In structural steel fabrication, controlling HAZ microstructure and properties is the central challenge of welding metallurgy, because weld-related failures most commonly originate within this narrow but metallurgically complex region.

The Weld Thermal Cycle and t_8/5 Cooling Parameter

During welding, every point in the HAZ experiences a unique thermal history defined by three parameters: peak temperature (T_peak), heating rate, and cooling rate. Of these, cooling rate in the 800–500°C range is the single most influential parameter governing HAZ microstructure, because the principal solid-state phase transformations in structural steel — austenite decomposition to ferrite, bainite, or martensite — all occur within this window.

The t_8/5 cooling time (delta-t_8/5, in seconds) is defined as the time for the HAZ to cool from 800°C to 500°C. Transformation start and finish temperatures, and hence the resulting phase mixture, are a direct function of t_8/5 for a given steel composition.

Factors Controlling t_8/5

For a given joint geometry and base metal, t_8/5 increases with:

• Higher arc energy (heat input) — more energy deposited per unit length slows the cooling rate.
• Higher preheat and interpass temperature — reduces the thermal gradient driving heat extraction.
• Greater section thickness — heat flow transitions from three-dimensional (thin plate) to two-dimensional (thick plate), reducing the effective heat sink.
• Joint geometry — fillet welds on thin plate cool faster than butt welds on thick plate at the same nominal heat input.

Arc energy is calculated as:

Thermal efficiency factors: SMAW η ≈ 0.8; GMAW/FCAW η ≈ 0.8–0.9; GTAW η ≈ 0.6; SAW η ≈ 1.0. The MetallurgyZone calculators hub includes a dedicated welding heat input calculator.

HAZ Sub-Zones: Microstructure, Grain Size, and Properties

Moving outward from the fusion boundary, four structurally distinct sub-zones form in response to decreasing peak temperature. Each has characteristic grain size, phase constitution, and mechanical property profile.

1. Coarse-Grain HAZ (CGHAZ) — T_peak: 1,100–1,500°C

Immediately adjacent to the fusion line, the steel is heated well above Ac3. At temperatures above approximately 1,100°C, microalloying carbides and nitrides (Nb(C,N), TiN, VN) that normally pin austenite grain boundaries dissolve progressively. Without Zener pinning restraint, grain boundary mobility increases dramatically and prior austenite grains grow rapidly — typical CGHAZ grain sizes are 50–200 µm, compared with 10–30 µm in the parent plate.

On cooling, the coarse austenite grains present few grain boundary nucleation sites per unit volume, so transformation products are coarser and less interlocked than in fine-grained zones. The resulting microstructure depends on cooling rate (t_8/5):

• Short t_8/5 (<8 s): Fully lath martensite or mixed martensite-bainite. Hardness 380–520+ HV depending on carbon equivalent. Hard and potentially susceptible to cold cracking and hydrogen embrittlement.
• Intermediate t_8/5 (8–25 s): Lower bainite + some lath martensite. Hardness 300–400 HV. Best compromise for toughness and hardness in structural steel CGHAZ.
• Long t_8/5 (>25 s, high heat input): Upper bainite or coarse polygonal ferrite + upper bainite. Lower hardness, but significantly reduced toughness due to coarse carbide-ferrite aggregates and upper bainite sheaf morphology.

The CGHAZ is consistently the most critical region for structural integrity. It combines large grain size with a high probability of brittle transformation products, susceptibility to cold cracking, and vulnerability to local brittle zone formation under impact or low-temperature loading. Fracture mechanics assessments of weld joints for offshore or pipeline applications always focus on CGHAZ defect scenarios. See the full guide to martensite formation in steel for the crystallographic and thermodynamic basis of martensitic transformation in low-alloy steels.

2. Fine-Grain HAZ (FGHAZ) — T_peak: 900–1,100°C

Immediately outboard of the CGHAZ, peak temperatures exceed Ac3 but remain below the dissolution temperature of most carbonitride pinning particles. Complete reaustenitisation occurs, but grain growth is limited by residual microalloying precipitates. The resulting fine austenite grains (typically 10–25 µm) transform on cooling to a fine mixture of polygonal ferrite, acicular ferrite, and pearlite, or to fine bainite at higher cooling rates.

FGHAZ properties routinely exceed those of the original base metal in HSLA steels, particularly in Charpy impact toughness at low temperatures. This zone does not generally govern weld qualification requirements.

3. Intercritical HAZ (ICHAZ) — T_peak: Ac1–Ac3 (730–900°C)

The ICHAZ experiences only partial reaustenitisation. Austenite nucleates preferentially at carbide dissolution sites — grain boundary carbides and pearlite colonies — producing carbon-enriched austenite pools within a ferrite matrix. The fraction of austenite at peak temperature depends on both the peak temperature and the original microstructure.

On cooling, the carbon-enriched austenite pools transform to martensite-austenite (M-A) constituents — hard martensite islands (650–900 HV) surrounded by retained austenite. The high carbon content (often 0.3–0.6 wt%) suppresses the M_s temperature, so transformation is incomplete and retained austenite is trapped alongside fresh martensite. These brittle islands reduce impact toughness, particularly in thermomechanically processed (TMP) and normalised steels.

For offshore structural steels per DNV-ST-F101 and BS 7910, CTOD (crack tip opening displacement) testing of the ICHAZ is often specified as a mandatory qualification requirement, reflecting the significance of M-A constituent formation.

4. Subcritical HAZ (SCHAZ) — T_peak: <Ac1 (<730°C)

No phase transformation occurs in the SCHAZ. The dominant metallurgical effects are:

• Tempering of existing martensite — critical for quenched and tempered (Q&T) steels. If T_peak exceeds the original tempering temperature, over-tempering reduces yield strength and hardness. For high-strength Q&T steels (S460Q, S690QL, HY-100, BISPLATE 80), SCHAZ softening can reduce local yield strength to below the plate minimum specified value. The extent of softening increases with heat input and is a critical consideration in repair welding of high-strength steel structures.
• Carbide coarsening — in normalised and TMP steels, carbide coarsening in the SCHAZ reduces dispersion hardening and slightly reduces yield strength.
• Stress relief — some relaxation of pre-existing residual stresses.

HAZ Hardness: Measurement, Traverses, and Acceptance Criteria

HAZ hardness is the primary quality indicator measured during weld procedure qualification (PQR) and production weld acceptance testing. High HAZ hardness indicates a susceptible microstructure — hard martensite or bainite — with elevated risk of cold cracking, hydrogen embrittlement, and brittle fracture.

Hardness Traverse Method

Laboratory Vickers hardness traverses are performed on sectioned, mounted, and polished weld macrograph sections per ISO 9015-1. Standard traverse patterns include:

• Cap HAZ traverse at 1 mm below the cap surface, running from base metal through HAZ into weld metal and back.
• Mid-wall or body HAZ traverse at approximately half plate thickness.
• Root HAZ traverse at 1 mm from the root fusion line.

Test load is typically 10 kgf (HV10) for general structural work. NACE/sour service applications require HV10 specifically, as lighter loads can give falsely high readings on hard surface layers. For in-service measurement, calibrated portable Leeb/Equotip or portable Vickers instruments are used.

Acceptance Criteria by Application

Cold Cracking (Hydrogen-Induced Cracking) in the HAZ

Cold cracking — also termed hydrogen-induced cracking (HIC), hydrogen-assisted cracking (HAC), or delayed cracking — is one of the most serious weld defect modes in structural steel fabrication. It initiates in the CGHAZ of hardenable steels at temperatures typically below 150°C, and may be delayed by hours to days after welding completion, making it particularly insidious in quality assurance terms.

The Cold Cracking Triangle: Three Necessary Conditions

Cold cracking requires all three of the following conditions to be simultaneously satisfied:

1. Susceptible microstructure: Hard martensite or bainite with hardness >350 HV in the CGHAZ. Soft polygonal ferrite-pearlite microstructures do not support cold crack propagation.
2. Sufficient diffusible hydrogen: Hydrogen absorbed during welding from moisture in flux, electrode coatings, shielding gas impurities, or base metal surface contamination. Diffusible hydrogen content H_D is measured in ml/100 g deposited weld metal per ISO 3690.
3. Tensile residual stress: Welding introduces residual stress fields due to differential thermal contraction and restraint. The stress must be sufficient to drive crack propagation under the combined influence of applied loads and hydrogen-assisted reduction of cohesive strength.

Elimination of any one of these three conditions prevents cold cracking. This provides the engineering framework for prevention: control microstructure via preheat and heat input, control hydrogen via consumable selection and baking, control stress via joint design and PWHT.

Mechanism of Hydrogen-Induced Cracking

Atomic hydrogen generated at the arc enters the weld metal and diffuses rapidly through BCC martensite (diffusivity ≈ 10^-9 m²/s at room temperature, orders of magnitude faster than in FCC austenite). Hydrogen concentrates at triaxial stress points — fusion boundaries, weld toes, inclusion-matrix interfaces, and grain boundaries — where it:

• Reduces the cohesive strength of grain boundaries (decohesion mechanism)
• Promotes localised plasticity ahead of crack tips (hydrogen-enhanced localised plasticity, HELP mechanism)
• Interacts with vacancies to form platelet-type defects at grain boundaries

At sufficient local hydrogen concentration and stress intensity (K > K_ISCC), a crack initiates and propagates subcritically. Cracks are typically intergranular along prior austenite grain boundaries in the CGHAZ, but can be transgranular in hard lath martensite. See the dedicated hydrogen-induced cracking guide for detailed mechanistic treatment.

Carbon Equivalent and Cold Cracking Susceptibility

Carbon equivalent (CE) formulae aggregate the contributions of all alloying elements to HAZ hardenability, providing a single index of cold cracking susceptibility. Two principal formulae are used.

IIW Carbon Equivalent — CE(IIW)

Developed by the International Institute of Welding for C-Mn and C-Mn-Cr-Mo structural steels:

Cold Cracking Susceptibility Index — P_cm

The P_cm (or CEN) formula by Ito and Bessyo is preferred for modern microalloyed steels with carbon content below approximately 0.16 wt%, where CE(IIW) overestimates susceptibility:

Boron (B) appears in P_cm with a coefficient of 5 — reflecting its disproportionate effect on hardenability even at additions of 0.001–0.003 wt%. Boron segregates to austenite grain boundaries, blocking ferrite nucleation and dramatically raising the hardenability of the CGHAZ. Modern HSLA structural plates with boron additions require careful preheat assessment using P_cm, not CE(IIW).

Preheat Calculation — EN 1011-2 Annex C (CEN Method)

The most rigorous preheat calculation for structural steel welding is the EN 1011-2 Annex C method, which accounts for all four primary cold cracking variables:

T_p = 700 × CEN + 160 × tanh(d/35) + 62 × HD&sup0;·³5 + (53 × CEN − 32) × Q − 328

where:
  T_p  = minimum preheat temperature (°C)
  CEN  = P_cm value (carbon equivalent for preheat, wt%)
  d    = section thickness (mm)
  HD   = diffusible hydrogen level (ml/100 g deposited metal)
       = 5 for H5, 10 for H10, 15 for H15 (ISO 3690 designations)
  Q    = arc energy / heat input (kJ/mm)

This formula captures the four-variable interaction: hardenability (CEN), section thickness (thicker sections cool more slowly in 3D but have greater restraint), hydrogen content (higher H_D requires higher preheat to diffuse H out before cracking), and heat input (higher Q reduces hardness but the interaction with CEN is non-linear). The MetallurgyZone preheat calculator implements this formula with dropdown alloy selection.

Prevention of Cold Cracking: Engineering Measures

Multi-Pass Weld HAZ and the ICCGHAZ Problem

In multi-pass welding — necessary for all structural section welds above approximately 8–10 mm — successive passes thermally cycle the HAZ deposited by earlier passes. The interaction of these overlapping thermal cycles produces composite microstructures that may be significantly worse than either cycle alone.

Intercritically Reheated CGHAZ (ICCGHAZ)

The most damaging interaction occurs when the CGHAZ of an early pass is reheated to the intercritical range (Ac1–Ac3, approximately 730–900°C) by a subsequent pass. The resulting zone is termed the intercritically reheated coarse grain HAZ (ICCGHAZ), sometimes called the “local brittle zone” (LBZ) in offshore structural engineering literature.

The ICCGHAZ mechanism proceeds in two cycles:

1. First pass: Generates a coarse-grain CGHAZ with prior austenite grain size of 50–150 µm. On cooling, transforms to lath martensite, bainite, or a mixed microstructure.
2. Subsequent pass reheat to Ac1–Ac3: The coarse martensite/bainite is partially re-austenitised. Austenite nucleates preferentially at grain boundary carbide dissolution sites in the coarse structure. Because the prior austenite grain boundaries are coarse and the nucleation sites are widely spaced, the resulting austenite pools have high local carbon enrichment (0.3–0.7 wt% C vs the nominal steel carbon of 0.08–0.15 wt%). On re-cooling, these carbon-enriched pools transform to M-A constituents.

The combination of a coarse prior austenite grain matrix (from the first cycle) and brittle M-A constituent islands (from the second cycle) produces dramatically reduced toughness. Reported Charpy impact values in the ICCGHAZ range of structural offshore plate steels include 5–20 J at −40°C, compared with base metal requirements of 50–70 J at −40°C and 27 J at −60°C for demanding North Sea applications.

Characterisation by Gleeble Simulation

Because the ICCGHAZ occupies a physically narrow band (often <0.5 mm), it is impossible to extract full Charpy or CTOD specimens from actual welds without contamination from adjacent zones. Thermal cycle simulation using a Gleeble thermo-mechanical simulator reproduces the two-cycle thermal history on bulk test coupons, from which standard-size Charpy and CTOD specimens are machined.

Gleeble-simulated CTOD testing of the ICCGHAZ is mandated for welding procedure qualification in offshore structural applications per DNV-ST-F101 Annex D, NORSOK M-120, and similar standards. Typical acceptance criteria: CTOD ≥ 0.15 mm at the design temperature (often −10°C for splash zone, −20°C for submerged structure).

Industrial Significance and Quality Assurance

HAZ control is the central technical challenge in fabrication of pressure vessels, offshore structures, pipelines, bridges, lifting equipment, and naval vessels. Key industrial implications include:

Pressure Vessel Fabrication (ASME VIII, EN 13445)

Mandatory PWHT above certain shell thicknesses and material P-Numbers (e.g., P1 Group 1 steels above 38 mm per ASME VIII Div.1 UCS-56) tempers the HAZ martensite, reduces residual stress, and facilitates hydrogen escape. Hardness limits per UHX and NACE requirements govern sour service pressure vessels. The relationship between the HAZ microstructure and PWHT effectiveness is central to pressure vessel integrity management.

Pipeline Welding (API 1104, DNV-OS-F101)

Pipeline girth welds in sour service (H₂S-containing production fluids) must meet NACE 250 HV10 limits. For X70–X80 pipeline steels with thermomechanically processed microstructures, achieving 250 HV HAZ hardness while maintaining adequate weld metal strength is a major challenge, requiring careful heat input control, consumable selection, and sometimes temper bead techniques.

Offshore Structural Fabrication

Jacket structures for oil and gas platforms use heavy section S355 and S460 plates down to −60°C service temperatures. Charpy and CTOD qualification of the CGHAZ and ICCGHAZ per DNV GL rules is standard practice, alongside comprehensive hardness surveys. The bainite microstructure guide provides context on transformation products and their toughness implications.

Repair Welding of Q&T Steels

Repair welding on high-strength Q&T steels (Domex 700, BISPLATE 80, HY-100) requires particular attention to SCHAZ softening. The tempered martensite microstructure, responsible for the high base metal strength, is irreversibly over-tempered if heat input is excessive. Minimum strength requirements at repair zones must be verified by mechanical testing. Reference the quenching and tempering guide for the metallurgical basis of Q&T steel properties.