5.2 Structure of the Welded Joint & HAZ Zones

5.2 Structure of the Welded Joint & HAZ Zones

Quick Answer: A fusion weld is a gradient of thermal cycles, not a single material. Inspectors must distinguish weld metal, fusion line, HAZ sub-zones (CGHAZ, FGHAZ, ICHAZ, SCHAZ), and base metal. Solidification starts at the fusion boundary; the coarse-grain HAZ is often the toughness and hardness concern. Hardness and toughness in the HAZ link directly to cracking risk and acceptance testing.

Module WT2.4 treats the structure of the welded joint so inspectors can interpret metallographs, hardness traverses, and procedure qualification results. Production inspection rarely includes a full metallurgical lab on every joint, but the conceptual map is essential when reviewing WPQR hardness surveys, Charpy locations, or cracking investigations.

Regions of a Fusion Welded Joint

For a single-pass fusion weld on steel, picture a cross-section from weld centreline outward:

Dilution is the fraction of weld metal that came from melted base metal. High dilution changes deposit chemistry (for example carbon pick-up from high-C base metal into a low-C consumable) and can alter hardenability or corrosion resistance. Inspectors see dilution effects when cladding, buttering, or joining dissimilar thicknesses/grades with a single consumable.

HAZ Sub-Zones Overview

The HAZ is subdivided by peak temperature relative to the steel’s critical temperatures. Names and exact temperature bands vary slightly by textbook, but IWI-S candidates should know the following working map for carbon and low-alloy steels:

CGHAZ — Coarse-Grain HAZ

• Peak temperature well above A₃ (often approaching solidus near the fusion line)
• Austenite grains grow rapidly; carbides dissolve
• On cooling, the coarse prior-austenite grain size can produce hard transformation products (bainite/martensite) and lower toughness
• Often the zone of maximum hardness adjacent to the fusion line in hardenable steels
• Multipass welds may reheat CGHAZ regions into refined or embrittled substructures (local brittle zones)—important in thick-section fracture discussions

FGHAZ — Fine-Grain HAZ

• Peak temperature above A₃ but lower than CGHAZ, or with less time for grain growth
• Austenite grains remain finer; subsequent cooling often yields better toughness than CGHAZ
• Still fully transformed; properties differ from original base metal

ICHAZ — Intercritical HAZ

• Peak temperature between A₁ and A₃
• Partial austenitisation: mixture of austenite and ferrite at peak temperature
• On cooling, the austenite islands can transform to hard constituents while ferrite remains, creating local hard spots and toughness variability
• Relevant when discussing dual-phase-like structures in the HAZ of some modern steels

SCHAZ — Subcritical HAZ

• Peak temperature below A₁
• No bulk austenitisation; may still temper, age, precipitate, or relieve/rearrange residual stresses depending on alloy and time
• Softening of quenched-and-tempered or cold-worked base metals can occur here
• Hydrogen diffusion and residual stress fields still matter even without full transformation

Ordering toward the fusion line (typical single-pass picture): unaffected BM → SCHAZ → ICHAZ → FGHAZ → CGHAZ → fusion line → weld metal. Real multipass joints overwrite this map repeatedly.

Solidification of the Weld Pool

When the arc moves on, the weld pool cools and solidifies. Key points for inspectors:

1. Heterogeneous nucleation at the fusion boundary — solid grows epitaxially from partially melted base-metal grains; crystallographic orientation continues into the weld metal.
2. Solidification modes — planar, cellular, cellular-dendritic, or equiaxed dendritic depending on temperature gradient (G) and growth rate (R). High G/R near the fusion line often favours columnar structures toward the centreline.
3. Segregation — solute rejected into liquid can enrich last-to-solidify liquid films; centreline segregation contributes to solidification cracking risk when restraint and wide freezing range coincide.
4. Shape of the pool — process, travel speed, and heat input change pool geometry; teardrop pools at high speed can promote centreline cracking tendencies.
5. Filler addition — consumable chemistry and deoxidisers influence inclusion content, toughness, and solidification behaviour.

Inspectors do not select G and R on the shop floor, but they recognise that excessive travel speed, incorrect parameters, or unsuitable consumables can produce solidification-related imperfections (hot cracks, centreline porosity associations, coarse columnar structures linked to toughness issues).

Grain Growth and Reheating Effects

Grain growth in austenite is thermally activated: higher peak temperature and longer time above grain-coarsening temperatures produce larger grains. Large prior-austenite grains:

• Reduce toughness after transformation
• Can increase hardenability slightly (transformation kinetics)
• Make hydrogen-assisted cracks more continuous along prior-austenite boundaries in severe cases

Heat input (related to arc energy and process efficiency) is the production lever that changes HAZ width and peak-temperature time. High heat input widens the HAZ and can coarsen CGHAZ more; very low heat input can increase cooling rate and hardness. Procedure windows balance these extremes.

In multipass welding, each bead heat-treats previous beads and HAZ regions:

• Reheating CGHAZ into the intercritical range can create local brittle zones
• Tempering of martensite in previous passes can reduce hardness and improve toughness
• Interpass temperature control keeps successive thermal cycles within qualified limits

WPQR hardness maps and Charpy notch locations (weld metal, fusion line, HAZ offsets) exist because properties vary sharply over millimetres.

Why HAZ Hardness and Toughness Matter for Inspection

Hardness

Elevated HAZ hardness is a practical indicator of hard microstructures (often martensite or lower bainite) that:

• Increase susceptibility to hydrogen cold cracking when diffusible hydrogen and tensile residual stress are present
• May violate contractual or code hardness limits (for example sour service or client specifications)
• Signal that preheat, heat input, or consumable hydrogen control may have been inadequate relative to carbon equivalent and thickness

Inspectors may witness hardness testing on production joints or production test plates when specified. They compare results to WPS/contract limits and investigate high readings with NDT for cracks and process review—not by “grinding hardness away” as a silent fix.

Toughness

Toughness (commonly Charpy V-notch energy at a design temperature) measures resistance to brittle fracture initiation/propagation under impact-like loading. CGHAZ and certain reheated HAZ regions can fall below base-metal toughness. Critical structures (pressure equipment, low-temperature service, fracture-critical bridges) therefore require WPQR toughness tests with notches in weld metal and HAZ locations defined by the application standard.

Cracking risk linkage

Three classic factors for hydrogen-assisted cold cracking in ferritic steel welds are often summarised as:

1. Susceptible microstructure (hard HAZ/weld metal) — hardenability + cooling rate
2. Hydrogen — from moisture, residual oil, cellulosic electrodes, poor baking of basic electrodes, humid conditions
3. Tensile stress / restraint — joint design, fit-up, thickness, residual stress

Section 5.3 connects carbon equivalent and material groups to preheat decisions that reduce the first factor’s severity. Chapter 6 expands cracking phenomena. For this section, remember: the HAZ is where many cold cracks initiate, often just outside the fusion line in the coarse-grain region under high restraint.

Practical Inspection Scenarios

• Hardness traverse on a WPQR coupon: peaks near fusion line in CGHAZ; confirm test locations and indent spacing match the standard’s requirements.
• Crack found by MT after cooling: map location—HAZ toe crack vs weld metal crater crack tells different metallurgical stories.
• High heat input SAW on thick plate: wide HAZ, possible toughness loss in CGHAZ; verify heat-input records against WPS max.
• Low heat input root pass on high-CE steel without preheat: rapid cool, high hardness, hydrogen crack risk—stop and escalate if parameters violate WPS.

Exam Focus for IWI-S

Expect questions that ask you to order HAZ sub-zones by peak temperature, identify which zone is most associated with grain coarsening and toughness loss, explain epitaxial solidification from the fusion boundary, or state why hardness testing near the fusion line is relevant to cold-cracking risk.

Exam tip: CGHAZ = highest peak temperature in the HAZ, coarsest grains, often highest hardness and lowest toughness adjacent to the fusion line. SCHAZ = below A₁, no bulk austenitisation.