2.2 The Heat-Affected Zone (HAZ)
Key Takeaways
- The HAZ is base metal that was heated enough to change microstructure but did not melt
- The coarse-grained HAZ (CGHAZ) near the fusion line is the most critical — largest grains, fastest cooling, highest hardness
- CGHAZ is most susceptible to hydrogen-induced cracking due to martensite, residual stress, and hydrogen migration
- Higher heat input produces a wider but softer HAZ; lower heat input produces a narrower but harder HAZ
- Thicker base metal acts as a greater heat sink, producing faster cooling and harder HAZ
- ESW has the widest HAZ; electron beam and laser have the narrowest
2.2 The Heat-Affected Zone (HAZ)
The Heat-Affected Zone (HAZ) is the region of base metal adjacent to the weld that has been heated to temperatures high enough to cause microstructural changes but not melted. The HAZ is often the weakest link in a welded joint and is a primary focus of welding inspection.
Zones of a Welded Joint
A complete welded joint consists of several distinct regions:
| Zone | Description | Temperature Experienced |
|---|---|---|
| Weld metal (fusion zone) | Melted and resolidified filler + base metal | Above melting point (~2,800°F / 1,540°C) |
| Fusion line (fusion boundary) | Interface between melted and unmelted metal | At melting point |
| Coarse-grained HAZ (CGHAZ) | Grains grew very large from extreme heat | Well above A3 (~2,000–2,700°F) |
| Fine-grained HAZ (FGHAZ) | Grains refined by proper austenitizing temperature | Just above A3 (~1,600–2,000°F) |
| Intercritical HAZ (ICHAZ) | Partially transformed — between A1 and A3 | Between A1 and A3 (~1,333–1,670°F) |
| Subcritical HAZ | Tempered or aged but not transformed | Below A1 (< 1,333°F) |
| Unaffected base metal | No metallurgical changes | Below any significant temperature |
Why the HAZ Matters
The coarse-grained HAZ (CGHAZ) nearest the fusion line is the most critical region because:
- Large grain size — high peak temperature caused grain growth, reducing toughness
- Fastest cooling rate — closest to the weld pool, it cools rapidly, promoting martensite
- Highest residual stress — thermal contraction creates high tensile stresses
- Hydrogen migration — diffusible hydrogen migrates to the CGHAZ from the weld metal
This combination of coarse grains + martensite + residual stress + hydrogen is exactly what causes hydrogen-induced cracking (cold cracking) — the most common and dangerous cracking mechanism in structural steel welding.
Factors Affecting HAZ Size and Properties
| Factor | Effect on HAZ |
|---|---|
| Heat input | Higher heat input → larger HAZ, slower cooling, softer |
| Preheat/interpass temperature | Slows cooling rate → softer HAZ, less martensite |
| Base metal carbon content | Higher carbon → greater hardenability → more martensite in HAZ |
| Base metal thickness | Thicker material → faster heat extraction → harder HAZ |
| Welding process | SAW (high heat) → large HAZ; GTAW (low heat) → small HAZ |
| Joint geometry | More material around joint → faster heat sink → harder HAZ |
HAZ Width by Process (Approximate)
| Process | Typical HAZ Width |
|---|---|
| Electron beam / Laser | < 0.04" (1 mm) |
| GTAW | 0.08–0.20" (2–5 mm) |
| SMAW | 0.12–0.30" (3–8 mm) |
| GMAW / FCAW | 0.12–0.40" (3–10 mm) |
| SAW | 0.20–0.60" (5–15 mm) |
| ESW | 0.60–2.0"+ (15–50+ mm) |
For the Exam: The CGHAZ is the most susceptible region to hydrogen-induced cracking because it has the largest grains, highest hardness, and highest residual stress. This is why preheat and low-hydrogen practices are essential.
Which zone of the HAZ is most susceptible to hydrogen-induced cracking?
How does increasing heat input affect the HAZ?