Heat Affected Zone (HAZ): Microstructure and Properties
Introduction
The Heat Affected Zone (HAZ) is the region of the base metal adjacent to the weld that did not melt but was heated to temperatures high enough to alter its microstructure. It is the most critical region of the welded joint, particularly in high-strength or heat-treated steels.
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HAZ Sub-zones (hypo-eutectoid steels)
1. Partial Fusion Zone (>1490°C)
Boundary between deposited metal and HAZ. Very coarse grain, high risk of cracking.
2. Coarse Grain HAZ (1100–1490°C)
- Austenite with pronounced grain growth
- After cooling: martensite (high CE) or upper bainite (slower cooling)
- Properties: high hardness, very low toughness → most critical HAZ sub-zone
3. Fine Grain HAZ (900–1100°C)
- Temperature slightly above A₃ → fine-grained austenite
- Properties: excellent — better than the original base metal
4. Intercritical HAZ (727–900°C)
- Partial transformation: mixed microstructure (ferrite + martensite/bainite islands)
- In Q&T steels: degradation of tempered properties (ICHAZ softening)
5. Subcritical HAZ (<727°C)
- No phase transformation
- Effect: stress relief and over-tempering (softening) in Q&T steels
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t8/5 Time — The Key Parameter
t8/5 is the cooling time between 800°C and 500°C:
- Short t8/5 (rapid cooling) → more martensite → high hardness, low toughness
- Long t8/5 (slow cooling) → more ferrite/pearlite → low hardness, higher toughness
EN 1011-2 provides formulas and nomograms to calculate t8/5 as a function of heat input, preheat temperature and thickness.
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HAZ Hardness Limits
| Material | Maximum hardness (HV10) |
|---|---|
| Structural steels (EN 1011-2) | 380 HV |
| Low-temperature steels | 350 HV |
| Martensitic stainless steels | 400 HV |
Hardness above the limit → risk of cold cracking and embrittlement.
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Measures to Control the HAZ
- Limit heat input: reduces grain growth and HAZ volume
- Preheat: increases t8/5 → avoids martensite in steels with CE > 0.40%
- PWHT: stress relief and tempering of formed martensite
- Fine-grain steels (V, Nb, Ti): inhibit austenitic grain growth → narrower HAZ
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Reference Standards
- EN 1011-2:2001 — Welding. Recommendations for welding of metallic materials. Part 2: Arc welding of ferritic steels
- EN ISO 15614-1 — Specification and qualification of welding procedures for metallic materials
- EN ISO 9015-1 — Destructive tests on welds in metallic materials. Hardness testing
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