Heat-Affected Zone (HAZ)
The Heat-Affected Zone (HAZ) is the region of the base metal that, without having reached melting, undergoes irreversible microstructural changes as a result of the thermal cycle imposed by the welding process. Its extent and characteristics largely determine the mechanical integrity of the welded joint.
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1. The Welding Thermal Cycle
Every point in the HAZ is subject to a specific thermal cycle — rapid heating followed by cooling — whose intensity depends on the distance from the fusion line and on the process parameters.
The key parameter is the effective heat input (ET_ef):
$$ET_{ef} = \frac{\eta \times U \times I}{v}$$
where:
- η = thermal efficiency of the process (e.g. 0.80 for GMAW; 0.65 for GTAW)
- U = arc voltage (V)
- I = welding current (A)
- v = welding speed (mm/s)
The unit is J/mm.
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2. Peak Temperature — Rykalin Formula
$$\frac{1}{T_p - T_0} - \frac{1}{T_f - T_0} = \frac{ET_{ef}}{4{.}13 \times \rho c \times t \times y^2}$$
Numerical example
Given: ET_ef = 720 J/mm; T₀ = 20 °C; t = 10 mm; y = 1.5 mm → T_p ≈ 1204 °C
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3. HAZ Sub-zones
| Sub-zone | T_p (°C) | Typical microstructure | Properties |
|---|---|---|---|
| Coarse-grained (CGHAZ) | 1100–1500 | Very large austenitic grain; martensite or upper bainite | High strength; reduced toughness |
| Fine-grained (FGHAZ) | 900–1100 | Refined austenitic grain | Acceptable toughness |
| Intercritical (ICHAZ) | 723–900 | Partial transformation; local heterogeneity | Variable hardness; embrittlement risk |
| Subcritical (SCHAZ) | 200–723 | No phase change; stress relief | Slight strength reduction |
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4. Cooling Rate and Critical Thickness
Thin plate: $\frac{dT}{dt} = \frac{2\pi \times \lambda \times (T-T_0)^3}{ET_{ef}^2 \times \rho c}$
Thick plate: $\frac{dT}{dt} = 2\pi \times \lambda \times \rho c \times \left(\frac{T - T_0}{ET_{ef}}\right)^2$
The resulting Δt₈/₅ is compared with the CCT curves of the steel to predict the final microstructure.
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5. Carbon Equivalent — EN 1011-2
$$CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}$$
| CE | Cold cracking risk | Recommended action |
|---|---|---|
| < 0.35 | Low | No preheat |
| 0.35–0.45 | Moderate | Preheat 75–150 °C |
| 0.45–0.60 | High | Preheat 150–300 °C |
| > 0.60 | Very high | Preheat > 300 °C |
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6. Multi-pass Welding
Each successive bead reheats the HAZ of previous passes. This can refine the coarse grain (improved toughness), create local intercritical zones (risk in high-carbon steels) or temper martensite from the previous pass. Pass sequence and interpass temperature must be defined in the WPS per EN ISO 15614-1.
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7. Practical Control Measures
- Heat input — Comply with WPS limits.
- Preheat — Essential when CE > 0.35.
- Interpass temperature — Keep within WPS limits.
- PWHT — Stress relief at 550–650 °C for C-Mn steels.
- Low hydrogen consumables — HD ≤ 5 ml/100 g deposited metal.
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References
- EN 1011-2:2001 — *Welding. Recommendations for welding of metallic materials. Part 2: Arc welding of ferritic steels.* CEN, Brussels.
- Lancaster, J.F. — *Metallurgy of Welding.* 6th ed. Abington Publishing, Cambridge, 1999.
- Chiaverini, V. — *Tecnologia Mecânica: Estrutura e Propriedades das Ligas Metálicas.* Vol. 1. McGraw-Hill, São Paulo.
- Rykalin, N.N. — *Berechnung der Wärmevorgänge beim Schweissen.* VEB Technik, Berlin, 1953.