DEFINITION
The Heat Affected Zone (HAZ) is the region of base material which is altered—but not melted—during a heat-intensive welding process. This is where the true strength of the joint is decided. Not in the puddle. Not in the filler. In the gradient of temperature that passes through the parent metal, rearranging its crystal lattice without liquefaction.
THE DEFINING EQUATION
Heat input governs the width of the HAZ. Too much, and the grain coarsens. Too little, and the transformation is incomplete. The formula is non-negotiable:
Q = ((V × I × 60) / (S × 1000)) × Efficiency
Q:
Heat Input (kJ/mm)
V:
Voltage (volts)
I:
Current (amps)
S:
Travel Speed (mm/min)
Efficiency:
Process efficiency factor (0.8 for SMAW, 0.9 for GTAW)
In the warehouse, we measure travel speed by the mark the torch leaves. In the lab, they measure it by the grain boundary shift. Both are watching the same fire.
METALLURGICAL REGIONS
From fusion line outward, the HAZ contains distinct zones, each with its own microstructure:
- Fusion Boundary: Peak temperature reached melting point. Grain growth uncontrolled.
- Coarse-Grained HAZ: 1200°C+. Austenite grains grow large. Toughness drops. This is the danger zone.
- Fine-Grained HAZ: 900–1200°C. Recrystallization occurs. Optimal hardness.
- Intercritical HAZ: 727–900°C. Partial transformation. Mixed ferrite/austenite.
- Subcritical HAZ: Below Ac₁. Tempering effect only. Softest region.
FAILURE MODES
The HAZ is where cracks initiate when the protocol fails:
- HAZ Cracking: Hydrogen diffusion along grain boundaries in coarse regions. Prevented by low-hydrogen electrodes and preheat.
- Softening: Over-tempered martensite in quenched-and-tempered steels. Load capacity collapses.
- Brittle Fracture: Untransformed retained austenite shearing under impact.
OPERATIONAL PROTOCOL
This is not theory. This is how I stack the pallets of heat:
- Preheat to 150°C: Slows cooling rate. Reduces hydrogen trap formation.
- Maintain interpass temp ≥ 100°C: Prevents rapid quench in subsequent layers.
- Travel speed ≤ 150 mm/min: Keeps Q within bounds. Faster burns widen the HAZ.
- Post-weld heat treat: Stress relief at 620°C for 2 hours. Reclaims ductility.