Understanding Heat Affected Zone (HAZ) Microstructure
The HAZ is the region where base material experiences thermal cycles but does not melt. HAZ microstructure and properties depend on peak temperature, cooling rate, and material composition, critically affecting weld joint performance.
HAZ Sub-Zones (Carbon Steel)
- Fusion Zone (FZ): Melted and solidified weld metal
- Peak temp: >1700°C (melting point)
- Columnar dendritic structure
- As-cast properties
- Coarse Grain HAZ (CGHAZ): 1200-1500°C
- Grain growth above AC₃ (910°C for steel)
- Critical zone: Martensite forms if cooling >300°C/s
- Highest hardness (450-650 HV) → crack susceptible
- Lowest toughness
- Fine Grain HAZ (FGHAZ): 910-1200°C
- Complete austenization, fine grain size
- Best balance of strength and toughness
- Moderate hardness (250-400 HV)
- Intercritical HAZ (ICHAZ): 727-910°C
- Partial austenization (AC₁-AC₃ range)
- Mixed ferrite + new martensite/bainite
- Variable properties
- Subcritical HAZ (SCHAZ): <727°C
- Below AC₁ (eutectoid temp)
- Tempering/stress relief if pre-hardened
- Softening in quenched steels
Cooling Rate (t₈/₅) and Microstructure
t₈/₅ is the time to cool from 800°C to 500°C - the critical range for phase transformation in steel.
| Cooling Rate | t₈/₅ (s) | Microstructure | Hardness (HV) |
|---|---|---|---|
| >300 °C/s | <1 | 100% Martensite | 450-650 |
| 100-300 °C/s | 1-3 | Martensite + Bainite | 350-450 |
| 30-100 °C/s | 3-10 | Bainite | 280-350 |
| <30 °C/s | >10 | Ferrite + Pearlite | 180-250 |
⚠️ Martensite Formation:
Martensite (hardness >450 HV) is brittle and crack-sensitive. For steels with CE > 0.4%, preheat to 100-200°C to reduce cooling rate below 100°C/s.
Stainless Steel HAZ Issues
Sensitization (650-850°C zone):
- Chromium carbides (Cr₂₃C₆) precipitate at grain boundaries
- Local Cr depletion → intergranular corrosion
- Prevention:
- Use L-grade SS (304L, 316L) with low carbon (<0.03%)
- Post-weld solution annealing (1050°C + water quench)
- Fast cooling through 650-850°C range
Sigma Phase Formation (>600°C, long exposure):
- Fe-Cr intermetallic, very brittle
- Reduces toughness and corrosion resistance
- Avoid multi-pass welding with slow cycles
Preheat Temperature Guidelines
| Material | Carbon Equivalent (CE) | Preheat Temp |
|---|---|---|
| Low carbon steel | CE < 0.35% | No preheat |
| Medium carbon | CE 0.35-0.45% | 100-150°C |
| High carbon | CE 0.45-0.60% | 200-300°C |
| High alloy/thick | CE > 0.60% or >25mm | 300-400°C |
Carbon Equivalent (IIW formula):
CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
HAZ Width Optimization
- Narrow HAZ (<2mm):
- Achieved by: High speed, low power, focused beam
- Benefits: Less distortion, smaller softened zone
- Trade-off: May reduce penetration
- Wide HAZ (>4mm):
- Caused by: Low speed, high power, defocus
- Issues: More grain growth, higher crack risk, distortion
- Solution: Increase speed, reduce power, optimize focus
Post-Weld Heat Treatment (PWHT)
For critical applications:
- Stress Relief (550-650°C, 1-2h): Reduce residual stress
- Tempering (550-650°C, 1h): Reduce martensite hardness to <350 HV
- Normalizing (900-950°C + air cool): Refine grain structure
- Annealing (full cycle): Restore base metal properties