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, so compare it with weld joint performance requirements.
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)
- Martensite risk may increase at very fast cooling rates
- High hardness band (450-650 HV) can indicate crack-susceptibility risk
- Toughness may be reduced depending on composition and cooling path
- Fine Grain HAZ (FGHAZ): 910-1200°C
- Complete austenization, fine grain size
- Often a more favorable strength/toughness screening band
- 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, a common screening range for steel phase-transformation checks.
| Cooling Rate | t₈/₅ (s) | Microstructure | Hardness (HV) |
|---|---|---|---|
| >300 °C/s | <1 | Martensite-dominant indication | 450-650 |
| 100-300 °C/s | 1-3 | Martensite + bainite indication | 350-450 |
| 30-100 °C/s | 3-10 | Bainite | 280-350 |
| <30 °C/s | >10 | Ferrite + Pearlite | 180-250 |
Martensite Screening Note:
Martensite at hardness above about 450 HV can be crack-sensitive. For steels with CE > 0.4%, a 100-200°C preheat trial can test whether the cooling-rate band moves lower.
Stainless Steel HAZ Checks
Sensitization (650-850°C zone):
- Chromium carbides (Cr₂₃C₆) precipitate at grain boundaries
- Local Cr depletion can increase intergranular-corrosion concern
- Common controls:
- Specify L-grade SS (304L, 316L) with low carbon (<0.03%) where appropriate
- Post-weld solution annealing may be specified for some procedures
- Check thermal cycle through the 650-850°C range
Sigma Phase Formation (>600°C, long exposure):
- Fe-Cr intermetallic, very brittle
- Reduces toughness and corrosion resistance
- Check multi-pass welding when thermal cycles are slow
Preheat Temperature Planning Ranges
| Material | Carbon Equivalent (CE) | Preheat Temp |
|---|---|---|
| Low carbon steel | CE < 0.35% | Often no routine 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 Checks
- Narrow HAZ (<2mm):
- Often associated with: high speed, lower power, focused beam
- Potential benefits: less distortion, smaller softened zone
- Trade-off: May reduce penetration
- Wide HAZ (>4mm):
- Often associated with: low speed, high power, defocus
- Key risks: grain growth, crack risk, distortion
- Process check: compare speed, power, and focus changes
Post-Weld Heat Treatment (PWHT)
For code-governed or high-consequence work:
- 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