Here's a detailed breakdown:
1. When PWHT is Not Required
Thin Sections (≤25 mm):
SMA490AW's low carbon content (≤0.20%) and alloy design minimize weld cracking risk.
Natural air cooling is sufficient if proper preheating (100–150°C) and low-hydrogen electrodes (e.g., E7018) are used.
Non-Critical Applications:
For structures with moderate stress (e.g., architectural façades, non-load-bearing components).
2. When PWHT May Be Needed
Thick Sections (>25 mm):
Residual stresses from welding can promote cracking.
PWHT Recommendation: Stress relief at 600–650°C (hold time: 1 hour per 25 mm thickness).
High-Restraint Joints:
Examples: Heavy girders, bridge nodes, or dynamically loaded structures.
PWHT reduces residual stresses and improves fatigue life.
Harsh Environments:
If the weld will face cyclic loading (e.g., bridges) or subzero temperatures, PWHT enhances toughness.
3. Alternatives to PWHT
Controlled Slow Cooling:
Cover welds with insulating blankets to reduce cooling rate and stress.
Peening:
Light mechanical peening of weld passes can relieve residual stresses.
4. Key Considerations
Material Trade-off: PWHT may soften the heat-affected zone (HAZ), slightly reducing strength.
Cost vs. Benefit: Evaluate based on project criticality and thickness.
