Preheating means raising the base metal to a specified temperature before welding. The purpose is to slow the cooling rate, reduce the risk of cold cracking (hydrogen-induced cracking), facilitate hydrogen diffusion and prevent hard microstructures.
General rule: Preheat is mandatory for materials with high carbon equivalent (CEV), heavy sections and low ambient temperatures.
EN ISO 1011-2: Two Methods
- Method A: CEV (Carbon Equivalent Value) based. Used for general structural steels.
- Method B: CET (Compositional Equivalent for Tempered steels) based. Designed for high-strength steels (S460–S960).
Method A: CEV-Based Calculation
CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
| Steel | CEV (typical) | Preheat Requirement |
|---|---|---|
| S235JR | 0.35–0.40 | Usually not required (thickness ≤30 mm) |
| S355J2+N | 0.40–0.45 | Recommended for heavy sections (≥25 mm) |
| S460M / S500MC | 0.40–0.50 | Typically 50–100°C |
| S690QL | 0.55–0.65 | 100–150°C mandatory |
| S960QL | 0.65–0.82 | 150–200°C mandatory |
Method B: CET and Practical Formula
CET = C + (Mn+Mo)/10 + (Cr+Cu)/20 + Ni/40
Preheat temperature (Tp):
Tp = 700·CET + 160·tanh(d/35) + 62·HD0.35 + (53·CET - 32)·Q - 330
- d: Plate thickness (mm)
- HD: Diffusible hydrogen (ml/100g weld metal)
- Q: Heat input (kJ/mm)
Practical Example — S690QL
Composition: C=0.16, Mn=1.50, Mo=0.40, Cr=0.30, Cu=0.20, Ni=0.40
- CET = 0.16 + 0.19 + 0.025 + 0.01 = 0.385
- 20 mm thickness, H5 consumable, 1.0 kJ/mm:
- Tp = 269.5 + 82.4 + 110.0 − 11.6 − 330 = 120.3°C
Result: Preheat must be applied at ≥120°C. If the steel maker recommends 100–150°C, use 150°C (safety margin).
Low Ambient Temperature Correction
- Ambient +5°C to 0°C → add +25°C
- Ambient 0°C to −10°C → add +50°C
- Ambient below −10°C → postpone welding or erect shelter
Practical Application: Preheat Methods
- Resistance blanket: High accuracy, ideal for pipes and heavy plates.
- Induction: Fast, clean. Preferred for production lines.
- Gas torch: Common in field welding. Use Tempilstick to verify temperature.
- Tempilstick: Practical temperature verification tool.
Relationship with Interpass Temperature
- Interpass temperature ≥ preheat temperature (must never fall below)
- Interpass temperature ≤ manufacturer recommendation (≤220°C for S960QL)
- Measure with Tempilstick or digital pyrometer before each pass
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