Preheat and Interpass Temperature in Welding
What is preheat?
Preheat consists of heating the base material before welding, reducing the thermal gradient between the weld bead and the surrounding metal. This is essential to prevent cold cracking (hydrogen-induced cracking), which occurs in the HAZ after cooling.
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Minimum preheat temperature (T₀)
Depends on:
- Carbon equivalent (CE) of the steel
- Material thickness — thicker plates cool more rapidly
- Hydrogen level of the consumable (HD)
- Degree of restraint of the joint
EN 1011-2 Method B — Séferian formula
``` T₀ (°C) = 697 × CET + 160 × tanh(t/35) + 62 × HD^0.35 + (53 × CET − 32) × Q − 328 ```
Where: CET = Carbon Equivalent, t = thickness (mm), HD = diffusible hydrogen (ml/100g), Q = heat input (kJ/mm).
Diffusible hydrogen classification (EN ISO 3690)
| Class | HD (ml/100g) |
|---|---|
| H2 | ≤ 2 |
| H5 | ≤ 5 |
| H10 | ≤ 10 |
| H15 | ≤ 15 |
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Maximum interpass temperature — typical values
| Steel type | Max Tp |
|---|---|
| S235–S355 | 250°C |
| S460–S690 | 200°C |
| Austenitic stainless | 150°C |
| Special high-strength | Per WPS |
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Temperature measurement
Measure in the base metal at 75 mm from the joint edge using thermal crayon, contact thermometer or infrared pyrometer (calibrated for correct emissivity).
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WPS requirements (EN ISO 15614-1)
T₀ and Tp max are essential variables of the WPS — any deviation outside the approved limits invalidates the procedure.
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