Carbon Equivalent: How to Calculate It and What It Means for Weldability
What is Carbon Equivalent?
Carbon equivalent (CE) is a calculated value that summarises the influence of all alloying elements in a steel on its tendency to harden and crack in the Heat-Affected Zone (HAZ) during welding.
Higher CE → greater tendency to form martensite → higher risk of cold cracking.
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Calculation Formulae
IIW Method (International Institute of Welding)
Applicable to C-Mn steels with C > 0.18%:
``` CE(IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 ```
CET Method (Seyffarth)
Applicable to modern micro-alloyed steels with C < 0.25%:
``` CET = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40 ```
All values in mass %. Consult the technical data sheet or material certificate (EN 10204) for each element content.
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Interpreting the Result
| CE (IIW) | Weldability | Recommended action |
|---|---|---|
| < 0.35 | Excellent | No preheat required |
| 0.35 – 0.45 | Good | Light preheat (75–150°C) for thickness > 25 mm |
| 0.45 – 0.60 | Moderate | Preheat 150–250°C mandatory |
| > 0.60 | Poor | High preheat + strict control |
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Practical Example
S355J2 steel with typical composition:
- C = 0.20%, Mn = 1.50%, Si = 0.40%, Cr = 0.02%, Ni = 0.01%, Mo = 0.01%, Cu = 0.01%, V = 0.01%
CE(IIW) = 0.20 + 1.50/6 + (0.02 + 0.01 + 0.01)/5 + (0.01 + 0.01)/15
CE(IIW) = 0.20 + 0.25 + 0.008 + 0.001 = 0.459
Result: moderate weldability → preheat recommended from 150°C for higher thicknesses.
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Other Factors Affecting Risk
CE is only one indicator. The actual cold cracking risk also depends on:
- Diffusible hydrogen in deposited metal (consumable HD: < 5 ml/100g = low, > 15 = high)
- Restraint stresses of the joint (geometry, fixturing, thickness)
- Cooling rate (heat input, thickness, ambient temperature)
Preheat acts on all three factors: reduces cooling rate, promotes hydrogen escape and reduces residual stresses.
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Normative Reference
EN 1011-2 (Recommendations for welding of ferritic steels) provides detailed methods to calculate the minimum preheat temperature based on CE, thickness and consumable hydrogen level.
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