The IIW Carbon Equivalent Formula
The **IIW (International Institute of Welding) Formula** assigns a weighting factor to alloying elements based on their approximate influence on the **Hardness** and **Martensite formation** in the heat-affected zone (HAZ) during the cooling phase of welding.
Calculation Formula
The CE value is calculated as the sum of Carbon (C) and the scaled contributions of other elements:
$$\mathbf{CE_{\text{IIW}} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}}$$
Weldability and CE Values
The calculated CE value is a key indicator for determining required welding procedures, particularly the need for **preheat** to slow the cooling rate and prevent cracking.
- **CE $\le 0.35\%$**: Generally considered excellent weldability. Welding is possible without special precautions.
- **$0.35\% < \text{CE} \le 0.45\%$**: Good weldability. Simple preheating might be required for thicker sections.
- **CE $\ge 0.45\%$**: Moderate to poor weldability. Preheat, controlled interpass temperature, and slow cooling are typically mandatory to avoid cold cracking.
Alternative Formulas (e.g., Pcm):
For high-strength, low-alloy steels (HSLA) with lower carbon content (C $\le 0.12\%$), the **Pcm (Cracking Parameter)** formula is often preferred as it is more sensitive to these small changes and provides a more accurate prediction of cold cracking susceptibility.