What is Carbon Equivalent?
Carbon Equivalent (CE) is a calculated value that predicts the weldability of steel based on its chemical composition. Higher carbon and alloy content increases hardness in the heat-affected zone (HAZ) during welding, which increases the risk of hydrogen-induced cracking (cold cracking). CE provides a single number to assess this risk.
The CE value helps welding engineers determine whether preheat is needed, what welding processes and consumables to use, and what post-weld heat treatment may be required. It is essential for developing welding procedure specifications (WPS) in structural steel construction.
CE Formulas
The IIW formula (International Institute of Welding) is used for steels with C > 0.18%:
The PCM formula (Ito-Bessyo) is preferred for low-carbon steels (C < 0.11%):
Weldability Classification
| CE (IIW) | Weldability | Preheat Temperature |
|---|---|---|
| < 0.35 | Excellent | Generally not required |
| 0.35 - 0.40 | Good | 50-100°C (thin sections may not need) |
| 0.40 - 0.45 | Fair | 100-200°C recommended |
| 0.45 - 0.50 | Poor | 200-300°C, controlled procedures |
| > 0.50 | Very Poor | 300°C+, special welding procedures needed |
Common Structural Steel CE Values
| Steel Grade | Typical CE (IIW) | Weldability |
|---|---|---|
| A36 | 0.25 - 0.35 | Excellent |
| A572 Gr. 50 | 0.35 - 0.42 | Good |
| A992 | 0.40 - 0.45 | Good to Fair |
| A514 (T-1) | 0.45 - 0.55 | Fair to Poor |
Frequently Asked Questions
Which CE formula should I use?
Use the IIW formula for steels with carbon content above 0.18% (most structural steels). Use the PCM formula for low-carbon, high-strength steels with C below 0.11%. When in doubt, calculate both and use the more conservative (higher) value.
Does a low CE guarantee no cracking?
No. CE is a guide, not a guarantee. Other factors including hydrogen content, residual stresses, joint restraint, and plate thickness also affect cracking risk. Low-hydrogen welding practices should always be followed for structural steel.
How does plate thickness affect preheat needs?
Thicker plates cool faster (higher cooling rate), which promotes hardening and cracking. The required preheat temperature increases with plate thickness. AWS D1.1 provides specific preheat tables based on CE, plate thickness, and welding process.