Weld Carbon Equivalent Calculator

Carbon equivalent (CE) is a single number that combines a steel’s alloying elements into an estimate of its hardenability and susceptibility to hydrogen-induced cracking (cold cracking) during welding.

The IIW (International Institute of Welding) formula:

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

All values are weight percentages. Carbon is the most potent hardenability element, followed by manganese, chromium, molybdenum, and vanadium in the second group, and the less-effective nickel and copper in the third.

Interpreting CE:

• CE < 0.40: Good weldability, no preheat normally needed
• CE 0.40-0.45: Marginal; preheat may be needed depending on heat input and plate thickness
• CE 0.45-0.60: Preheat required
• CE > 0.60: Very high preheat needed; special procedures required

Minimum preheat temperature. One common empirical formula:

T_preheat = 350 x sqrt(CE - 0.25) degrees C (for CE > 0.25)

That puts a CE of 0.45 at about 157°C and a CE of 0.60 at about 207°C, which is the range real welding procedures actually specify. Note that it disagrees with the bands above at the low end: it returns roughly 111°C at CE 0.35, where the guidance says no preheat is normally needed. The two are answering different questions. The bands assume ordinary thickness and restraint; the formula is a worst-case floor. For thin plate at low restraint you can usually skip preheat below CE 0.40, and for thick, heavily restrained joints you should not.

The purpose of preheat is to slow the cooling rate after welding, giving dissolved hydrogen time to diffuse out before the heat-affected zone (HAZ) becomes fully martensitic. Cold cracking typically occurs hours or days after welding, not during it, which is why a joint that looked perfect on Friday can be cracked on Monday.

Other formulas. The Pcm (Japanese) formula weights carbon more heavily and is better suited for low-carbon, high-strength steels:

Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B

The IIW formula is standard for structural and pressure vessel steels, so it is the one this page leads with. It is also known to overstate the risk on modern low-carbon steels, because the Mn/6 term punishes manganese as heavily on a 0.08% carbon plate as on a 0.25% one, and the whole point of a low-carbon high-strength grade is that it hardens less than its alloy content suggests. That is the gap Pcm was written to close, so the calculator reports both and tells you which to trust for the composition you entered. The usual dividing line is 0.16% carbon: below it, believe Pcm.

Pcm bands: below 0.23 is generally weldable without preheat; 0.23 to 0.28 is marginal; above 0.28 needs preheat and low-hydrogen practice.

Note that silicon and boron appear in Pcm and not in IIW. Boron especially: at 0.003%, a rounding error on most mill certificates, the 5B term adds 0.015 to Pcm on its own. Boron-treated steels are far more hardenable than their carbon content suggests, and IIW cannot see that at all.