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Carbon equivalent calculator
Enter one mill-certificate composition and get the three carbon equivalents an engineer is actually asked for: CE(IIW), the Ito-Bessyo cracking parameter Pcm, and the CET that EN 1011-2 feeds into its preheat equation. They disagree on purpose, because each was fitted to a different family of steels.
No acceptance band is built in. Type the maximum your specification names and the tool reports the margin against it; the calculation runs entirely in your browser.
Three formulas, three purposes
CE (IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15
Pcm = C + Si/30 + Mn/20 + Cu/20 + Cr/20 + Ni/60 + Mo/15 + V/10 + 5B
CET = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40
All contents in weight percent from the heat analysis of the parent metal.
Each formula answers the same question — how readily does this steel form a hard, crack-sensitive microstructure when it cools quickly — with a different set of weights. CE(IIW) is the classical index and the one most contracts still name. Pcm was derived for low-carbon high-strength steels, where CE(IIW) reads pessimistically. CET exists because EN 1011-2 needs a carbon equivalent it can put inside an equation rather than a chart.
How to calculate a carbon equivalent
From a mill certificate to a number you can defend in a procedure review.
- Find the heat analysis on the certificate. Use the analysis of the actual heat supplied, not the composition range printed in the grade standard. The grade range is a worst case and usually costs you a preheat step you did not need.
- Enter the elements in weight percent. Carbon, silicon, manganese, chromium, molybdenum, nickel, copper, vanadium and boron. Silicon and boron affect Pcm only; vanadium does not appear in CET.
- Read all three equivalents. CE(IIW) for plain carbon and carbon-manganese steels, Pcm for low-carbon high-strength grades, CET when the next step is the EN 1011-2 preheat equation.
- Apply the maximum your specification names. Choose which of the three the specification refers to, type its limit, and the tool reports the margin or the overshoot in points.
- Carry CET forward to the preheat calculation. A carbon equivalent on its own is a hardenability index, not an instruction. The preheat temperature calculator turns it into a shop-floor number together with thickness, hydrogen and heat input.
- A carbon equivalent is an index, not an instruction. It ranks how the steel will respond to fast cooling. What to do about it — preheat, control the hydrogen, slow the cooling with heat input — comes from the preheat temperature calculator and the t8/5 cooling time that follows from it.
- The deposit has its own composition. Dilution mixes parent metal into the weld pool in a ratio the joint geometry decides, so the bead is neither the plate nor the consumable. The weld dilution calculator puts a number on that mix.
- Boron is the trap in Pcm. It carries a factor of five, so a boron content that looks like a rounding error on the certificate moves Pcm materially. Enter it if the certificate reports it at all.
- Say which formula you quoted. Three numbers from one certificate, all correct, all different. A weld procedure review that records "CE = 0.42" without the formula has recorded nothing — the same discipline the heat input guide argues for on arc energy versus heat input.
- Hardenability is only half the risk. The other half is what the arc did on the day, which is a measurement rather than a calculation — see WeldTrace for how the parameters get recorded.
For how hardenability turns into a crack in service, read hydrogen-induced cracking prevention.
Questions engineers ask about this calculation
Which carbon equivalent formula should I use for my steel?
It depends on the carbon level and on what the specification names. CE(IIW) = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 is the classical formula and works well on plain carbon and carbon-manganese steels. Pcm = C + Si/30 + Mn/20 + Cu/20 + Cr/20 + Ni/60 + Mo/15 + V/10 + 5B was derived for modern low-carbon high-strength grades, broadly below 0.17 % C, where CE(IIW) over-predicts hardenability. CET = C + (Mn+Mo)/10 + (Cr+Cu)/20 + Ni/40 exists because it is the input to the EN 1011-2 preheat equation. Calculate all three, then quote whichever the contract asks for.
Do the three carbon equivalents give the same number?
No, and they are not meant to. They weight the alloying elements differently because they were fitted to different steels and different failure mechanisms, so on the same certificate CE(IIW) is typically the largest and Pcm the smallest. A number quoted without saying which formula produced it is unusable — that is the single most common mistake in a welding procedure review, and it is why this page shows all three together rather than one.
Should the carbon equivalent be calculated on the parent metal or the weld metal?
On the parent metal, from the mill certificate of the actual heat, because it is the heat-affected zone that the equivalent is predicting. The weld metal has its own composition, diluted by the parent metal in a ratio the joint decides, and it can be the more sensitive of the two — EN 1011-2 assumes the parent metal CET exceeds the weld metal CET by at least 0.03 % and tells you to work from the weld metal value plus 0.03 % when it does not. A specified maximum written for the plate does not automatically cover the consumable.
Why does a certificate maximum matter if the plate is within the grade?
A grade specifies a composition range and a mill fills it heat by heat, so two plates of the same grade can sit far apart within it. A carbon equivalent computed from the grade limits is a worst case; one computed from the actual certificate is the real number, and it is often low enough to drop the preheat requirement by a step. That difference is worth money on a long seam, which is why the certificate — not the grade — is the input.
The certificate says what the steel is. Nothing on a WPS says what the arc did.
WeldTrace records voltage, current and wire feed speed at 25 kHz on every pass, so the heat input that goes with a carbon equivalent into a preheat decision is a measured figure from the joint rather than a nominal one from the sheet.
