Carbon Equivalent and Weldability: The CEV and Pcm Formulas

CEV compresses a full steel analysis into one weldability index. This guide gives the standard IIW formula, the Pcm alternative for modern low-carbon steels, and the limits of what either number can tell you.

Why a single number?

The hardness of the heat-affected zone (HAZ) after welding — and with it the risk of hydrogen-induced cold cracking — rises with carbon and alloying content acting together. Carbon equivalent formulas compress the ladle analysis into one index that correlates with HAZ hardenability and cracking risk, so procedures can be screened before any test weld is made.

The IIW formula (CEV)

CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 — all elements in weight percent.

This is the formula adopted in EN and ISO practice: EN 10025 declares a maximum CEV for each structural grade, and EN 1011 uses CEV in its welding-procedure guidance. It was developed for conventional carbon-manganese steels. Widely used interpretation bands are:

CEV up to roughly 0.40 — generally weldable without preheat under ordinary conditions.
Roughly 0.40–0.50 — preheat commonly required, increasing with thickness, restraint and hydrogen level; low-hydrogen practice applies.
Above roughly 0.50 — high cracking risk; preheat, controlled interpass temperature and low-hydrogen consumables are standard, with procedure qualification essential.

Worked example (illustrative mill values): C 0.16, Mn 1.50, Cr+Mo+V 0.10, Ni+Cu 0.15 gives CEV = 0.16 + 0.25 + 0.02 + 0.01 = 0.44 — squarely in the preheat band.

Pcm: the formula for modern low-carbon steels

Thermomechanically rolled and HSLA steels carry low carbon but a spread of alloying elements, and the IIW formula overstates their cracking risk. The Ito–Bessyo Pcm weights the elements differently:

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

Pcm is standard in line-pipe practice (API 5L caps it for PSL2 grades, typically at 0.25) and in Japanese standards. As a rule of thumb, Pcm up to roughly 0.20–0.25 indicates good weldability for low-carbon steels. EN 1011 offers the related CET formula as another alternative for procedure calculations.

What CEV does not tell you

Preheat temperature is not read off CEV alone: EN 1011-2 method A combines CEV with combined thickness, the hydrogen scale of the process and consumable, restraint and heat input.
Two steels with equal CEV can behave differently — a higher-carbon/low-alloy heat is not the same welding problem as a low-carbon/higher-alloy heat.
Use the actual ladle analysis from the certificate for procedure work. The grade maximum (for example S355J2 at typically ≤ 0.45 in common thicknesses) is a ceiling the heat must stay under, not the value of the heat in front of you.
Austenitic stainless steels and hardened tool steels sit outside the scope of both formulas entirely.
Finally, CEV is not a property of the steel in service: it predicts nothing about the strength or toughness of a finished joint, only about HAZ hardening tendency during welding. A low-CEV steel can still produce a poor weld through bad fit-up, damp consumables or an unqualified procedure — the index screens chemistry, not workmanship.