CEV vs Pcm: Which Carbon-Equivalent Formula Applies?

CEV and Pcm both reduce steel chemistry to a single index related to weldability/hardenability, but they were developed for different composition ranges and they weight alloying elements differently.

The practical rule is:

Use the formula required by the governing material or project specification. Do not choose CEV or Pcm simply because one gives a lower number.

The formulas

IIW carbon equivalent (CEV / CEIIW)

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

Pcm

A commonly specified Pcm equation is:

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

All elemental values are mass percentages.

Why do two formulas exist?

The traditional IIW carbon equivalent works well as a general compositional hardenability indicator for many conventional carbon-manganese and low-alloy steels.

As steelmaking evolved toward lower-carbon, microalloyed, high-strength steels, formulas such as Pcm were developed to characterize hydrogen-cracking susceptibility in those lower-carbon systems more effectively. TWI notes that Pcm is generally associated with modern low-carbon steels, including many pipeline steels.

Pcm therefore gives relatively different weighting to alloying elements and includes boron, which can have a strong metallurgical effect even at very small concentrations.

A numerical example

Using the same illustrative chemistry:

• C = 0.12
• Si = 0.25
• Mn = 1.30
• Cu = 0.20
• Ni = 0.20
• Cr = 0.15
• Mo = 0.05
• V = 0.03
• B = 0.0002

The results are approximately:

• CEV = 0.409
• Pcm = 0.222

The smaller Pcm number does not mean Pcm declares the steel “twice as weldable.” The formulas have different scales and intended interpretation. Each result must be compared only with the limit or welding method that was developed for that formula.

How API/ISO line-pipe specifications use carbon equivalent

API 5L / ISO 3183 line-pipe requirements are a common reason buyers encounter both formulas. The API/ISO framework distinguishes carbon-equivalent treatment according to composition and product requirements, and project specifications often impose additional maximum values.

A particularly important 2026 point is that API Spec 5L has moved to the 47th edition, while ISO/EN ISO 3183:2019 still supplements API 5L 46th edition. Exact formula triggers and acceptance limits for a new order should therefore be verified against the edition named in the contract.

Do not carry a limit copied from an older API edition into a 47th-edition order without checking the current standard.

What if the project specification asks for both CEV and Pcm?

Calculate and report both.

Offshore/project specifications frequently impose requirements such as:

• maximum CEV;
• maximum Pcm;
• maximum carbon;
• limits based on both heat and product analyses.

Even if the base product standard would normally emphasize one carbon-equivalent formula for a particular composition, the purchaser can add a stricter contractual requirement for both. The mill must then demonstrate compliance with the actual PO/specification.

CEV and Pcm are not interchangeable

Do not:

• compare a Pcm result with a CEV acceptance limit;
• convert one to the other using a simple ratio;
• assume the lower numerical value is automatically preferable;
• omit one calculation when the PO explicitly calls for both;
• use either index as the sole basis for a welding procedure.

Which should I ask for in an RFQ?

Start with the product standard and project requirements.

For a standard structural-steel RFQ, the product standard may specify CEV limits directly. For modern low-carbon line pipe or offshore material, Pcm may also be highly relevant. If the client specification includes both, reproduce both requirements exactly in the RFQ rather than simplifying them.

A good MTC review should also check that all elements required by the specified formula — especially B for Pcm — are actually reported. If a required element is missing, a reliable calculation may not be possible from the certificate alone.

MetalMate takeaway

CEV and Pcm are different empirical tools, not alternative names for the same number. CEV is the traditional IIW-style index; Pcm was developed for lower-carbon modern steels. Use the formula and limit required by the governing standard/project specification, and calculate both when the contract asks for both.

References

• TWI, Carbon equivalent formulae in relation to hydrogen cracking: https://www.twi-global.com/technical-knowledge/faqs/faq-what-is-the-difference-between-the-various-carbon-equivalent-formulae-used-in-relation-to-hydrogen-cracking
• ISO, ISO 3183:2019 — Steel pipe for pipeline transportation systems: https://www.iso.org/standard/76676.html
• API, API announces API Spec 5L 47th edition (2026): https://www.api.org/products-and-services/standards/important-standards-announcements/api-announces-47th-edition-of-foundational-line-pipe-standard-5l