Which carbon equivalent formula does AWS D1.1 use?

AWS D1.1 Annex B (the alternative preheat guideline) is based on the IIW carbon equivalent formula: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. The standard's mandatory prequalified preheat temperatures in Table 5.11 are assigned by material category and thickness and do not require the user to calculate CE.

What CE threshold indicates preheat is needed?

A CE(IIW) above roughly 0.40 is a common engineering indicator that preheat is warranted to reduce hydrogen-induced cracking risk, though the required temperature also depends on base metal thickness, filler metal hydrogen designation, and restraint level. Use Annex B or Table 5.11 to determine the actual preheat temperature.

When is Pcm more accurate than IIW CE?

The Pcm formula performs better for low-carbon microalloyed steels with C below about 0.18%, such as modern HSLA and TMCP grades. For these materials, IIW CE can overestimate required preheat; Pcm gives a closer match to actual cracking behavior in laboratory testing.

Can I use the CE value printed on a mill certificate for Annex B preheat calculations?

Only if the mill certificate states that the value is CE(IIW). Mills sometimes report Pcm or a proprietary formula without labeling it. If the formula is unlabeled or unclear, calculate CE(IIW) yourself using the chemical analysis values and the IIW formula.

Nov 5, 20266 min read

IIW vs. Pcm Carbon Equivalent: Preheat Calculations for Structural Steel

Compare the IIW and Pcm carbon equivalent formulas for structural steel preheat decisions, understand when each applies, and use them correctly with AWS D1.1.

AWS D1.1 Preheat Carbon Equivalent WPS PQR Base Metal CWI

New to procedures? Start with WPS welding: what a welding procedure specification is, with a filled-in example.

Why Carbon Equivalent Matters for Structural Welding

Preheat is the first line of defense against hydrogen-induced cracking (HIC) in structural steel welds. The purpose is straightforward: a warmer base metal slows the cooling rate after welding, giving dissolved hydrogen more time to diffuse out of the weld metal and heat-affected zone before the microstructure becomes too brittle to absorb it.

The factor that most reliably predicts a steel's susceptibility to HIC is its hardenability — how rapidly its HAZ transforms to martensite when cooled. Carbon equivalent (CE) formulas translate a steel's chemical composition into a single number that correlates with hardenability, giving engineers and CWIs a quick way to assess whether preheat is needed and at what temperature.

Two CE formulas dominate structural steel practice: the IIW formula used in AWS D1.1 Annex B and in most North American preheat guidelines, and the Pcm formula developed by Ito and Bessyo for modern low-carbon steels. Understanding when each applies, and what their outputs mean, is a practical skill for any CWI or QC manager writing or reviewing WPS documents.

The IIW Carbon Equivalent Formula

The International Institute of Welding formula is:

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

Each element in the formula is expressed as weight percent from the chemical analysis. The formula weights carbon most heavily — it appears alone in the first term — because carbon has the strongest individual effect on hardenability. Manganese follows, then the carbide-forming elements chromium, molybdenum, and vanadium as a group, then the austenite-stabilizers nickel and copper at the lowest weighting.

AWS D1.1 Annex B, the alternative preheat guideline, uses the CE(IIW) framework. This annex offers a more engineering-intensive path to preheat determination than the table-based approach in Table 5.11: the engineer calculates CE from the base metal chemistry, selects a hydrogen level for the filler metal, and accounts for base metal thickness to arrive at a minimum preheat temperature. The result can sometimes be lower than the Table 5.11 default, which is useful when standard preheat requirements would create production difficulties.

CE(IIW) is well-validated for carbon-manganese steels and low-alloy steels with carbon contents above about 0.18%. Common structural grades where it performs reliably include A36, A572, A992, and A588. The formula was developed on traditional steelmaking compositions and tends to overpredict the preheat needed for very-low-carbon microalloyed steels produced by modern controlled-rolling processes.

The Pcm Formula

The Pcm formula, also called the Ito-Bessyo formula after its developers, is:

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

The key differences from IIW: silicon appears in this formula (it promotes martensite hardening at low-carbon compositions), boron gets a large coefficient because even trace boron dramatically increases hardenability, and carbon receives a lower relative weighting because at very low carbon levels the other elements dominate cracking susceptibility.

Pcm was specifically developed for steels with carbon below about 0.18% — a range that covers most modern high-strength low-alloy (HSLA) and thermo-mechanically controlled process (TMCP) steels that deliver yield strengths of 50 ksi and above from relatively low carbon contents. For A913 Q&ST steel, A1085 HSS, and similar grades, Pcm often gives a better match to actual weld cracking behavior in test programs than CE(IIW) does. These steels achieve their strength through microalloying additions and controlled rolling, not through elevated carbon, and IIW's heavy carbon weighting leads it to underestimate the total alloy contribution to hardenability.

Reading CE Values on Mill Certifications

Certified material test reports (CMTRs) from domestic mills often include a CE value. Before using this number, verify that the mill states which formula it represents. Some CMTRs calculate CE(IIW) and label it as such; others report Pcm, which is common for offshore-platform structural steel mills; some use a proprietary formula the mill developed internally.

If the CMTR is silent on which formula applies — a frequent occurrence with foreign-mill material — the safest approach is to calculate CE(IIW) yourself from the chemical analysis column. The chemical composition values are always present on the CMTR; it takes thirty seconds to run the IIW calculation. Never carry a mill's unlabeled CE number into an Annex B preheat determination without confirming it matches the formula Annex B expects.

Practical Application: Which Formula to Use

For most structural steel work governed by AWS D1.1, CE(IIW) is the appropriate formula:

When working with A36, A572 Gr. 50, A992, A588, or similar conventional structural grades, CE(IIW) is the right tool and matches Annex B requirements.
When the CMTR reports a CE(IIW) value explicitly and it falls below 0.40, preheat may not be needed for thin-to-medium material under normal conditions — though Table 5.11 or a formal Annex B calculation should confirm this, not the CE threshold alone.

Pcm is appropriate when:

The steel is a modern HSLA or TMCP grade with C ≤ 0.18% and the manufacturer recommends Pcm for preheat guidance.
The engineer of record has specified Pcm-based preheat calculations in the project specification or WPS scope document.
You are evaluating preheat requirements for a boron-containing steel (even trace boron at 0.0005% can significantly increase hardenability, and only Pcm captures this).

If neither the project specification nor the WPS scope document specifies which formula to use, default to CE(IIW) for a prequalified WPS under AWS D1.1. For a tested WPS, the PQR base metal chemistry determines the actual preheat qualification, so the formula is a planning tool rather than a code requirement.

Limitations of CE Formulas Alone

CE is a useful screening tool, not a substitute for the full preheat determination in AWS D1.1. Two factors CE does not capture:

Base metal thickness. Thicker sections cool faster and retain hydrogen longer. A CE of 0.42 on a 1/2-inch plate might require no preheat; the same CE on a 3-inch plate might require 150°F (65°C) or more. AWS D1.1 Table 5.11 and Annex B both account for thickness explicitly. See the prequalified minimum preheat guide for the table-based approach.

Filler metal hydrogen level. An E7018-H4 electrode (maximum 4 mL/100g diffusible hydrogen) produces significantly lower hydrogen levels than a standard E6010. Annex B factors in the hydrogen designation of the filler metal; a lower-hydrogen process can reduce or eliminate preheat that a higher-hydrogen process would require. This is why switching from E7018 to E7018-H4 or to a low-hydrogen FCAW-G wire can be a viable alternative to increasing preheat temperatures.

For the CE calculation to work in Annex B, you also need to verify preheat temperatures using calibrated equipment at the correct distance and location per AWS D1.1 preheat verification methods. A calculated preheat is only as good as the measurement that confirms it has been achieved.

Connecting CE to Your WPS Documentation

When writing or reviewing a WPS, the base metal section should identify the ASTM designation, grade, and either the CE(IIW) or the Table 5.11 category on which the preheat is based. If the WPS relies on Annex B, the supporting calculation — CE value, hydrogen level, thickness, resulting minimum preheat — should be part of the WPS package or the PQR file.

If a project introduces a base metal not on Table 5.6 (the prequalified base metal list), CE-based Annex B calculations become even more important because there is no preassigned Table 5.11 preheat category to fall back on. The carbon equivalent and preheat reference covers how preheat interacts with prequalification eligibility in more detail.

For shops managing preheat documentation across multiple base metal grades and WPS documents, tracking CE values alongside material certificates reduces the risk of applying the wrong preheat to a heat of steel that arrived with a different chemistry than the previous order. Purpose-built WPS management tools that link CMTR data to WPS preheat requirements can flag this automatically — see wpswelding.com/pricing for available features.

Rule library based on AWS D1.1:2025; verify against your governing edition.

FAQ

Frequently asked.

Which carbon equivalent formula does AWS D1.1 use?: AWS D1.1 Annex B (the alternative preheat guideline) is based on the IIW carbon equivalent formula: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. The standard's mandatory prequalified preheat temperatures in Table 5.11 are assigned by material category and thickness and do not require the user to calculate CE.
What CE threshold indicates preheat is needed?: A CE(IIW) above roughly 0.40 is a common engineering indicator that preheat is warranted to reduce hydrogen-induced cracking risk, though the required temperature also depends on base metal thickness, filler metal hydrogen designation, and restraint level. Use Annex B or Table 5.11 to determine the actual preheat temperature.
When is Pcm more accurate than IIW CE?: The Pcm formula performs better for low-carbon microalloyed steels with C below about 0.18%, such as modern HSLA and TMCP grades. For these materials, IIW CE can overestimate required preheat; Pcm gives a closer match to actual cracking behavior in laboratory testing.
Can I use the CE value printed on a mill certificate for Annex B preheat calculations?: Only if the mill certificate states that the value is CE(IIW). Mills sometimes report Pcm or a proprietary formula without labeling it. If the formula is unlabeled or unclear, calculate CE(IIW) yourself using the chemical analysis values and the IIW formula.