clause5.io/ welding/ questions/ carbon-equivalent-explained

Why Carbon Equivalent Exists

Steel is not pure iron. Every alloying element — manganese, chromium, molybdenum, nickel, vanadium, copper — changes how the heat-affected zone (HAZ) responds to welding. Some elements increase hardenability, which means the HAZ can form hard, brittle martensite during rapid cooling. Hard martensite plus trapped hydrogen equals cracking.

Carbon equivalent collapses all of those alloying contributions into one number. A higher CE means the steel is more hardenable, more susceptible to hydrogen cracking, and more likely to need preheat to slow the cooling rate. D1.1 uses CE as the entry point for the alternative preheat methods in Annex B.

The Two Formulas

D1.1:2025 Annex B defines two separate carbon equivalent formulas, each tied to a different preheat determination method. They are not interchangeable.

CE(IIW) — Hardness Control Method (Annex B6.1.1)

D1.1 adopted a modified version of the International Institute of Welding (IIW) formula that groups silicon with manganese as (Mn+Si)/6. The pure international IIW formula omits silicon — which is why many online references show C + Mn/6. When working to D1.1, always use the Annex B version. This formula drives the HAZ hardness control method per Annex B6.1, which is restricted to fillet welds (B3.1) and is based on keeping HAZ hardness below a critical value — either 350 HV or 400 HV (B6.1.2). The CE value determines the critical cooling rate from Figure B.2, which in turn determines the minimum heat input needed to prevent excessive hardening.

Note that boron does not appear in CE(IIW). If you need boron accounted for, use Pcm, which includes it as 5B.

Pcm — Hydrogen Control Method (Annex B6.2.1)

The composition parameter Pcm drives the hydrogen control method per Annex B6.2. This method is based on controlling the quantity of hydrogen remaining in the joint after it cools to about 120 °F [50 °C] (B4.1). Unlike the hardness method, the hydrogen method applies to both fillet welds and groove welds, and it is the only method for Zone III steels and quenched and tempered steels.

Pcm includes silicon (Si/30) and boron (5B), making it more sensitive to low-carbon micro-alloyed steels where those elements significantly affect hardenability. The Pcm value feeds into Table B.1 to determine a susceptibility index grouping (A through G), which then combines with restraint level and thickness in Table B.2 to produce the minimum preheat temperature.

Zone Classification: Which Method Applies

Annex B5.1 uses the CE(IIW) value together with carbon content to classify steels into three zones per Figure B.1. The zone determines which preheat method to use:

For Zone II steels with high carbon, both methods may be needed: a minimum energy input to control hardness, and preheat to control hydrogen for both fillet and groove welds (B5.2(2)).

This zone system is what makes Annex B more precise than Table 5.11. Table 5.11 assigns preheat based on steel specification, welding process, and thickness alone. Annex B considers the actual chemistry from your mill test report, the hydrogen level of your consumables, and the restraint of your joint. Two heats of A572 Grade 50 with different chemistries can have very different CE values and very different preheat requirements under Annex B, even though Table 5.11 treats them identically.

Table B.1: From Pcm to Susceptibility Index

When using the hydrogen control method, your Pcm value and hydrogen level determine a susceptibility index grouping per Table B.1. The hydrogen levels are defined in Annex B6.2.2:

Lower groupings (A, B) mean lower cracking susceptibility and lower preheat requirements. Higher groupings (F, G) mean the steel and consumable combination is highly susceptible to hydrogen cracking. The jump from H1 to H3 at the same Pcm pushes you two groupings higher — electrode hydrogen control is as important as steel chemistry.

Table B.2: Susceptibility Index to Preheat Temperature

The susceptibility index grouping from Table B.1 feeds into Table B.2 along with restraint level and thickness. Table B.2 provides minimum preheat and interpass temperatures for three restraint levels: low, medium, and high. Restraint is classified per Annex B6.2.5 based on engineering judgment, research, or calculation.

For example, a steel with susceptibility index D, medium restraint, and thickness over 3/4 through 1-1/2 inch requires 230 °F preheat per Table B.2. The same steel with low restraint at the same thickness needs only 175 °F. With high restraint, it jumps to 280 °F. Restraint level can shift your preheat requirement by over 100 °F in either direction.

When to Use Annex B vs Table 5.11

Table 5.11 is the default. Every prequalified WPS references it through Clause 5.7.1. Annex B is the alternative, and D1.1 Clause B1 states it provides optional methods when the Table 5.11 requirements appear overly conservative or not sufficiently demanding.

In practice, Annex B is most valuable when:

Table 5.11 seems too conservative. Your steel specification falls into a high-preheat category, but the actual mill test chemistry shows low CE and low Pcm. Annex B can justify lower preheat based on the real numbers. This saves time, gas, and labor cost on every joint.

Table 5.11 may not be conservative enough. An unlisted steel or a steel with unusually high alloy content may have cracking susceptibility beyond what Table 5.11 anticipates. Annex B catches this because it uses the actual chemistry rather than the specification limits. If you use the preheat calculator and your steel is not in Table 5.11, Annex B is your path.

You need to account for hydrogen level. Table 5.11 uses welding process categories (non-low-hydrogen SMAW vs low-hydrogen processes) as a proxy for hydrogen. Annex B uses actual hydrogen levels (H1, H2, H3) from consumable testing, which is more precise.

D1.1 vs AISC: Carbon Equivalent in Context

AISC 360 does not directly reference carbon equivalent for preheat. AISC relies on D1.1 for welding requirements through its general reference to the applicable welding code. When D1.1 is the contract welding code, Annex B is available regardless of whether the design standard is AISC 360 or another specification. The Engineer must approve the use of Annex B as an alternative to Table 5.11.

Common Mistakes

Using the wrong CE formula. D1.1 Annex B uses (Mn+Si)/6, but the pure international IIW formula uses Mn/6 without silicon. Many online calculators and references use the wrong version. If you are working to D1.1, your CE must include silicon. Always confirm which formula applies to your contract welding code before calculating.

Using specification maximums instead of actual chemistry. The power of Annex B is that it uses real MTR values. If you plug in the specification maximum carbon and manganese, you lose the benefit. Annex B6.1.1 lists four sources for chemical analysis: mill test certificates, typical production chemistry, specification chemistry, and user tests. Mill test certificates give the most accurate result.

Ignoring restraint level. Table B.2 has three restraint columns. Using low restraint when the joint is actually a highly restrained repair weld in thick material underestimates the preheat by up to 120 °F. Annex B6.2.5 defines the three levels: low restraint allows reasonable freedom of movement, medium describes members already attached to structural work, and high describes almost no freedom of movement such as repair welds in thick material.

Applying the hardness method to groove welds. Annex B3.1 states that the hardness control method is restricted to fillet welds. For groove welds, the hydrogen control method shall be used (B5.2(2) for Zone II, B5.2(3) for Zone III). A CJP butt joint requires Pcm, not CE(IIW).

Carbon equivalent is one piece of the D1.1 compliance chain. For an overview of how CE fits into the prequalified and non-prequalified WPS paths, see the AWS D1.1:2025 overview.