Carbon equivalent CEV -
what it says about steel weldability and what it does not

The carbon equivalent is a number describing the combined effect of carbon and alloying elements on the hardenability of steel, and therefore on the risk of cold cracking in the heat-affected zone. EN 10025-2 adopts the formula of the International Institute of Welding and sets maximum values for individual grades: 0.35 for S235JR up to 40 mm thick, 0.40 for S275 and 0.45 for S355 up to 30 mm thick.

A misunderstanding that comes up regularly in the industry is worth clearing up at the outset. The carbon equivalent has nothing to do with carbon dioxide emissions or the carbon footprint of a product. The similarity in name arises solely because the word carbon appears in both cases, once as a chemical element and once as shorthand for carbon dioxide. These are two independent subjects, described in different documents and expressed in different units.

The formula and its components

The basic formula, referred to in EN 10025-2 and EN 1011-2, is:

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

All values are given in percent by mass, from the cast analysis. The structure of the formula shows the hierarchy of influences: carbon counts in full, manganese at one sixth, chromium, molybdenum and vanadium at one fifth, and nickel and copper at only one fifteenth. This reflects their actual contribution to increasing hardenability.

The formula has a defined range of application. EN 1011-2 provides for it in method A for CEV between 0.30 and 0.70, with carbon content of 0.05-0.25 percent, manganese up to 1.7 percent, chromium up to 0.9 percent, nickel up to 2.5 percent and molybdenum up to 0.75 percent. Outside this range, other formulas are used.

For higher-strength steels, with yield strength up to around 1,000 MPa, the standard provides method B, based on a different equivalent:

CET = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40

In offshore structures and in certain welding applications, the Pcm parameter is also used, intended for low-carbon steels under rapid cooling.

Maximum values in product standards

EN 10025-2 gives maximum carbon equivalent values broken down by grade and thickness range.

The standard also provides a correction: with very low silicon content, the permissible CEV rises by 0.01 or 0.02, which matters for products intended for hot-dip galvanising, where silicon content is deliberately restricted.

The increase in the permissible value with thickness is not a concession but a reflection of the technology: in larger sections it is harder to keep the equivalent low while maintaining the required mechanical properties. At the same time, it is precisely at greater thicknesses that the welding risk increases, because a thicker section conducts heat away from the joint faster.

Where to find the value for a specific batch

The carbon equivalent of a given heat results from its chemical analysis and is stated in the inspection certificate 3.1 according to EN 10204. Product data sheets of European producers contain separate tables of CEV values by thickness, alongside tables of chemical composition and mechanical properties.

This is of practical importance when planning welded production. The standard value is a maximum, not the actual value. A specific heat is usually below it, sometimes significantly, and it is the heat that determines the choice of welding procedure. For this reason, for structures requiring procedure qualification, it is advisable to order material with an inspection certificate rather than with a declaration of compliance, which does not contain values for the specific batch.

Calculation example

For a structural steel with a composition within the limits of EN 10025-2, with carbon 0.16 percent, manganese 1.35 percent, chromium 0.03 percent, nickel 0.02 percent and copper 0.01 percent:

CEV = 0.16 + 1.35/6 + 0.03/5 + 0.03/15 = 0.16 + 0.225 + 0.006 + 0.002 = 0.393

The result is well below the 0.45 limit specified for S355 up to 30 mm thick. At typical thicknesses and with low-hydrogen electrodes, such material usually does not require preheating, although the final decision requires a full calculation taking into account the joint thickness and welding conditions.

For a more highly alloyed material with the composition: carbon 0.20 percent, manganese 1.55 percent, chromium 0.25 percent, molybdenum 0.05 percent, vanadium 0.02 percent, nickel 0.10 percent and copper 0.15 percent:

CEV = 0.20 + 1.55/6 + 0.32/5 + 0.25/15 = 0.20 + 0.258 + 0.064 + 0.017 = 0.539

The difference is 0.15, and its consequence is a change of welding procedure, not merely an adjustment of parameters.

Both examples are illustrative and are based on compositions within the standard limits, not on a specific inspection certificate.

From a number to a welding procedure

Technical literature contains a rule of thumb according to which steels with an equivalent below about 0.40 are considered readily weldable without special measures, in the 0.40-0.60 range preheating may be necessary, and above 0.60 it is necessary, often together with post-weld heat treatment. This is, however, a preliminary guideline, not a provision of the standard.

EN 1011-2 approaches the subject differently and more precisely: the preheating temperature depends not only on the chemical composition but on four parameters together.

Diffusible hydrogen content is covered by five classes: scale A is more than 15 ml per 100 g of weld metal, B from 10 to 15, C from 5 to 10, D from 3 to 5, and E below 3. Low-hydrogen filler materials make it possible to lower the required preheating temperature, or to dispense with it entirely, for the same parent material.

In method B, the standard gives the relationship as a closed formula in which the preheating temperature is a function of the CET equivalent, combined thickness, hydrogen content and heat input. Method A is based on nomograms read for a given hydrogen class and equivalent value.

The practical conclusion is clear. The carbon equivalent alone is not enough to plan a welding procedure. It is one of four inputs to the calculation, and the only one that comes with the material.

How the carbon footprint of a product is measured

Since CEV does not describe emissions, it is worth knowing what does, as questions on this subject appear more and more often in requests for quotation.

An environmental product declaration, prepared according to EN 15804 and ISO 14025, contains a set of environmental indicators, including global warming potential expressed in kilograms of carbon dioxide equivalent per tonne of product. For example, one European producer's declaration for hot-rolled strip gives a value of 2,080 kg CO2 equivalent per tonne for the stages from raw material extraction to the factory gate.

A narrower tool is ISO 14067, which measures only the carbon footprint of a product. At regulatory level, emissions embedded in imported goods are covered by the Carbon Border Adjustment Mechanism, CBAM, operating in its definitive phase since 1 January 2026.

Three documents, three different purposes. None of them uses the CEV value, and the value from an inspection certificate should never be quoted as environmental data.

What this means when buying material

For a plant engaged in welded production, three things matter. First, the grade and thickness determine the maximum permissible equivalent value, so already at the specification stage it is known within what limits the material lies. Second, the actual value for a batch comes from the inspection certificate and is often significantly lower than the limit. Third, for structures requiring welding procedure qualification, it is advisable to ensure consistency of supply within a narrow range of chemical composition, because a change of supplier under the same grade designation may mean a different CEV value and different process requirements.

Available steel grades, coating types and formats of coils, slit strip and sheets are presented in the HEN-STOL steel sheet offer, with separate pages for galvanized steel sheet, cold-rolled steel sheet and hot-rolled steel sheet.

See how we can help

HEN-STOL helps select material taking into account the requirements of the welding procedure. Send us the specification and a description of the application, and we will check availability and propose the most favourable option.

+48 664 700 160hs@henstol.pl

Frequently asked questions

Sources

1. EN 10025-2, Hot rolled products of structural steels - non-alloy structural steels
2. EN 1011-2, Welding - recommendations for welding of metallic materials, arc welding of ferritic steels
3. EN 10204, Metallic products - types of inspection documents
4. British Steel, product data sheets S235JR, S275JR, S355JR and S355J2
5. EN 15804 and ISO 14025, environmental product declarations
6. European Commission, Carbon Border Adjustment Mechanism