Carbon Equivalent (CEV): Formula, Interpretation and Limitations

Carbon equivalent converts the effects of carbon and several alloying elements into a single composition-based index. In carbon-manganese and low-alloy steels it is widely used as an indicator of hardenability and susceptibility to hydrogen-assisted cold cracking during welding.

CEV is useful — but it is frequently overinterpreted. It is not a complete weldability calculation, and there is no universal CEV number below which every steel is automatically “safe to weld.”

The common IIW CEV formula

A widely used formula is the IIW carbon equivalent:

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

All elements are entered as mass percent.

Example

For an illustrative carbon steel with:

• C = 0.12%
• Mn = 1.30%
• Cr = 0.15%
• Mo = 0.05%
• V = 0.03%
• Ni = 0.20%
• Cu = 0.20%

CEV is approximately:

0.409

The numerical result is normally reported without the percent sign as a compositional index, although MTCs often display it as a percentage-style value.

What does a higher CEV generally indicate?

Other factors being equal, increasing carbon/alloy contribution generally increases steel hardenability. During welding, faster cooling can then form harder microstructures in the heat-affected zone, increasing susceptibility to hydrogen cracking.

CEV therefore helps engineers assess how much attention may be needed for:

• preheat;
• heat input;
• interpass temperature;
• hydrogen control;
• material thickness;
• joint restraint;
• welding procedure qualification.

But “other factors being equal” is crucial.

Why CEV is not a complete weldability verdict

TWI identifies several factors that influence hydrogen cracking, including:

• material composition;
• section thickness;
• heat/arc energy;
• degree of restraint;
• hydrogen from the welding process/consumables.

A steel with a relatively low CEV can still present welding difficulties in a thick, highly restrained, low-temperature joint with poor hydrogen control. Conversely, a higher CEV material can often be welded successfully under a qualified procedure with appropriate controls.

That is why statements such as “CEV below 0.40 means no preheat” are unreliable unless they come from the applicable welding code/procedure and its defined conditions.

Heat analysis or product analysis?

The answer depends on the governing standard and project specification.

Some orders set a CEV limit on the heat/ladle analysis. Others also control CEV calculated from product analysis and may restrict how far the product value can deviate from the heat value.

When auditing an MTC, therefore, check:

1. Which chemical analysis is being shown?
2. Which analysis does the PO use for the CEV limit?
3. Does the product standard define a formula and maximum value?
4. Does a project specification add a stricter limit?

Do not recalculate CEV using a formula chosen from memory if the governing specification defines another carbon-equivalent method.

CEV versus Pcm

CEV/CEIIW is only one carbon-equivalent formula. Modern low-carbon, microalloyed steels — especially line-pipe steels — often use Pcm because it was developed for lower-carbon compositions where the traditional IIW formula is less representative.

The two numbers are not interchangeable and should not be compared against the same acceptance limit.

See also: CEV vs Pcm: Which Carbon-Equivalent Formula Applies?

Is CEV meaningful for stainless steel?

Usually not in the way it is used for carbon and low-alloy structural steels.

The conventional IIW CEV equation was developed around carbon/low-alloy steel hardenability and hydrogen-cracking behavior. Austenitic stainless, duplex stainless and highly alloyed materials have different welding metallurgy and different controlling mechanisms.

For example, stainless/duplex weldability may depend on issues such as:

• ferrite/austenite phase balance;
• intermetallic precipitation;
• heat input and interpass control;
• filler selection;
• corrosion resistance;
• solidification cracking.

Applying the carbon-steel CEV formula to a highly alloyed stainless composition can produce a number, but that number is not a meaningful universal weldability index for the alloy family.

If a client specification explicitly requests a particular calculation, follow that requirement; otherwise do not impose CEV on stainless steel simply because it appears routinely on carbon-steel MTCs.

Common mistakes

Treating CEV as a material grade

Two heats of the same grade can have different CEV values within the permitted chemistry range.

Assuming one acceptance threshold applies everywhere

Maximum CEV is specification-, thickness-, grade- and application-dependent.

Using CEV alone to set preheat

Preheat determination also depends on thickness, hydrogen level, heat input, restraint and the applicable welding method/code.

Mixing CEV and Pcm

They are different empirical formulas with different intended application ranges.

Applying CEV blindly to stainless/duplex

The conventional formula is not a general alloy weldability score.

MetalMate takeaway

CEV is a composition-based weldability/hardenability indicator for carbon and low-alloy steels, not a universal welding pass/fail test. Calculate it using the formula required by the governing standard, then interpret it together with thickness, heat input, hydrogen, restraint and the qualified welding procedure.

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
• TWI, Weldability of materials — carbon manganese and low alloy steels: https://www.twi-global.com/technical-knowledge/job-knowledge/weldability-of-materials-carbon-manganese-and-low-alloy-steels-019
• TWI, Welding of HSLA steels: https://www.twi-global.com/technical-knowledge/job-knowledge/welding-of-hsla-steels-098