What is Carbon Equivalent (CE)?
Carbon Equivalent is an empirical value that converts the effects of various alloying elements (like Manganese, Chromium, and Molybdenum) into an equivalent percentage of carbon. By doing this, engineers can assess a complex steel alloy on a simple, single-number scale of weldability.
If the CE is low (typically under 0.35), the steel has excellent weldability and requires no special precautions. If the CE is high (over 0.45), the rapid cooling of the weld pool will form martensite, a hard, brittle grain structure highly susceptible to Hydrogen-Induced Cold Cracking (HICC).
The IIW and Pcm Formulas
There are several formulas for calculating CE, but two are dominant in structural codes like AWS D1.1 and API 1104. The International Institute of Welding (IIW) formula is the global standard for medium-carbon steels. For modern, low-carbon, high-strength steels (like pipeline steel), the Ito-Bessyo (Pcm) formula is more accurate.
- IIW Formula is used when actual Carbon content > 0.12%.
- Pcm Formula is used when actual Carbon content is ≤ 0.12%.
How to Calculate the IIW Carbon Equivalent
To calculate the CE, you need the Material Test Report (MTR) or Mill Cert for the steel plate, which lists the exact percentages of each chemical element.
The Formula: CE = %C + (%Mn / 6) + [(%Cr + %Mo + %V) / 5] + [(%Cu + %Ni) / 15].
Worked Example: A steel plate has the following chemistry: C = 0.15%, Mn = 1.20%, Cr = 0.20%, Mo = 0.05%, V = 0%, Cu = 0.15%, Ni = 0.15%. First calculate the fractions: Mn/6 = 0.20. (Cr+Mo+V)/5 = 0.25/5 = 0.05. (Cu+Ni)/15 = 0.30/15 = 0.02. Add them up: 0.15 + 0.20 + 0.05 + 0.02 = 0.42 CE. Because the CE is 0.42, it is in the 'fair' weldability range and will likely require a moderate preheat.
Determining Preheat Requirements
Preheating the base metal before striking an arc serves two purposes: it slows down the cooling rate (preventing brittle martensite formation), and it allows diffusible hydrogen gas to bake out of the weld joint rather than becoming trapped in the crystalline matrix.
Codes like AWS D1.1 Annex H use the calculated CE, combined with the thickness of the steel and the hydrogen level of the welding consumable (e.g., Low-Hydrogen E7018 electrodes), to prescribe an exact minimum preheat temperature, often ranging from 150°F to over 400°F.
Frequently asked questions
What is Hydrogen-Induced Cold Cracking (HICC)?
Also known as underbead cracking, HICC occurs hours or even days after the weld has cooled. It is caused by three factors occurring simultaneously: a brittle microstructure, high residual stress, and trapped hydrogen gas.
Do I need to calculate CE if I'm welding mild steel (A36)?
Yes. While A36 is considered mild steel, its chemistry can vary wildly from the mill. A thick piece of A36 with an unexpectedly high manganese content can crack if welded in freezing temperatures without preheat.
What happens if I skip the preheat on a high CE steel?
The rapid thermal shock of the weld will cause the heat-affected zone (HAZ) to harden. This localized hard zone will be brittle, and as the weld contracts during cooling, internal stresses will tear the grain boundaries apart, causing invisible subsurface cracks.
Can I use low-hydrogen electrodes instead of preheating?
Using low-hydrogen processes (like SMAW with E7018 or GMAW with solid wire) significantly reduces the risk of cracking, but it does not completely replace the need for preheat on high CE or highly constrained, thick joints.