Carbon equivalent and weld preheat

Why one number for weldability

Hydrogen-assisted cold cracking in the HAZ needs three things at once: a susceptible (hard, martensitic) microstructure, diffusible hydrogen, and tensile stress. The first of those is governed by hardenability, which scales with carbon and the alloying elements that shift the CCT diagram. Carbon equivalent bundles that hardenability into a single figure so it can be compared against a threshold.

The three formulas

The IIW carbon equivalent is the most widely quoted, suited to carbon and C-Mn steels:

For low-carbon micro-alloyed steels the Ito–Bessyo cracking parameter weights carbon more heavily and tracks behaviour better:

The EN 1011-2 preheat route uses CET:

From carbon equivalent to preheat

Two mainstream routes turn the chemistry into a preheat temperature. AWS D1.1 offers a hardness-control and a hydrogen-control method keyed to CE/Pcm, restraint and hydrogen category. EN 1011-2 Method B computes a preheat from CET, combined plate thickness, diffusible hydrogen and arc energy (heat input). Both are screening tools — high restraint, thick sections and wet conditions push the requirement up.

Putting it to work

Compute the carbon equivalent from the actual heat composition on the mill certificate, not the spec midpoint — the difference can cross a preheat threshold. Then size preheat with the method that matches your code, and pair it with hydrogen control and, where required, post-weld heat treatment.