Carbon equivalent (CE)

Carbon equivalent compresses a multi-element composition into one number that ranks how readily a steel forms hard, crack-susceptible martensite when it is welded and cooled. Carbon dominates hardenability, which is why it names the index, but manganese, chromium, molybdenum, vanadium, nickel and copper each contribute — some strongly, some only at the margin — and a single formula folds them together with weightings chosen to match how each element actually behaves under a welding thermal cycle rather than in slow furnace cooling.

There is no one carbon equivalent formula; there are several, built for different steel families and different failure mechanisms. CE(IIW), published by the International Institute of Welding, is the most widely quoted general-purpose version. Pcm (the Ito-Bessyo formula) weights carbon far more heavily and is the one generally preferred for the lower-carbon, higher-strength steels where CE(IIW) is a poor predictor. CET, defined directly in EN 1011-2, is the version that standard’s own preheat equation is built around. The three do not produce comparable numbers from the same composition, so a specification naming a maximum carbon equivalent has to name which formula the maximum applies to — a number without its formula attached is not verifiable.

The practical use of the index is as an input, not an output: it feeds directly into preheat temperature calculations, because a higher carbon equivalent means the joint needs to cool more slowly — a higher starting temperature — to avoid forming untempered martensite in the heat-affected zone. The same number also flags material where hydrogen cracking is a live risk, because susceptible microstructure is one of the three conditions that mechanism requires alongside hydrogen and tensile stress. A material certificate that omits an element the chosen formula needs — vanadium for CE(IIW), boron for Pcm — cannot be run through that formula honestly, and treating a missing element as zero understates the result for exactly the alloying choices that make hardenability worse.