Carbon equivalent formula for welding
Carbon equivalent (CE) folds the alloying elements in a steel into one number, as if they were extra carbon. Every element is a weight percent from the heat analysis, so 0.20% carbon goes in as 0.20.
- IIW carbon equivalent: CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15.
- Pcm (Ito-Bessyo): Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B.
The IIW formula is the one most welding courses teach and the one most people mean by "carbon equivalent". Pcm adds silicon and boron and weights the other elements differently, which is why it suits the lower-carbon, microalloyed steels that the IIW formula was not built around. Both formulas are published by TWI and the Australian Welding Institute. Confirm the formula your governing code or WPS names before you rely on a result.
How the calculator works
- Copy the weight percent of each element from the mill test report. Carbon is required; leave any element the report does not list blank and it counts as zero.
- The tool divides each element by its factor, adds the terms and shows each step, so you can check it against a hand calculation.
- Read the two results as separate scales. A Pcm of 0.28 and an IIW CE of 0.41 can describe the same steel.
What a higher carbon equivalent means for weldability
A higher carbon equivalent means the steel hardens more easily as the weld and the heat-affected zone cool. Harder, more brittle microstructure plus hydrogen in the weld plus restraint on the joint is the recipe for hydrogen cracking, and that is the problem carbon equivalent is a shorthand for. The higher the number, the more hardenable the steel, the less forgiving it is, and the more careful the procedure has to be about heat input, cooling rate and hydrogen.
A low number does not mean a steel is safe to weld cold, and a high number does not mean it cannot be welded. It is one input, and it ignores thickness, joint restraint, hydrogen level and process.
Why the code or WPS sets preheat, not a calculator
Preheat slows cooling so hydrogen can escape and the structure stays tougher. How much you need depends on composition, section thickness, the hydrogen level of the process and consumable, restraint and the heat input you run. Codes and procedure qualification records fold those together, and the result is written into the WPS. That is why this page stops at the number: the temperature is set by the document that governs your job, and an inspector checks the work against that document. If you are studying for the CWI exam, the useful skill is knowing which factors push preheat up and where to look it up, not memorizing a temperature.
Worked example 1: a conventional carbon-manganese steel
Heat analysis, in weight percent: C 0.20, Si 0.25, Mn 1.00, Cr 0.10, Mo 0.03, V 0.01, Ni 0.10, Cu 0.15, B 0.
- IIW: 0.20 + 1.00/6 + (0.10 + 0.03 + 0.01)/5 + (0.10 + 0.15)/15 = 0.20 + 0.1667 + 0.0280 + 0.0167 = 0.4113.
- Pcm: 0.20 + 0.25/30 + (1.00 + 0.15 + 0.10)/20 + 0.10/60 + 0.03/15 + 0.01/10 + 0 = 0.20 + 0.0083 + 0.0625 + 0.0017 + 0.0020 + 0.0010 = 0.2755.
Manganese is the biggest contributor after carbon, which is typical for this kind of steel.
Worked example 2: a low-carbon microalloyed steel
Heat analysis, in weight percent: C 0.08, Si 0.30, Mn 1.40, Cr 0.03, Mo 0.01, V 0.04, Ni 0.03, Cu 0.02, B 0.0005.
- IIW: 0.08 + 1.40/6 + (0.03 + 0.01 + 0.04)/5 + (0.03 + 0.02)/15 = 0.08 + 0.2333 + 0.0160 + 0.0033 = 0.3327.
- Pcm: 0.08 + 0.30/30 + (1.40 + 0.02 + 0.03)/20 + 0.03/60 + 0.01/15 + 0.04/10 + 5 × 0.0005 = 0.08 + 0.0100 + 0.0725 + 0.0005 + 0.0007 + 0.0040 + 0.0025 = 0.1702.
Notice that boron is tiny in weight but is multiplied by 5 in Pcm, so a few thousandths of a percent still count. These are original made-up analyses for practice, not data from a real heat or an exam.
Carbon equivalent questions candidates ask
What is the difference between IIW carbon equivalent and Pcm?
They are two scales for the same idea. IIW uses carbon, manganese, chromium, molybdenum, vanadium, nickel and copper. Pcm also counts silicon and boron and uses different divisors, so its numbers run lower for the same steel. Use the formula your code or WPS names and never compare one scale against the other.
What carbon equivalent is too high to weld without preheat?
There is no single number to memorize. Whether a steel needs preheat, and how much, depends on composition together with thickness, hydrogen level, restraint and heat input. The governing code and your WPS set it, so read the preheat requirement there for the job in front of you.
Do I need to memorize the carbon equivalent formula for the CWI exam?
Part A is closed book, and metallurgy and welding-related calculations are both Part A content areas. Carbon equivalent sits where the two meet, so know the IIW formula and what a higher result means. The CWI Part A exam guide shows where the points are, and the CWI math calculator drills the rest of the Part A arithmetic.