Carbon Equivalent (CE) & Weld Preheat Calculator
Carbon equivalent calculator for steel weldability: IIW CE and Ito-Bessyo Pcm from the mill certificate chemistry, hardenability rating, and a recommended preheat temperature band from CE and combined plate thickness for low-hydrogen welding.
How it works
CE(IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15; Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B; CET = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40
The carbon equivalent collapses a steel's alloy chemistry into one number that tracks hardenability — how readily the heat-affected zone transforms to brittle martensite as the weld cools. Higher CE, thicker sections (faster heat sinking) and more diffusible hydrogen together raise the risk of hydrogen-induced cold cracking hours after welding, and preheat counters all three by slowing the cooling rate and letting hydrogen escape. The IIW formula is the general-purpose measure written into AWS D1.1 and most structural codes; Pcm was developed for modern low-carbon microalloyed steels where the IIW formula overstates the risk, and CET drives the EN 1011-2 preheat equation. The bands here are conservative starting points — the governing WPS and the material specification always take precedence.
Worked example
A 25 mm S355 plate at 0.18% C, 1.2% Mn, 0.1% Cr, 0.02% Mo, 0.01% V, 0.05% Ni and 0.1% Cu has CE = 0.18 + 0.200 + 0.026 + 0.010 = 0.416 — fair weldability — so a 100–125 °C preheat with low-hydrogen consumables is the safe start; Pcm is 0.263.
Frequently asked questions
Which formula should I use?
IIW CE for carbon-manganese and low-alloy steels with C above about 0.12%; Pcm for low-carbon HSLA, TMCP and pipeline steels; CET when working to EN 1011-2. If the material spec quotes a maximum CE, that is the one the mill controls to.
Does preheat replace low-hydrogen practice?
No — dry, low-hydrogen consumables reduce the hydrogen available to cause cracking, and preheat reduces hardness and gives hydrogen time to diffuse out. Thick, restrained or high-CE joints need both, and sometimes post-heating or PWHT.
Related calculators
Guidance values from published welding references (AWS D1.1 / EN 1011 style preheat bands, typical deposition efficiencies). Always follow the qualified welding procedure specification (WPS), the consumable data sheet and the machine nameplate; verify preheat and interpass temperatures with a contact thermometer or temperature crayon.