Carbon equivalent calculator
One number that says how the steel will behave when it cools: whether the heat-affected zone hardens, and whether preheat is needed before the first run.
Chemical composition, % by mass
Welding conditions — used for the preheat figure
Two formulae, two uses
CE (IIW) = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. This is the classic index quoted on certificates and in steel standards; it works well for carbon-manganese steels above about 0.18 % C.
CET = C + (Mn+Mo)/10 + (Cr+Cu)/20 + Ni/40. This is the one EN 1011-2 method B uses for the preheat calculation, and it fits modern low-carbon fine-grained steels better.
Reading the result
Below 0.40 % CE, ordinary structural steel welds without preheat. From 0.40 to 0.60 the risk of a hard heat-affected zone is real and preheat depends on thickness, hydrogen and restraint — feed the CET into the preheat calculator. Above 0.60 preheat and controlled cooling are not optional, and post-weld heat treatment usually enters the procedure.
The composition has to come from the mill certificate for that heat, not from the grade designation. Two plates both stamped S355J2 can differ by 0.05 % C and 0.3 % Mn and still be within specification — which is enough to move CE across a decision boundary.
Updated: