The most common carbon equivalent (CE) formulas to evaluate weldability depends if the metal is an alloy steel or modern carbon steel. Here are both common CE equations;
for low alloy steel, elements are expressed in weight percent amounts;
CIIw = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
for modern low carbon steels or microalloy steels, elements are expressed in weight percent amounts;
Also from the Welding Journal for low carbon, microalloyed steels, the Ito-Besseyo carbon equivalent
Ceq = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 +
5*B expressed in weight percent amounts
For CE%<0.14 Excellent weldability, no special precautions necessary
0.14<CE%<0.45 Martensite is more likely to form, and modest preheats with low hydrogen electrodes become necessary
CE%>0.45 Extreme complications, weld cracking is very likely, hence preheat in the range 100-400¦C and low hydrogen electrodes are required
