C eqivalent potential

A measure of whether a steel can be easily welded is to determine the carbon equivalent (C_equiv or C^D_equiv) of the steel. There are a number of different equations that can be used for this, the most appropriate depends on the type of steel being considered. For low alloy C-Mn steels with a ferrite + pearlite structure the appropriate equation is:

(alloying additions in wt%). For modern low carbon steels then we need to use:

(alloying additions in wt%).
For the steel to be easily weldable then C_equiv needs to be less than about 0.4 or the C^D_equiv needs to be less than about 0.25. 
There are a number of different welding techniques that can be used for steels. Selecting the most appropriate technique depends upon the final product, plate thickness, geometry of the weld required.

The equivalent carbon content concept is used on ferrous materials, typically steel and cast iron, to determine various properties of the alloy when more than just carbon is used as an alloyant, which is typical. The idea is to convert the percentage of alloying elements other than carbon to the equivalent carbon percentage, because the iron-carbon phases are better understood than other iron-alloy phases.

Higher concentrations of carbon and other alloying elements such as manganese, chromium, silicon, molybdenum, vanadium, copper, and nickel tend to increase hardness and decrease weldability. Each of these materials tends to influence the hardness and weldability of the steel to different magnitudes, however, making a method of comparison necessary to judge the difference in hardness between two alloys made of different alloying elements.

Carbon equivalent (CE)	Weldability
Up to 0.35	Excellent
0.36–0.40	Very good
0.41–0.45	Good
0.46–0.50	Fair
Over 0.50	Poor