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. Typical examples of welding procedures and their applications are given below:

Fusion Welding
Spot Welding
Friction welding