When should steel be preheated before welding?
First, we need to determine the Carbon Equivalent:
Carbon Equivalent = C + Mn/4, if exceeds 0.58, then the steel may be crack sensitive and should be preheated before welding in ambient temperatures below 40 F.
Examples: If steel pipe having a carbon content of 0.25 and Manganese Content of 0.70, is to be welded in springtime temperatures, ranging from 40F to 80F, is preheat necessary?
Carbon Equivalent = 0.25 = 0.7/4 = 0.25 + 0.175 = 0.425. So it is not necessary to preheat this particular steel before welding it.
But, another example:
Carbon = 0.20
Manganese = 1.60
Carbon Equivalent = 0.2 + 1.6/4 = 0.60 , then this steel should be preheated particularly for early welding.
Why does preheating prevent cracking? Because it slows the cooling rate and reduces the amount of austenite retained as the weld cools. This prevents microcracking.
sources: Pipeine Rules of Thumb Handbook, E. W. McAllister, Editor, Elsevier, GPP.