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From the chemistry of the steel determine the carbon equivalent using the following formula:
Carbon Equivalent = C + Mn/4
If Carbon Equivalent exceeds 0.58, the steel may be crack sensitive and should be preheated before welding in ambient temperatures below 40° F.
Example: 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 40° F to 80° F, is preheat necessary?
Carbon Equivalent = 0.25 + (0.70/4) = 0.425
It is not necessary to preheat this particular steel before welding it.
Let's look at another example: Carbon content = 0.20; Manganese = 1.60
Carbon Equivalent = 0.20 + (1.6/4) = 0.60
This steel should be preheated, particularly for early morning welding.
Why does preheating prevent cracking? It slows the cooling rate and reduces the amount of Austenite retained as the weld cools. This prevents micro-cracking. Other alloying elements and pipe wall thickness may also influence when joints of high strength pipe should be preheated.
Offshore welded pipes should meet the requirements of the main body of the specification "Steel Line Pipe for Gas and …