Welding Inspection & Metallurgy API ICP Self Study Notes

Typically,
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Steels with a CE less than 0.35 require no preheat.
Steels with a CE of 0.35 – 0.55 usually require preheating, and
Steels with a CE greater than 0.55 require both preheating and a PWHT.
However, requirements for preheating should be evaluated by considering
other factors such as hydrogen level, humidity, and section thickness.