depending on material and thickness. But is is rare under normal circumstances. Except some rare occasions, no preheating has ever been necessiated in my career. And post heating - unlss you call burning the pipes in furnace to remove the residues before solvent clean/pickling as post heat, never.
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People say between two opposed opinions the truth lies in the middle. Not at all! Between them lies the problem, what is unseeable,eternally active life, contemplated in repose. Goethe
It depends upon the alloy of the pipe, and the temperature of the site. They had to preheat the Alaska pipeline to get the metal to a safe welding temp. If a weld cools too quickly, it could crack. A post heat would allow the stresses to relax in a more uniform nature. Soft metals don't care too much, hard, or brittle metals, like cast iron, require a lot of experienced technique.
welding
Re: welding
