Hot pass is a term which originated with pipeline welding. In pipelines the root pass is mostly applied using downhill welding and cellulosic electrodes. Now cellulosic electrodes need high misture to the tune of 5-10% if I remember correctly to hold the cellulose together, which means the hydrogen in weld metal of root pass is also very high. The second pass in such cases is deposited within a very short time of depositing the root pass typiclly three to five minutes. So the root is still hot when the first fill pass deposited. So the root pass hydrogen gets time to diffuse out, the root pas gets tempered, and defects in the root pass get burned out. That is why the first fill pass is normally called hot pass because it is deposited when the root pass is still hot.

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