Interpass temperature refers to the temperature of the steel just prior to the depositing of an additional weld pass. It is identical to preheat, except that preheating is performed prior to any welding.

When a minimum interpass temperature is specified, welding should not be performed when the base plate is below this temperature. The steel must be heated back up before welding continues.

A maximum interpass temperature may be specified to prevent deterioration of the weld metal and heat affected zone properties. In this case, the steel must be below this temperature before welding continues. "