To control the mechanical and microstructural properties of weldments, interpass temperature is just as important as, if not more important than, preheat temperature. Yield and ultimate tensile strengths of the weld metal depend greatly on interpass temperature. A high interpass temperature can reduce weld strength and at the same time result in a finer grain structure and improved Charpy V notch-toughness transition temperatures. However, when interpass temperatures exceed approximately 500 F, this trend is reversed. In fact, the American Welding Society Position Statement on the Northridge Earthquake recommends that interpass temperature not exceed 550 F when notch toughness is a requirement.
There are other times when a designer may want to limit the maximum interpass temperature. For example, if he expects a minimum strength level for a particular component that could experience extremely high interpass temperatures, due to its size or welding procedures, he would specify a maximum interpass temperature. Otherwise, weld strength may be unacceptably low. A maximum interpass temperature is also necessary for quenched and tempered steels. Due to their heattreating characteristics, engineers must control interpass temperature within limits in order to provide adequate mechanical properties in the weld metal and HAZ.