1. Reduce the risk of hydrogen crack and martensitic grains structure
2. To eliminate heat affected zone hardness due to sudden cooling cause martensite structure. By pre heating rate of cooling will be less that gives toughness and ductility
-Purpose of the preheat is to remove moisture,slowdown the colling rate to ensure the hydrogen has diffuse out from the weldment.it give the resistance from hydrogen cracking.preheat can be determine by the thickness of the material and carbon equivalent. Interpass temperature means the temperature during welding.the relations between interpass temperature and preheat temperature is when perform welding process, we need to contol the interpass temperature not lower than the preheat temperature and not exceed to the maximum interpass temperature.as example in the wps state that for material F22(low alloy) the preheat temperature 150celcius and the max. interpass tmperature is 270celcius, the welding process should perform in range of the preheat temperature and maximum interpass temperature.if exceed the max. interpass temp. it will induced the problem at the HAZ region.