Pre-Heating is, as a subsidiary process of welding work, before
and during welding, the operation of heating the part of the
base metal on which welding metal is melted and attached. The
fundamental purpose of putting in operation of Pre-Heating is
to prevent from cracking at Welding zone and the Heat affected
The effects of Pre-Heating when welding are as below
1) Make heat loss decrease so that the hardness level of the
material can decrease by slowing the cooling velocity of welding
2) Repress one-side sitting of impurities can make cracking.
3) Make thermal transforming volume by welding derease.
4) Make easy to emission of hydrogen and repress the happening
of the crack by hydrogen..
5) By eliminating water on the base metal on which welding metal
is melted and attached, make welding metal prevent from incoming
hydrogen in the welding metal.
6) Make easy for welding metal to melt and attach.
There is Interpass Temperature related to Pre-Heating.
The welding part completed through Multi-Pass is affected
by the remaining heat of the last pass.
Interpass Temperature is the temperature of base metal at
welding part heated by the welding heat of the last Pass,
just before the Multi-Pass welding raises Arc.
If Interpass Temperature is beyond 200℃, hardness and impact
value may drop, and for stainless steel it happen to raise
Weld Decay. So Interpass Temperature should be below the appropriate
So, Interpass Temperature is presented as maximum temperature,
however, always minimum Pre-Heating temperature should be
kept as well, and this should be applied when temporary welding
of assemply cast, maintenance welding and Gouging as same
as the conditions of main welding.