By Robert Colman
Preheating is an important prep- aratory operation for certain
welding jobs. As the name implies, it
is the process of raising the temperature of a part before welding it.
This preheat temperature has to be
maintained throughout the welding
procedure. In some cases it might be
necessary to continue heating the
part during welding, but sometimes
the heat input from the welding
itself maintains that temperature.
The interpass temperature, which
is the base metal temperature
maintained between the first and
last welding passes, should not fall
below the preheat temperature.
Preheat is recommended for certain
jobs for several reasons. Among
these is that it helps stave off moisture from the base metal and weld,
which in turn makes the completed
part less susceptible to hydrogen
cracking. It also reduces shrinking
stresses in both the base metal and
the weld joint. Generally speaking,
preheating helps ensure that the
weld has the mechanical properties
it was designed to achieve.
Essentially three methods commonly
are used to preheat joints: propane
(gas) torch, induction heating, and
electrical resistance heating.
“Torch heating is certainly the
most portable of the three methods,”
said Scott Fong, owner and general
manager of KASI Technologies Inc.,
an Edmonton company that specializes in renting industrial heating,
welding, and bolting equipment.
“Basically, torch heating is like
using a big barbecue lighter. It’s just
a torch and you simply monitor its
temperature with temperature-indi-
cating sticks. Once you see the sticks
melt, you know you’ve reached the
minimum preheat temperature and
you start welding.”
While Fong appreciates the port-
ability and affordability of this
method, he also notes that it is, by
its very nature, not as accurate as
the other technologies available.
“It is very easy to overheat the
steel past your maximum interpass
temperatures,” he explained. “You
also don’t get even heating all the
way around the surface. You’ve got
to manipulate the torch a lot. Once
the welder starts welding, he may be
able to keep the weld at the min-
imum preheat temperature via the
welding process itself; however, on
thicker sections of steel, the heat
tends to bleed out very quickly. In
situations like that, it’s important
to have a more controlled preheat
The most common methods for
achieving a more controlled preheat
temperature are induction heating
42 | AUGUST 2017 www.canadianfabweld.com
Welding preheat practices:
choose the right tool for your job
While using a torch to preheat your welds may seem simple enough to
suit your job, is it doing the job correctly?
Preparing an electrical resistance preheat on a large piece of machinery. Image courtesy
of KASI Technologies Inc.