It's used to remove any residual moisture from the flux on the rod.
I seem to remember reading that the water in the flux can cause hydrogen
inclusions in the weld, but don't quote me on that. Charles will know
the right answer.
Some rods must be kept in a moisture free environment if out of their
packaging for any length of time.
To meet code, for example (at least according to the manual we use at
work), 7018 rod must be out of it's sealed package no more than 4 hours.
It takes considerably more heat than a rod oven will provide to drive
off the moisture, so prevention is the fix.
Other rods, like the 6011, actually require a bit of moisture in the
flux to work correctly, and will give all sorts of greif if too dry.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.