Do you use flux when you forge weld?
Is non-flux welding more difficult.
Good morning, FF Drew. Welcome to the addiction.
The answer might depend on more than one thing, but the first of which would be, what is your forging experience? And a close second would be what tools are you going too be using?
Forge welding is what's called, diffusion bonding, where you get the atoms/molecules close enough together that they share electrons. To do this, you need to avoid the outer layer molecules from developing a layer of oxides (scale) that will prevent them from getting close enough, which is why you heat the steel in what's called a 'reducing' atmosphere. This is also the main thing that the flux is for, to prevent scale formation.
Using a hydraulic press to set the weld makes it very easy to get the molecules close enough very quickly, so the chance on developing an oxide layer is relatively small. I'm lucky enough to have access to a 25-ton press and I rarely, if ever, use flux. Having the large dies and all that power allows one to 'squish' the steel enough to get the molecules really close in one shot, even at what some folks would consider temperatures that are too low (I've done a billet when my forge was topping out at 1880F).
However, if you are setting the welds by hand, then I'd say you should get used to flux, because it's hard to 'squish' the whole billet in one shot. When using a hammer, you squish where the hammer face strikes, but all around that area, the layers will tend to separate, allowing air in and oxides to form, and as mentioned before, the flux will prevent this scale formation.
You can do things to make this easier, like making sure all the pieces are cleaned/ground smooth and fit together as close as possible. But if I were doing everything by hand, I'd probably just get used to using flux and replacing the floor occasionally (depending on how often you weld), or buy a removable ceramic plate or fire brick to catch the flux when using flux, or having a dedicated welding forge.
