For a butt joint V it out on both sides and do many, many passes. Maybe have a few guys with rosebud torches working heat into it as you go. Big rods make it go faster (https://youtu.be/j61ezBX-EyA). For a lap joint it's the same idea, you're going for a great big fillet. But if you have a gigantic press at your disposal there are other options. For inspiration: https://youtu.be/k_LA_R4ifYk
The idea behind forge welding is you get both parts nearly molten (e.g. "welding heat") then your hammer blow (or the pressure from a huge press) puts enough energy into the weld area to briefly melt it.
Also, hot rivets might be a better option than welding if you can get away with it.
The idea behind forge welding is you get both parts nearly molten (e.g. "welding heat") then your hammer blow (or the pressure from a huge press) puts enough energy into the weld area to briefly melt it.
Also, hot rivets might be a better option than welding if you can get away with it.