Forges are cool because they're a great way to get twice as much stuff back when you craft metallurgy items.
How to Obtain
You can mine them from generated bunkers and dungeons, craft them, or trade for them in market worlds.
How to Use
Hook your forge up to steam power and then stand right next to it while you craft metallurgy items. Viola! Double your return!
List of items you can double up on by crafting them near a forge: