One forge is located in the Riken on the map "Desolation Point". It is one of at least two places where one may craft Arrowheads, Improvised Knife. or an Improvised Hatchet. A second is in the Forlorn Muskeg in the back left hand corner of the map. The furnace below any forge may also be used like a fire.
Metalwork can be done at the forge if possessing a Heavy Hammer, and by bringing the forge up to at least 150 degrees Celsius/270 degrees Fahrenheit. To get the forge up to this high level of heat, at least five pieces of Coal will be needed. As coal may not be added immediately to a fire, bringing a piece of Cedar Firewood, Fir Firewood, or a Firelog would be ideal. Forging can be maintained as long as this heat is maintained. Please note that the temperature of the forge/fire does not increase when adding wood to the fire after coal has already been placed in the furnace. You will also need Scrap Metal.
With the new 'Interloper' difficulty introduced in v.364, tools can no longer be found; therefore, the Riken is almost entirely essential to survival on this difficulty, as without tools, a character can only last so long.