So. Basically, in real life, forges can only get so hot. This is actually the driving force in forge designs--you have to make a very hot fire, keep the hot in the fire, and be able to put metals in and take metals out. There is also the question of temperature control, which is important for making swords and anything that needs to bend and not break.
My suggestion is that much later in the development, we have the forges you construct be rated in terms of max temperature, which takes into account time and skill of the designer, the quality of materials used in construction, and ventilation. Then specific forges would be used to forge specific metals. Perhaps if temperature control makes it in, then you could have one forge used to shape metal, and another for tempering metal. This would require splitting up forging into seperate "mini-jobs" that would all come under the heading of "forging sword."
This makes those willing to go the extra mile to make steel swords have to work a little harder on the construction side. But it also allows attentive players to set up a more complex industry that would possibly streamline tasks. Players who don't care for this could just set up everything to be able to make steel swords, and take the hit in terms of construction times.
So. Tell me why this shouldn't go in.