I want to reinforce the bluesman's variable: some of these forge designs are assembled so that the burner blows straight across the diameter of the tube. This will produce localized hot spots and can wear out the liner earlier than necessary. I recommend aligning the burner so that it enters the forge on a tangent and swirls the flame around the inside. The forge heats faster and it reduces the hot spot problem and early replacement of the liner.

But, there are a bunch of ways that people build forges and they will all get metal hot for the most part.