Atmospheric Gas Forge with Preheating


Steve Bloom, IronFlower Forge
(click on any figure to see it in more detail)
A variant on the pipe forge but using the same principles is the box forge. The unit shown here is my general shop forge and it has served me well for over four years. The basic concept is a refractory lining with a specific flue chamber through which pipes extend. Those pipes will connect to the burners and air drawn through the pipes will be preheated, thus increasing the effectiveness of the forge. The design starts with two sheets of sheet steel formed into the shapes shown at the right. They represent the enclosure of the forge with the exception of the from and back surfaces. The ends are mirror images of one another and simply cap the forge ends. All of these pieces are held together with sheet metal screws.

The base of the forge is a 1" thick layer of mizzou castable refractory (3000F). The first layer of refractory bricks (again 3000F material) consist of two bricks laid end-to-end on the right side of the forge and aligned on the left side (right side as you look into the forge) of the refractory base. These bricks are oriented with the narrow end dimension perpendicular to the base. On the other side of the forge, approximately 4 inches from the left edge of the base are three half bricks, arranged as shown to leave a two 1/4 brick length gaps (~2.25"). Those gaps are critical. The arch that forms the roof of the forge is composed of pairs of bricks as shown (the actual forge doesn't have the center brick as shown in the diagrams -- so sue me!). The bricks have been shaped with an old horseshoe rasp to get the right angles needed to form the arch. As you can imagine, you have to rough out the arch, make measurements, then actually make the steel sheet that will enclose the arch. The gaps are the openings betwen the forge chamber and the flue chamber.

Note that I have indicated the preheat pipe runs in the diagram above. You will need to decide just where you want the burners, then cut holes in the sheet steel to accomodate the pipes. I elelcted to use 2" blackpipe and place the burners at the 1/3 and 2/3 positions down the length of the forge.

Once the arch is formed and the steel enclosure fastened together, you'll need to line the flue with castable refractory. I first flipped the forge up on end such that the front was down. I then laid an inch layer of refractory on the 'bottom' surface in the flue chamber (actually the front veritical flue chamber wall). When it had set up, I reversed the forge and laid a layer on the back veritcal wall of the flue chamber. The preheat pipes were then extended through the holes and coffer-dammed in place with kid's modeling clay. The forge was laid on its left side (preheat pipe entrances down) and a layer of refractory laid down. When it had set up, the forge was flipped (for the last time!) and the remaining wall of the fluw chamber was covered.

The door was a slip unit running on two angle-iron brackets and operated by a chain and lever arrangement. There is also a door on the back of the forge to allow handling those occassion long pieces. I'ld recommend using a swing door - the slide door has a decided tendency to stick due to heat expansion and warpage. I originally lined the doors with refractory brick cut into 1" thick slices, then when to castable when the bricks broke (could that have been due to hammering on a stuck door? nah..). Eventually, the castable also broke free and I'm just running them bare now.

The burner assemblies are described in the other (pipe forge) project write-up - the only difference here is that there is a 90 degree adapter used to connect the preheat pipes to the burners. To acheive damascus welding temperatures, I use a small (3" diameter) squirrel cage blower on the forward prepheat pipe, i.e., the forge is very close to not needing a blower to get to welding temperatures. I suspect that if I had used slightly larger pipes (say 2.0" rather than 1.5" pipes for the burner tops and preheat tubes, it would not have required the blower. In any event, I rarely use this forge that hot. It can acheive forging temps (light yellow) when running on less than 5 psi, so the overall unit is reasonably economical (8 to 10 hours on a 20lb propane bottle).

As you can see in the interior shot of the unit, it has had a lot of borax flux in it and the castable refractory has stood up well to the abuse. Overall, I'm pleased with the unit - but will try to build a pre-combustion chamber into the next forge to eliminate hot spots. If any of this is unclear, feel free to drop me a line (e-mail address available from the main page).