Tempering in a small gas forge

[evfreek]

I wanted to temper a hammer punch that was forged for me as a demo. It is made of a piece of S-7 rod that I bought on EBay. When I got home, I snap temtered it with a propane torch, and the end still skated a file. So, it certainly was air hardening.

Tempering S-7 requires several minutes at at least 1000F. This is difficult to do in the traditional manner (letting the colors run, then quenching). This temperature is well past light blue, and there is a grey sheen oxide coating, but not scale. I tried using an old toaster oven. This was just too scary. To get to 1000F, I disabled the thermostat and filled in both sides with rock wool. This would supposedly protect the plastic sides which held the electrical connections to the quartz tube elements. The connectors on one side of the oven were connected to the hot side with plastic/ceramic insulators, so I added a tin can heat shield inside the oven compartment, and wrapped the whold contraption in a fiberglass blanket and stuck a K thermocouple in the side. It did reach 1000, but it was smoking and popping like popcorn. The sensitive side of the oven was getting so hot that the plastic sizzled water. The front selas around the glass window disintegrated, and the window came loose. But it did get to 1000 for a few minutes. Not good for pratical use, and I feared that the case could become energized.

Trying another approach, I decided to throttle down my mini-forge. This is made out of a 1 gal paint can with homemade refractory and a 1/2" naturally aspirated pipe burner. It works great for forging and occasional welding. I figured that I could throttle the flame down enough so that it would do 1000-1150F. I used a piece of 3/8" steel tubing as a thermocouple sheath along with single bead insulators on my K thermocouple and fired it up. There were two problems. First, it was very difficult to maintain 1000F with the regulator. There was a lot of pulastion at the low pressure (single stage regulator). The flame would flash back to burn in the venturi if I turned it down too low. Then, it would have to be reignited, since 1000F is too low to relight. Second, the opening, although scaled approximately to the burner rules (I plugged the back with a piece of refractory), lost too much heat. As a result, the tool, even though it fit completely in the forge, was cold on the end. Probing around with the thermocouple showed a nearly 250F variation in temperature along the tool Furthermore, the steel tubing did not help. Since it transferred appreciable heat down its length, the thermocouple read low by another 100-150 degrees. This made things kind of tough. At this temperature, the steel had a dim but visible dark red glow, so I knew it was in the ballpark.

After about 30 minutes, I shut off the gas and let it cool. One nice thing was that ther was no overshoot, but why should there be? I guess I was too used to the heat the other end, let the colors run method. The tool was covered with a fine grey sheen, and a file could mark the tip but not cut.. So, it sort of worked out. Probably the tip did not get much more than 1200F, at least not for long.

This was too hard! I think I need one of those heat treating furnaces with a door on lever hinges, like the Johnaon ones. How are the vents sized on these furnaces? 7 X the burner area? There is not much design data on the Internet. Only for the blacksmithing style forges. Maybe a kiln design would be appropriate. There are a couple of books at the library. Also, ithe more targeted configuration might work better. Burner spirals along the floor under a kiln shelf (or tile, if the temperautres are sufficiently low). Vent on the top keeps the cold spot away from the piece. Tile helps spred and make the heat more uniform. Another thing that I read was that at temperatures below 1100F, convection is the dominanant heat transfer mechanism, not radiation. So a fan is required to keep the temperature uniform. With a larger cavity, the burner could be turned up closer to its natural range. Does this sound like its worth the effort?