Even Heating Question - Too even

[reefera4m]

I've been working on Venturi Propane forge designs that more evenly distributes the heat within the forge. My 2nd forge has pretty mcuh accomplished this though I have some ideas for furthrer improvements.

So my problem is not with acheiving even heat in my forge so much as with acheiving too even heat. I find, like I'm sure everyone has experienced, that thinner parts of the blade heat faster than thicker parts when heat is evenly distributed throughout the forge.

In my case, where I made a blade with a distal taper from ricasso to tip, the thinner part of the blade naturally got hotter eariler that the thicker part around the ricasso as well as the tang (full tang). Since my forge is a 'Venturi' design I tried to adjust/reduce the temperature on the burner over the thin part of the blade by restricting the opening of it's venturi cone. This met with only partial success. I couldn't restrict the air enough to lower the heat so that the thin part of the blade heated at the exact rate of the thicker part. Closer, but still not exact. Further restricting the air produced a carburizing flame - too much propane not enough air). I finally ended up simply moving the blade around in the forge, placing the tang end of the blade where the most heat seem to be and making slight adjustments it the position of the blade in the forge. It didn't take much to heat the blade evenly, but definitely something that took paying attention to.

In the future I'm going to try individually regulating each burner by regulating the propane to each nozzle. I'll have to add some sort of valve just upstream of the nozzles but since the venturis operate well between about 6 psi and 12 psi, I think this may work. The only risk is that the differential pressure may cause the lower pressure burner to 'back-up'. I don't expect this will happen as the distance between the burners (4 inches), the size of the interior of the forge, and the lower pressure to begin with, will not produce enough over-pressure inside the forge, particularly if I keep one end open. Only way to know for sure is build it and try it.

Can anyone tell me how they deal with this type of eneven heat problem? Do commercial propane forges, those with flame diffusers, allow for creating differential heat patterns?

ps, I posted this question under the thread on scale problems but thought it should be treated seperately.

[DaveRuhlig]

In my little forge I just move the blade around quite a bit to try to get an even heat. I've thought about this before and I think if I had a bigger forge I think I would try putting a brick in the bottom of it with a sharp "V" cut into it. The idea being if I put the knife on the brick edge down and let the whole thing heat up the brick would insulate the edge a little bit so I might get a more even heat - or at least not heat the edge faster than the spine. I have no idea if this would work, but it might be worth a try if you have an extra brick laying around.
-Dave

[reefera4m]

Dave,

I tried something along the same line, I put a 1" wide strip of ceramic wool on the bottom of the forge and stood the knife spine up with the edge on the wool. It helped slow down the rate of heating for the edge but did nothing for the pointy 1/2 of the blade. The distal taper was causing the blade heat from point to ricasso (which was expected) so I did just what you did - moved the blade to achieve even heating.

I'm still trying to figure out a better way.

[GHEzell]

My forge also runs best hot, so it's great for forging but not so great for heat-treating. After quite a bit of experimentation and testing, and getting to see Michael Bell harden a katana in a rather small forge, I've come to the point where I like having a hot-spot. It is like using a large torch and moving the blade instead of the torch. The fellow with the katana, starting at the base of the blade in the hot-spot slowly drew the blade through the heat, gradually increasing the speed as he as he pulled the point through the hot-spot, then thrust the blade back quickly to the base and did it again. He did this many, many times, and as the blade heated he did it faster and faster until it was time to quench.

I try to do it the same way. I keep a magnet stuck close to the opening of the forge and start checking every dozen passes or so once it gets red. When the magnet begins to stop grabbing, I start watching for decalesence (I heat-treat at night with the lights out, some things are best done in the dark ), and quench once the transformation is complete. Fine pointed blades still make me a bit nervous, but not like they once did...

[Ray Rogers]

Never done this myself but I've read that some makers put a section of pipe (about 2" diameter, I think) in their forge to more evenly heat their blades. The pipe is sealed at one end, a few wood chips or a piece of charcoal is placed into the pipe to create a little smoke to reduce scale and then the blade is put inside the pipe. Since the flame can't touch the blade at all the blade heats more evenly and you have the bonus of reduced scaling. Probably takes longer but it's easy to implement so might be worth trying....

[son_of_bluegras]

It seems to me that you don't really have a problem. I would love to have a propane forge that gave even heat all around (I run coal). I think the solution is to get a pyrometer that will measure the temp of your forge and adjust the propane to get the correct temperature for the steel you use. Then it will not be much problem of the thin parts getting hot faster. From my understanding of the heat treating process, temperature has a stronger effect than time and letting a piece sit at or just above critical for a while won't hurt as much as getting it well above critical. And some steels benefit from some soak time.

Just my thoughts.

ron