Forge heat treat baffle design

[ymber]

I've been figuring out how best to build a baffle to use for hardening knives in a propane forge. I need to get the heat as even as possible across the blade and since I'll have a PID controller connected to a thermocouple and the main gas valve I want to be able to control the heat quite accurately. I'm thinking a 1/4" thick, 3" internal diameter round black iron pipe section should give me decent thermal mass and help even out the heat. I've seen a few people mention that the blade shouldn't be touching the inside of the pipe. Has anyone done this before and got an idea for how best to support the blade off the bottom of the pipe? The main things I've seen people talk about to reduce oxidation and decarb are welding a cap onto one end of the pipe, making the whole thing about 3" longer than the forge, and putting charcoal down the tube at the capped end when it's being heated. As far as I understand the charcoal will partly combust when it's heated, consuming some of the oxygen that would be reacting with the blade otherwise and the inside of the tube will be filled with CO and CO2 so the blade is in a reducing atmosphere. If I run a thermocouple through the cap on the back end of the pipe it should give a temperature for the PID controller to work with that's very close to the actual temperature of the steel. Is there anything else I can do to make it work better?

[Frosty]

This side of the Atlantic we call them a "muffle" but that's just a name. Use kiln furniture to support the blade. Support the muffle off the floor of the forge / furnace for the same reason as well. Seal the pipe nipple with screw on caps drill, a small pinhole drilled in one of the caps so it doesn't become a pressure bomb. Put a little lump charcoal in the muffle, it WILL consume the free oxy and so long as it isn't touching the blade won't effect it. Read the temp of the muffle, there will NOT be enough difference between the inside and outside of the steel walls to matter.

This is pretty simple process and has been done since steel has been used in a hardened state. To improve your process stop over thinking it. You can NOT control every aspect without maybe hundreds of thousands of dollars worth of equipment and instrumentation. 

Frosty The Lucky.

[Buzzkill]

In another thread you were specifically talking about heat treating stainless.  If you are using a good stainless alloy for knife blades you may need to bring the steel in the 1900 to 2050 degrees F range before quenching.  You also may need to hold it at a high temperature for an extended period of time depending on the specific alloy.   A muffle with a little bit of charcoal in it is not likely to prevent decarb in that scenario.  A lot of people who heat treat stainless alloys use a stainless foil pouch with as much air pressed out as possible and a small amount of something flammable to consume the remaining oxygen in order to protect the stock from decarb.

I'm not sure it's a problem to have the spine touch the muffle floor though.  In order to try to get even heat I can understand why you wouldn't want a blade leaning against a wall of the muffle, but there are fairly easy ways to accomplish that.  I've used small pieces of insulating fire brick in which I cut slots to hold blades where I wanted them.  As Frosty suggested, you can also use kiln furniture, and I believe there are some knife making websites that actually sell pre-made items to hold blades while heating for quench or during tempering cycles.

[ymber]

18 hours ago, Frosty said:

Seal the pipe nipple with screw on caps drill, a small pinhole drilled in one of the caps so it doesn't become a pressure bomb.

I've seen recommendations in a few places to cap one end to prevent it being a wind tunnel and leave the other end open. Capping both ends makes more sense to me intuitively when the point is to retain heat and keep oxygen out. What do you make of the idea of only capping one end? Seems like there's a reason people do it but I can't figure out what it is.

18 hours ago, Frosty said:

Read the temp of the muffle, there will NOT be enough difference between the inside and outside of the steel walls to matter.

You reckon the temperature of the muffle itself will be close enough to the temperature of the blade inside it that it's not worth running a thermocouple through to the inside?

15 hours ago, Buzzkill said:

If you are using a good stainless alloy for knife blades you may need to bring the steel in the 1900 to 2050 degrees F range before quenching.  You also may need to hold it at a high temperature for an extended period of time depending on the specific alloy.   A muffle with a little bit of charcoal in it is not likely to prevent decarb in that scenario.  A lot of people who heat treat stainless alloys use a stainless foil pouch with as much air pressed out as possible and a small amount of something flammable to consume the remaining oxygen in order to protect the stock from decarb.

I've seen the foil method, it looks pretty good. I'll definitely try it when I've got the thing up and running.

I'd not heard of kiln furniture before but it looks like exactly what I need, I'll get my hands on some.

[JHCC]

1 hour ago, ymber said:

Seems like there's a reason people do it but I can't figure out what it is.

There is one HUGE problem with closing (not sealing!) both ends of the pipe: getting the blade out for quenching. Getting the cap off one end of the pipe and removing the blade for quenching when everything is red-hot is tricky and potentially unsafe. Quenching the entire pipe is obviously no good, as it would keep the blade from contacting the quenchant.

The point of the muffle/baffle is to even out the heat first and reduce the decarb problem second. Set up your pipe with one end capped and the thermocouple inside, bring it to proper heat, add a little charcoal and the blade, bring it back up to heat again, hold at temperature for the recommended soak time, and quench the blade. That should be sufficient for your purposes; anything fancier than that, and you should be looking at a heat-treating oven.

 

[Frosty]

Right you are John!, Whoever suggested screw on pipe caps on both ends wasn't thinking when he wrote that!

Frosty The Lucky.

[ymber]

Thanks, that's an easy enough design. Now you've got me looking at heat treat oven designs though and that's a whole new thing for me to overbuild. It would let me do the hardening and tempering in the same oven too. I hadn't thought of building one yet but now I'm tempted.