Forge atmosphere in a SS pipe

Mecha

Mecha

Titanium Bladesmith
Knifemaker / Craftsman / Service Provider
Joined
Dec 27, 2013
Messages
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Hello all,

I've heard of putting a thick-walled stainless steel pipe into a propane forge, then running a blade through the hot pipe to heat treat it with a more even heat.

We have oxidizing, neutral, and reducing flame possibilities, but what sort of atmosphere would be expected within the red-hot pipe?

Thanks for your thoughts!

-mecha
 
I think it depends on where the ends are. If both ends of the pipe are closed off from the burner, say the end in the forge has a cap and the open end is several inches outside the dragon breath, I would expect a the air to be the same as air outside.
 
As Greenberg Woods says above, unless you are doing something to alter the atmosphere in the muffle pipe, it will be air (just as in most electric HT ovens).

At 20.9% Oxygen, Air is an Oxidizing atmosphere.

The muffle pipe does not (usually) need to be stainless, particularly thick-walled, or even round. Mild steel Square Hollow Section works well at the temperatures used for Carbon steel HT.
 
As said, the air in the muffle is atmospheric air .... at 1500F.

What people are leaving out of the equation is that the density of a gas is much lower at higher temperatures. I would have to do the exact math, but a close approximation is that there is 1/20th the amount of oxygen in the muffle at 1500F.

I think this is the proper formula - (p(v))/t = (P(V))/T (IIRC, T has to be in Kelvin)
 
Mecha said:
Hello all,

I've heard of putting a thick-walled stainless steel pipe into a propane forge, then running a blade through the hot pipe to heat treat it with a more even heat.

We have oxidizing, neutral, and reducing flame possibilities, but what sort of atmosphere would be expected within the red-hot pipe?

Thanks for your thoughts!

-mecha
Click to expand...
I have vertical gas oven /bottom side of SS pipe closed/ for HT carbon steel and I notice less decarburization ...................
 
Yes I need an oxidizing or neutral atmosphere. Oxidizing is best.
 
An open end muffle will have a very slightly oxidizing atmosphere. It is unaffected by the flame, so the actual chamber atmosphere is not really important.
I usually plug the back end with a piece of firebrick, and leave the front open for observation and access. If using a TC to read the exact blade temp. stick it in from the back through the firebrick. Just let it lay on the muffle floor and place the blade on or next to it.
 
Stacy E. Apelt - Bladesmith said:
An open end muffle will have a very slightly oxidizing atmosphere. It is unaffected by the flame, so the actual chamber atmosphere is not really important.
I usually plug the back end with a piece of firebrick, and leave the front open for observation and access. If using a TC to read the exact blade temp. stick it in from the back through the firebrick. Just let it lay on the muffle floor and place the blade on or next to it.
Click to expand...

Excellent, in that case it sounds like an open-ended square pipe would be perfect. I was also wondering of drilling some holes down the sides of the pipe could possibly make the pipe's atmosphere a bit more open and oxidizing.
 
You can also close one end of the pipe and put some wood shavings/similar inside to burn and consume the oxygen inside. On carbon steel, it minimizes the scaling. No idea what it would do for titanium.
 
Storm Crow said:
You can also close one end of the pipe and put some wood shavings/similar inside to burn and consume the oxygen inside. On carbon steel, it minimizes the scaling. No idea what it would do for titanium.
Click to expand...

In this case I want the pipe wide open. The right atmosphere for ti is the type that causes a lot of scaling on steel. A reducing atmosphere or flame is very detrimental for ti.

PS: Storm Crow I tried to send you a message but got an error that said, "You may not send member Storm Crow a message." :eek:
 
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You could place a tube at one end of the muffle and feed a small amount of Oxygen in. The regulators for breathing oxygen would be perfect, as they are for low volume and low pressure output. Using this method, you could oxidize as much as you want.
 
Stacy E. Apelt - Bladesmith said:
You could place a tube at one end of the muffle and feed a small amount of Oxygen in. The regulators for breathing oxygen would be perfect, as they are for low volume and low pressure output. Using this method, you could oxidize as much as you want.
Click to expand...

Yikes, that seems extremely dangerous. Can you clarify this? I haven’t seen it first hand, but after riding on an ambulance for years, a flame around an oxygen source could be deadly.

Maybe I’m misunderstanding the advise.


 
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I am amazed at how this forum magically provides just what i need. I'm working on my first pocket knife and, while I've done a number of fixed blades and used the differential heat treat method taught to me by a master blade smith, I've been concerned about heat treating this little sliver of steel without a Paragon oven. The idea of a muffle pipe may be just what I was looking for. Since this is my second attempt at a folder (the first attempt was a total failure) the results do not have to be perfect by any means. There will be plenty of flaws in this one. The downsize from fixed blade to folder and dealing with tiny pieces, tiny drills and taps, and precise dimensions has been a challenge for me. Thanks for the idea guys.
 
Thanks for the info Stacy. As long as the pipe is wide open and doesn't hold a reducing atmosphere it should be perfect. :]
 
Josh,
Adding a small amount of oxygen to the muffle will not cause a fire or explosion. There is no fuel in the muffle. As far as the flame outside the muffle, the dragons breath at the ports may show to be slightly more oxidizing as the flame reaches the most oxygenated air, but the amount of O-2 added is small. We add far more oxygen to the flame with a blown burner.


Mecha's intent was to have more oxygen available in the muffle to cause surface oxidation on the titanium blade. This is the opposite of what we do with steel, and may sound strange to some folks. All he wants is to increase the O-2 percentage, not replace the air with oxygen. Increasing the level of oxygen is what is done with a breathing cannula. It is safe, and no one goes up in flames when using an oxygen assist breathing apparatus. After all, t he air at sea level has more oxygen in a liter than a liter in Denver.
 
I’ve been in the fire service for 20 years so these last few posts are making my hair stand up thinking about the consequences. A blown burner (although I don’t have one) is blowing air into the forge, right? 100% O2 is not the same. When we wear our SCBA into a toxic environment our tanks are filled with air, not oxygen.

Although mecha may have a good handle on the techniques you’re describing and a safe way to do it, I’m not sure about the other 99.9% of the people who come across this thread will.

A cannula would be the fuel in this scenario and with oxygen pumping through it to fill the muffle and get ignited upon exiting the forge I can only assume it will travel against the air flow back to the tank with horrible consequences.
The guy in the video in my last post shows this.

Obviously it stresses me out thinking about this or I wouldn’t say anything.
 
Josh, I totally understand your concern. If too much oxygen was added the muffle tube would act like a cutting torch, and the hot metal would burn up.

The cannula was for reference to the rate I was talking about. The O2 would be piped into the muffle through a metal tube, probably stainless. The amount of O2 added is very small.It would be equivalent to blowing on coals to pep them up ... or to using a blower to add more air to a burner. Remember, air is 20% oxygen, so more air equals more oxygen. Just blowing some air down the muffe would work, but would require five times the volume, which may create temperature variations that are not desirable. Adding O2 will allow the atmosphere to change without any cooling issues.

The same method is used to pump nitrogen, argon, nitrous oxide, carbon monoxide, etc. into hot chambers. Oxygen is added to some processes the same way. It takes a low volume low pressure regulator capable of supplying the O2 at liters per hour rates. Medical regulators are perfect for this.
 
Mecha Mecha What is it about forging ti that makes an oxidizing environment beneficial? I would think just the opposite based on the pains taking to weld it entirely gas shielded.

But you've forged a million times more ti than I have, I'm just curious.
 
I thought possibly getting 'sploded in a raging titanium inferno was just part of my job! :D

At any rate, I wont be adding oxygen since an open-ended pipe isn't going to have a reducing atmosphere anyway. As long as it's at least neutral but preferably slightly oxidizing, all will be well. I couldn't be sure about it without asking the experts and folks who have used a muffle pipe before. It's going to make heat-treating a long thin sword blade a much more smooth and even process.
 
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