Blown Forge, How Low Can You Go?

DanF

DanF

Knifemaker / Craftsman / Service Provider
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Apr 17, 2017
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For the past couple weeks I’ve been experimenting with my 20lb propane tank blown gas forge to see how wide a temperature range I could achieve and hold for extended periods of time.
As built, the burner nipple was 1”, and this is would mostly stay from welding temps to down around 1700*f.
I removed the 1” burner, replacing it with a 3/4” burner, and realized temps from 1900 down to around 1250*f, still too hot for what I wanted to achieve.
So, after consulting a more experienced forge builder, I dropped down to a 1/2” burner.
This gave me temps from 1900 down to 600*f. I can even “sneak” the temp down to just above 500*f, if I work at it. With the 1/2” burner I can dial in at just over a thousand degrees f with only an 11 degree difference from 3/4 of the way inside the chamber to 1/4 of the way inside the chamber. This is all manual at this point in time.

My next step will be installing everything needed for the pid to control the temperatures, which should be able to hold the temps to closer tolerances than I can manually. Realizing a +/- temp range of 5 to ten degrees will be closer than my HT oven ever has been.
 
FDBF2946-AFC7-4061-95EE-537C1719AE63.jpeg Three burners, 1/2”, 3/4”,1”. With the appropriately sized bushing they are mostly plug and play for ease of change.

Internet speed sucks today, will have to post pics separately.
 
494E5ACA-CFC0-4EBF-8F09-960E044D24D9.jpeg
Lowest temp I have held at manually for at least 30 minutes
 
Following this, Dan. I'd love to hear if you could get that down into the 700F range for forging aluminum. I haven't been able to with my forge/PID set-up (although I haven't tried too hard yet). I'm thinking I'll need some sort of baffle to isolate part of the chamber from the burner...
 
Glad the changes to the burner solved then low temp issue. With the PID control, you can do spring tempering in the forge.
 
weo said:
Following this, Dan. I'd love to hear if you could get that down into the 700F range for forging aluminum. I haven't been able to with my forge/PID set-up (although I haven't tried too hard yet). I'm thinking I'll need some sort of baffle to isolate part of the chamber from the burner...
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I have a SS 2.5x2.5x10” piece of tubing on the way to use as a muffle, hoping that smoothes out the temps more than just open to the burner.
After today I can dial any temp down to 500*F and hold it at will, would love to be able to target anywhere in the lower 400’s with the muffle in place, but that may be asking a bit much.
 
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A couple observations (this applies to my forge, ymmv).
A good base layer of satanite covered over with ITC 100 goes a long way in getting the forge up to temperature fast. The ITC 100 is expensive, but worth it.

Sizing your burner to your air flow and chamber size and baffle placement seems to be critical for targeting lower temps.

It takes time to learn your individual forge’s idiosyncracys, and how to pull the most from it performance-wise.

In general, a blown forge can do more than most of us use them for. I can take this forge from 450*F to over 2100*F, and hold that temp as close as my kiln did (I did not hold the temps above 2000*F for any length of time out of concern for my TC).

weo weo , saw your wip thread and video from earlier this year, very nice.
 
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Thanks, Dan. It works pretty well, but I can't manage lower temps like I thought.
DanF said:
Sizing your burner to your air flow and chamber size and baffle placement seems to be critical for targeting lower temps.
Click to expand...
I think my burner is too big for forging aluminum temps. The forge, without the PID controller) was built over 10 years ago when I wasn't doing any bladesmithing. It was designed for larger sculptural pieces, as that's what my mentor at the time primarily does. If I had the space and money, I'd build another smaller one for blades.
 
What I told Dan was to make a second (and maybe third) blown burner that has lower output. Sometimes you can just add a smaller burner tube to an existing burner that has a speed control on the blower. A regulator that is made to run in the low pressure range is also important. A jewelers torch regulator is made to run the gas down to the 1/4PSI range. A BBQ regulator may also work fine.
This setup is where a cheap tiny blower made from a hairdryer or small surplus squirrel cage from some electronics device or microwave oven may be the best choice. Most of these simple blowers will run fine on a simple light dimmer speed control. Many are 12VDC and will do really well on a variable DC supply. You can buy one online for less tan $20, but your Personalized etching machine may be all you need to power the blower.

After the New Year, I will try and put together a photo tutorial on building a low output junk-box burner.
 
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My only concern running low temps and pid control is relight. My first forge had duel needle valves for a low idle and one for high. But i was never running that low so relighting was never an issue. In your case I would be half tempted to run a heat gun off the pid and blow it in through that burner port and call it a day. I do not like things I have to fiddle with. I use to back in the day but now a days I need flip it on and walk away.
 
Stacy E. Apelt - Bladesmith said:
If it is run on my two stage PID control, the flame never goes out.
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Yes that’s what I had said above that I had used. But to get to 500° your running tiny flame and putting all your trust in that everything will cycle properly. Just takes one weird-ness and the flame to go out and now you pumping flammable gas into your work environment. Wow look at me being the safety ranger lol. I’m normally the “here hold me beer” guy. But now that I have other people including my wife working in the shop full time I am more a tune to what if’s.
 
That is why you build a much smaller burner for HT.
Compare it to a welding torch. Try and throttle it down too low and it just pops out. But a small jewelers torch can run a stable flame 1mm long.

Before I made the two-stage burner control, I made a pilot light that ran from a solenoid valve, and had a piece of stainless steel tubing sticking just barely in the rear port.
 
I have a Graham blown forge from Atlas. I also have trouble running low temps. I've had flameouts and other issues.

I was able to get to down to heat treat temps recently. I had a bad TC and reader. Switching out the TC and the electronic unit helped.

Still having a bit of trouble getting consistent heat to stay level always seems to want to climb.

Im also not sure how to get a reducing atmosphere to reduce forge scale buildup. Less air from blower?
 
I've
weo said:
That's a good start. You might also have to bump up the propane/fuel PSI from the tank regulator as well.
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Fiddled a bit. I've forged two knives. One out of 1084 and one out of 5160. So I'm a rank amature/beginner.

I think one issue was also tank/line freeze.

Backing the gas/propane feed tube out a bit helped.

I have assumed a reducing atmosphere means the fuel to air mixture is high (more propane less oxygen) so that all the available oxygen is consumes, and the gas is looking for more oxygen at the edges/outside of fhe forge to finish combustion.
 
How large of a tank alleviates this?

I typically only forge one knife at a time, no more than 3 hours at a stretch. 20 lb tank. Probably easier for me at this point to put the tank in a tub of warm water.
 
I don't know how big it has to be, but I bought one of the 100lb one. I knew I wanted to do some damascus, and didn't want to run out!!
 
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