Heat Treat with PID controlled forge

Mimic Knife & Tool

Mimic Knife & Tool

The super part time maker
Knifemaker / Craftsman / Service Provider
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Mar 15, 2021
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Awhile back I built this PID controlled forge as a cheaper alternative to an electric oven for heat treating simpler carbon steels. It can hold temps within 25°F (So the thermocouple says) of my set point. And can reach 2000F
PXL_20251014_183801097.jpg
My question is

Is this accurate enough to treat some stainless steels such as AEBL?

Thanks
Andrew
 
AEB-L as well as other stainless steels have to stay at a certain temperature for a little while after it gets to temperature (I do 1950 degrees for AEB-L) so i'm not sure if the forge would stay steady for that time?
 
Linus Knives said:
AEB-L as well as other stainless steels have to stay at a certain temperature for a little while after it gets to temperature (I do 1950 degrees for AEB-L) so i'm not sure if the forge would stay steady for that time?
Click to expand...
I believe it can sustain that temp, but I'm wondering if the 25 degree variation would be ok. For example, the way my forge would work is it'll hit 1950 then gas will cut off, and turn back on again at 1925. So it'll stay in that range but won't hold level
 
In reality it may not even be that narrow. So let's say up to 50 degree variation
 
This might be a hot take, but in principle it should work. Thermostatic heat treating kilns that run on gas do exist. Wrap the blade in foil and place it near the thermocouple. You might be able to get the temperature swing tighter by increasing the gas going through the bypass. The PID itself has settings you can tune as well.
 
Mimic Knife & Tool said:
I believe it can sustain that temp, but I'm wondering if the 25 degree variation would be ok. For example, the way my forge would work is it'll hit 1950 then gas will cut off, and turn back on again at 1925. So it'll stay in that range but won't hold level
Click to expand...
I'll let someone that knows more about it than me say for sure, but I would think a 50 degree swing might be risky. As S SirSpice said, you can most likely tune the PID to change that swing to less but I'm not sure by how much...
 
A ±25°F degree swing is too much for "good" HT of any steel, that's from 1925F to 1950°F. Yes, it will harden, but it's just a bit "iffy" for good HT. If you've got the same swing at the 1475F range, that's still a bit "loose" for good HT on carbon steels. "BUT" if that's all you've got, it will make a decent knife. Need to hold the same temp for at least 10 to 15 minutes depending on alloy.

Be sure to use a muffler tube to place the blade in, and have the TC tip inside the muffler tube, right at the blade. Do some tuning with the PID and adjusting the bypass valve and I'd expect to close that spread some, perhaps to ±5F to ±10F degrees. Good luck and update the thread to let us know how it works.
 
It's not usually possible to switch a solenoid valve very fast, but it's usually worth getting your output cycle time as short as you realistically can, then run the autotune. I'd probably try a 10 second cycle, but wouldn't want to go much shorter unless the solenoid valve specs say it's ok.

Usually you'll get a saw-tooth temperature variation: rising while the valve is on, falling while the valve is off. I'm guessing you have a 30-second cycle time and that's giving 25 deg variations? If so, going to ten seconds will make the sawteeth one-third the size and give one third the variation: about 8 degrees?

Give it a try and please let us know what happens. If I'm right, it's an easy win and if I'm wrong, you'll save the next guy from getting useless advice.

There is scope for adjustment on the needle valves too. Increasing bypass flow should reduce the rate of cooling on the saw-teeth. Reducing flow at high-fire should limit the rate-of-rise. Once you have small sawteeth, you can adjust to make them flatter.
 
Load up the forge with as much mild steel as can fit for a heat sink. That'll cut down on the temp swings and get you closer to the target temp.
 
With no electronic control - only fuel pressure and air control - I can maintain a constant temp within 1-2 degrees for any temp between 1400 and 2300.
I've been doing it for years.
It has a smaller temp window than my Evenheat.
 
Yes, you can HT stainless in a PID control forge. Tim has given good info.
I find a blown forge can be regulated much better than a venturi burner.
Karl has a wonderful setup that regulates quite well. Here is a forge build thread he had with lots of good build info.
Karl B. Andersen

Thread 'Horizontal Forge Build - UPDATE'

I have four vertical forges of different sizes - and love them. I built my first one right at 18 years ago after seeing Don Fogg's forge at the Ashokan Seminar.
I had been using coal until then.
They do many things well, but nothing does everything well.
With some up-coming projects that might benefit from a horizontal forge I decided to add one to the arsenal.
Maybe something a person sees here can help them with their own ideas for their forge build.

  • Like
Other good forge build threads:
T

Thread 'My vertical forge build'

I thought I would make a thread as I slowly build my vertical forge. Maybe this will also help me from making too many major mistakes.

Right now I am still gathering materials. https://imgur.com/gallery/DjnYWyF

The body of the forge is an old air tank I got from the scrapyard (12” diameter). Inside that is going an 8” stovepipe wrapped in two inches of insulwool. Inside the stovepipe I have a 6” high density foam roller which will be a hollow form for the forge chamber. I’ll be pouring kasto-o-lite to from a 1” liner inside the stove pipe. All this of course after I’ve cut out and...

Fred Rowe's Vulcan's Chariot PID forge - https://www.bladeforums.com/threads/vulcans-chariot-is-ready-to-tune.540299/
Nick Anderson's PID control forge Plans - https://www.bladeforums.com/threads/pid-controlled-forge-heat-treat-oven-plans-wip.1385057/
There is a section in the Stickys about PID control forge building - https://www.bladeforums.com/threads/pid-control-forge-salt-pots-and-more.782574/

TIPS:
1) Wrap the blade in well sealed stainless foil.
2) Use a muffle pipe. I use a piece of very thick walled 3" stainless pipe. Place the muffle in te forge before turning it on and allow it to come up to heat with the forge.
3) Soak the forge for a good 15 minutes to get the refractory and muffle fully heated.
4) Tune the burner control. Start on manual (by-pass only) and adjust to get the forge running stable at around 10°F below the target temp.
5) Switch to PID mode and let the autotune zero in on the cycling. As mentioned, set the cycle time to a reasonable time span.

Some suggestions for a PID control build:
1) Only use a propane rated solenoid valve. They aren't cheap, but some shopping around the auction sites can snag one for a fraction of the regular price. Don't be tempted by the cheap On/OFF propane valves, They won't work for long. You want a good Red Hat or similar valve.

2) Mount the forge on a rolling cart. The metal shop carts and small welder carts are perfect. I like the $49 HF welder cart the best for a forge. Use the two lower holes so the top shelf is level not tilted. Mount the PID control on the back side. Put tongs and other welding stuff on the shelves. This cart is great for any forge, PID control or not.
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3) Don't hard mount the control piping and solenoid like the one shown in this thread. That puts a lot of strain on the tubing as well as exposes the parts to heat. Instead, build the control unit and mount it on a board or sheet of metal and mount it on the side of the bench or on a rolling forge cart.. Use flex gas line to connect the burner control to the forge. I like the Eastman 36" long 3/8" ID flex lines. They are very low price and work perfect for any forge build.

Eastman 36 Inch x 3/8 Inch OD Flexible Gas Line Connector with (2) 1/2 Inch MIP Fitting Ends for Natural Gas and Liquid Propane, Stainless Steel, Yellow,​


Eastman 36 Inch x 3/8 Inch OD Flexible Gas Line Connector with (2) 1/2 Inch MIP Fitting Ends for Natural Gas and Liquid Pr...

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