Maintaining Soak temps in gas forge

W

Walker Vitae

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Feb 23, 2021
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Ok guys...I need some clarification in how to achieve consistent temp for soak times normalizing and truly annealing as well as hardening. I had always smithed with bituminous coal and coke and had more success than failure due to my meticulousness or dumb luck. I’m not a complete novice but I am out of practice for one, and I’ve only had the opportunity to work with a gas forge a couple times. My wife purchased a hell’s forge Max a month ago in my response to purchasing her an electric kiln for her pottery. She’s amazing...I know. I’ve been drawing up blueprints for a rolling cart for the forge complete with refractory coated firebrick insulated folding doors/shelves on either end of the forge to both insulate heat, provide some safety measures, as well as allow me to play with the heat a bit. I had planned on using the thermocouple from a retired small dental furnace to help me dial in on temp but I’m deciding to purchase a dual channel thermocouple to get a more refined read. Now...I’m more interested in the chemical and structural changes needed to take place in order to get the most consistent product possible. I have every intention of getting my grain refined and the metal fully austentized prior to hardening and then flash tempering so as to homogenize the Martensite as much as possible. My mission is to run soak cycles for 10-15min(depending on size of material) at a sustained temp the HF Max without the aid of a $2000-5000 electric forge. My plan is to use a section of square tubing inside the forge with 2 thermocouples, 1 within the tubing and one outside the tubing. This will probably be my normal setup for normalizing, forging and heat-treating, and I don’t see this being a problem once I have the PSI dialed in. However annealing is another issue. Simply sticking the blade between two other pieces of steel heated at the same time, or heating and them submerging in vermiculite/ash and then letting cool slowly may be sufficient enough to soften certain steels/alloys, but it does not retain enough heat to allow cooling at a rate of 50-60F/hour. I want as homogenous austenite as possible. I’m planning on welding together a vestibule(probably internally coated in refractory cement) that is hinged on top and closed at each end, small enough to fit in to the forge and still allow the burners to function, and large enough to allow at two pieces of steel on either side of the knife as well as enough tolerance to pack with a vermiculite and ash mixture. I think this paired with a couple more fire bricks inside an enclosed forge, once extinguished, should cool at a sufficient rate. I know I’m long winded and there’s a lot to unpack, but I also know there are a fair amount of master smiths on this platform. Any input or criticism would be welcomed and greatly appreciated.
 
Short answer - you can't hold a constant temperature to do a 10-15 minute soak in a regular gas forge.

You can turn the burner down as low as it will remain stable, keep the blade moving all the time, move it to the sides, and pull it out occasionally, but that won't be a constant temperature. You can do that for a few minutes at most ( It will seem like 10 minutes, though).

The only way I know of to hold a constant temperature in a gas forge is to build my two-stage PID controlled burner system. There are many threads about it and there is info in the HT section of the stickies. With it and a blown burner, you can set the temp at 1475°F and it will hold right around there.
 
Unfortunately, you picked about the worst possible forge design to try to do this in. Really, just a horrible design with 75% of the heat blowing right out of the chamber. Even properly blocked up, with a muffle in use, you can't keep it running at 1475°F throughout the chamber. You will ALWAYS have a hot spot directly under each burner. You might get +/- 200°F with top-down burners. Even in my Graham forges, you still get +/- 50°F front to back.
 
Even with the forge running off a pid control your still not going to be even. It will be the temp it says in the pid where the TC is but the closer to the burner you get the hotter it will get. The same is true for heading towords the ends where the temp will drop. The only forge I have had be a consistent temp is my large ribbon burner forge. Once up to temp I close it up tight and it will be a very even temp inside. Not as even as a heat treat oven but a lot more then a standard forge.

i did get some consistent temps in my vertical forge. The TC was inserted in from the top and sat in the level where the openings where. I found that inside the small openings it is quite even but it’s a small area. I did heat treat an A2 blade in it once (many moons ago) and it came out rather good. It was a small blade that was short enough to fit completely inside the forge. Wrapped it 2 or 3 layers of stainless foil. In fact my wife still uses that knife to this day and it holds a great edge. But how much better would it have been if I did it the proper way? Those days are long gone now but there is no way I would ever do that again knowing what I know now.
 
Here is a cheap way to do it. It isn't perfect, but it works. I put a ball valve on the propane line. I place my thermocouple in a thick walled iron pipe inside the forge as a muffle. I watch the temp and when it equalizes and hits the target, I operate the ball valve open, close, open, close etc and I can maintain around 5 degrees of the target temp. This is on a blown forge with a burner placed to achieve a swirling action. I put the muffle pipe on a couple little ceramic stands so the flame swirl isn't blocked. In essence, you are the PID.

-Mike
 
That sounds like way to much work considering what it costs to buy a cheep pid, solenoid and SSR.
 
Yes, a PID, SSR, and a solenoid valve will make the forge run at a pre-set temperature. For HT, placing the blade and TC inside a heavy pipe muffle will fine tune it pretty tight. The two-stage burner control eliminates then WHOOF every time the burner gets turned back on. Instead of ON-OFF it cycles from a higher to a lower flame.
 
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