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heat treating with a home made propane kiln


Aug 31, 2017
So i have this huge beefy heavy steel cabinet that is lined with fire bricks, i think they may have used to preheat certain types of parts for welding cracks up for example cracks in cast iron, like an exhaust manifold. it doesn't have any forced air, so the highest temp i can achieve with the doors shut is around 400-500 before it runs out of air.

My goal is to do some heat treating for parts, for example i have some 1018 cold rolled that i want to pack carburize, which calls for 1800 degrees for 8 hours. What do i need to add to this setup to get to that temp for 8 hours.

i know i will need some sort of forced air. the bottom of the kiln has a 2" by 10" slot it in... no other openings.

there are four horizontal tubes with small holes that act as the burners

Most of the blacksmithing forges force air in with the fuel, either a ventrui or a blower fan of some sort.

I dont think the way that my burners are setup that i could do that.

i am thinking that i would need some sort of adjustable blower motor on the bottom of the kiln, and then some sort of adjustable stove pipe on top?

also trying to figure out how much propane that would require to maintain 1800 degrees for 8 hours.

see attached pictures, any input is greatly appreciated

Lots (like 6") lining from such as the light weight fire brick - all sides, top , and door - or you'll never get near 1800 F with out

It will gobble up the propane - in direct proportion to how fast you want it to to get to 1800

Propane ordinarily would go in with the air from a blower - and with such you naturally need to vent that air

It goes without saying that if you want to take little low volume stuff to 1800 - you need to start with far less size - unless you just enjoy buying Propane
Instead of a DIY solution have you contacted a regular heat treat shop? You might be surprised at the price. You will probably go through at least $100 worth or propane and you will have to babysit it the whole time.

Another option would be to try to find a place that fires ceramics although they might be leery of sticking a can of charcoal in there kiln.

Or possibly a shop that makes glass art pieces remember they have to keep the furnaces hot 24hr a day but, contamination of their equipment might be a concern.
I second the suggestion about finding a heat treat shop. Years ago in one shop I worked in we took 3 inch round bar about a foot long and dished one end like a saucer then sent them out for heat treat. They were put in a carburizing atmospheric furnace. I did one of the deliveries and the guy showed me around. They actually created a carbon rich atmosphere and soaked the bars then took them out and quenched the ends.
The bars, mild steel became pile driving tips. The hardened saucer shaped end would not slip off rocks and would shatter the rock rather than shifting off to one side.
I did something like that a few years back. I made a vise from 1018 I wanted to case harden. I welded up a box from 1/4" diamond plate big enough to hold the vise and bone charcoal. I dry stacked fire brick into a kiln of sorts, stuck an old oil furnace gun into a hole in the side and let er rip. After a few hours everything got toasty red and I let it cook for about 6 hours. I dumped the vice, charcoal and all, into a watering trough.
Worked pretty good. I still have the vise but I wouldn't do it again. A whole days work before I even started grinding........Bob
I think for heat treat work with propane you would want a muffle furnace. Yours is not. If you really want to do it yourself regardless of cost, buy an electric kiln. They are much easier to regulate the temperature.
Switch to waste oil ,and the cost comes right down......a vacuum cleaner can provide enough blow for a small furnace ......I know a few guys who regularly melt cast iron with a oil and a vacuum.cleaner............one of the whole house vac units is good too,single phase.
I heat treat tool steel and cast iron in an electric kiln on a weekly basis. I also have a shop-built forced air diesel burning furnace in which I melt iron on a weekly basis. For heat treating on a small scale, a couple hundred pounds or less, there is no way I would even think about a fuel-burning oven. It is way more complicated to regulate and a lot of work to build correctly using some combination of INSULATING firebrick (not common stove liners) and ceramic wool. On the other hand, a used electric kiln can be had for very little money, comes with a programmable thermostat, and you just plug it in, set the ramp temps, goal temps, hold time, and walk away.

Not all heat treating profiles can be easily done in an electric kiln or shop built fuel oven. I have never tried HSS, for example, as I have had no need, but it’s program is not simple and I can’t conceive of a need.

If the OP insists on the build, he should visit thehomefoundry.org where there are a whole lot of folks who have built furnaces of various designs ( usually each person has experience with several furnace builds and use) and are familiar with the many burners and fuels available. They also have had experience with the classes of insulating firebrick, ceramic wool, Satanite, castable or rammable refractories, etc. YouTube is a very poor source of info on this subject, let me assure you. Lot’s of “look at me, I melted aluminum cans and poured muffin tins of useless aluminum.”

I will add an additional comment on pack carburizing that does not require precise temperature control. This is a process that has been done for centuries using a clay or metal container, an open fire, and a carbon source. The OP would spend a lot less time and money getting an appropriate steel (or more durable for multiple uses but more expensive stainless) pipe or box in which to pack carburize. I have limited experience with this, but for small to medium parts simply boxing them and putting them in a well-fired wood stove is adequate.

We opted to use tanned goat skin and plant oils as our carbon-rich material. Goat skin and fat from an old pig were the ingredients used for the case hardening described by the monk Theophilus in c. 1120, in his textbook De diversis artibus (volume III, chap. 19). He describes the method in connection with the carburising and hardening of files (Hawthorne and Smith 1979:94-95):l