Home made heat treat oven information

Roy, I do a lot of emergency machining and repair work, and have found that if air-hardening A2 tool steel is used, an even heat to glowing red from a torch, air cooling, followed by a tempering with the torch works just fine. Scale is buffed off, beadblasted, or left as is. I have a foot-powered turntable to apply the heat as evenly as possible by spinning the part slowly while heating. I probably do a hundred or more small parts a year like this with great success.

RJ

I do not have a O/A torch and would prefer to have a self contained oven if possible. Besides it gives me something else to play with and work on to keep me from getting bored. I just have these urges to make things. YOu know how it is ;-)

Regards

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Are you making tools, or are you making something that needs to be tough? I am also interested in building a heat-treat furnace similar to what Roy wants, but it is for things like small crankshafts that will be receiving a pounding. Over hardened steel would just snap.

I would say, carve some grooves in the side walls for heater wire, *maybe* use it edge-wise instead, that is, 4.5" walls - *great* insulation that way, but will use up a lot quickly.. (would be good if you have a long heat to do... heh maybe anneal some white cast iron? Mmm, malleable iron!). Otherwise, electricity is cheap. :)

If you know the thermal conductivity of the stuff, you can estimate power consumption required for a certain temperature. Would suck to load it full of wire and realize you can only get to 1400°F from 240V! (Psst... find a

240:480V transformer of good power rating :)

Only other thing I would suggest is to put more wire in the corners, and put it on the largest walls if possible. Any wall that isn't contributing heat is sucking it down, and corners, being surrounded by cooling wall on three sides, will suck it down a lot.

Tim

Roy, I was searching for something similar about a year ago, and found good information glassworking enthusiasts. It seems home glassworkers are even tighter than home machinists. (G)

Jim

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???? For what reason? Ohm's law says it makes no difference aside from losing a little in heat to the transformer. Is there something I'm missing here?

Harold

My plans are about the same. So far I have the controller and thermocouple, but will have to wait until I can get into the nearest big city to see about firebrick. I plan to document my project with picures on my webpage. Meanwhile, I've found this book of interest:

This source always seems to have it up for sale on e-bay but Busy Bee Tools sels it for less. It's full of good info.

A little of both. You might be able to use S7 high shock tool steel, but hell, some engine cranks are cast material, some forged.

RJ

If you have too much resistance, you won't be able to dissipate the required amount of heat at a given voltage. (Likewise, if you used too little, you may have to run it off 120V - at an excess of current.) If you put in too much wire, it may well be that the ultimate temperature will be too low at

240V! However, a stepup transformer will put more voltage on it = more current = more power.

Tim

-- "That's for the courts to decide." - Homer Simpson Website @

Naturally one would want the higher voltage lines. Lower current for the same power. Smaller wire size at lower cost still over the top in rating is cheaper.

I suspect there are motor elements also - not sure. Current is typically called out...

Martin

Understood. I was going on the assumption that one would select heating elements for the voltage at hand, so it made no sense. Chromolox makes a wide variety of heating elements, many of which would likely lend themselves to the building of a furnace. One might be wiser to pay for the right element instead of involving a transformer, if for no other reason, economics and space savings. Running the elements at the recommended voltage might be a good idea to prevent over-heating them, shortening their useful life considerably. I'd also question the safety of higher and higher voltages, although one of my machines runs off 480, achieved by the use of a three phase 240/480 delta transformer. I had no alternative, though, the motors didn't lend themselves to being rewired to our standard voltages. It is a German precision grinding machine.

Harold

Gingerly gives you enough info to do this. See "big bertha" or suchlike. Its for a round furnace, but the idea can be extrapolated to anything.

What I would be really impressed by is a homemade neutral atmosphere furnace !!!

Absolutely, or a controlled atmosphere one (same thing?). If you've seen parts that have come from such a furnace, it puts things right in perspective. The heat treat facility that I used to use controlled their atmosphere with natural gas, which seemed to work beautifully. How I hate the scaling and the decarburizing of heating in the typical furnace after seeing how nice it can be.

Harold

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Yup. But Roy is the master scavenger, so who knows, he may just rip off an electric dryer or two! ;)

Tim

-- "That's for the courts to decide." - Homer Simpson Website @

I bet you could do by first sealing the outside box very well (welded seams I guess), making very certain of the leakage points (i.e. door), then filling it with a few CFM of natural gas (as you say), or propane since Roy's a gas nut, :) which could be applied with something like a torch. Maybe one using compressed air as well so you can completely turn off the gas and get an oxidizing/well-ventilated atmosphere (say if you wanted to do pottery? err I mean make your own crucibles ;). Or even hook it up to your MIG bottle and get a completely inert atmosphere... not sure what would need that though.

For toasting steel, somehow I imagine best would be something with orange flames coming out around the seals ;-) (Although... that might make for better case hardening than heat treating...)

Tim

-- "That's for the courts to decide." - Homer Simpson Website @

That is one method for case hardening, packing in carbon being another. Coked peach pits were the carbon of choice for pack hardening where I worked as a young man.

Harold

Well, if the latter were the case, then putting in *more wire* in

*parallel* with the existing wire would solve the problem without resort to a transformer. There's nothing which says you have to run all the elements in series.

If the former, then just inserting *more wire* in series with the existing wire would resolve the issue without the need to drop the supply voltage.

In principle, you can run with any voltage you like, as long as you adjust the total resistance of the furnace by either choosing a single wire of the correct length, or by using multiple wires connected in series or parallel to achieve the desired resistance.

Gary