The Home Machinist!

I guess this topic goes here.
Just built an annealing oven. Trying to address the super hard mystery metal I posted about elsewhere here. My thought was to see if I could anneal the metal and thus not be held hostage again.
Found fire brick by accident at the local stone supply store, and reasonably priced. $1.49 each for either 1.25 or 2.0 inch thick bricks. Used some scrap 2" X 2" X 1/4" angle iron to weld up a box shaped base with the oxy-acetylene (don't have a proper buzz box yet). I set three bricks side by side to form the bottom of the oven.
I then used 3/4" X 3/4" angle to weld up a similar box for the lid and set three more bricks side by side. The base and the lid now measure 9" X 13.5" around the outside. The lid was completed by welding on a ¾” X 1/16” thick flat, bent into a squared off U shape, with about 12” of clearance above the bricks.
I bored a ¾” diameter hole in the center of a 3” X 3” X 1/8” plate and welded it to one end of the base. This is for my air supply. I set 4 bricks on end around the inside of the base (on top of the bottom bricks) and this left me with two 2” spaces around the periphery. A hack saw blade (cheap, and disposable) quickly cut two pieces which fit surprisingly well in the gaps.
I marked the location of the hole in the end plate onto the mating brick and bored a matching ¾” diameter hole in the brick. I first piloted with a ½” masonry bit, and finished up with a ¾” flat wood bit that was rusting away in the bottom of a file drawer.
The inside of the box is now 4.5” X 11” X 7”.
Next, I brazed a ¾” NPT coupling onto the flat plate over the hole, then mounted a 12” nipple, ball valve, adapters, and a ¼” air hose disconnect.
I filled the contraption with charcoal briquettes and connected the shop air. Lit the charcoal and added the steel. It must have been quite a sight to see me out there minding an oven on the snowstorm we had yesterday.
After the charcoal lit I turned on the air and ran it for about 30 minutes.
Never got the steel hot enough to lose it’s magnetism. No surprise that the steel didn’t anneal either. I also ran too much air judging from the fact that the compressor ran non-stop and the tank was too hot to touch.
I figure I need a needle valve and an euductor set up to draw in much more air at a much lower pressure. More charcoal would also probably be in order.
Thoughts and comments appreciated.

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Kelly Jones, PE
A life spent making mistakes is not only more honorable, but more useful than a life spent doing nothing.
George Bernard Shaw
(1856-1950)

With my very limited knowledge and total lack of experience, the only suggestion I have is to try a fan/blower instead of using the compressed air. You need quite a bit of air at very low pressure. Some vacuum cleaners with a blowing connection might work.

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Don Young

Yeah, what Don said, but do use some wisdom. What you have built has the potential to melt steel, assuming all things come together properly. That's the basic principle of operation for cupolas and blast furnaces.

You might be a little happier with the idea of losing the charcoal and going to a combustible gas. Propane works great, but if you have natural gas service it's wonderful for such use. I've built several melting furnaces through the years, primarily for use in melting gold and silver, but they are perfect for melting copper, copper alloys, aluminum and for heating, such as you wish to do.

Harold

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Wise people talk because they have something to say. Fools talk because they have to say something.

I would suggest using an electric leaf blower for the blast. More air than you could possibly need, and they are cheap enough to buy.

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Dan Watson
Building IC mikado in 1.5" scale

Kelly, I have a Johnson heat treat furnace that I got out of a school shop that I might be willing to part with. It is 5" X 8" X 14" deep it runs on any kind of gas. PM me and we can work something out. Dave C

Hmm... my post did not show up. Must be operator error. I'll try again, and hope it doesn't post twice.

Did a postmortem on the oven today. Lots of unburned charcoal. A friend suggested I put way too much air in and did not allow for proper flow through the unit. The ball valve is not going to cut it for air control. Got to get a needle valve.

Cross drilled the inlet nipple to allow the high pressure jet to pull in more low pressure air.

Found some sanded grout on a shelf in the shop and tested it with the propane torch. It will be fragile, but will plug up most of the leaks. If I offset the lid an inch then I should get more even flow through the unit.

Going to give it another try tomorrow.

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Kelly Jones, PE
A life spent making mistakes is not only more honorable, but more useful than a life spent doing nothing.
George Bernard Shaw
(1856-1950)

Which kind of charcoal did you use? Lump will give you substantially more heat then briquettes.

That is right. Briquettes are up to 30% cement and binders that add nothing to the fire or heat value. Cowboy or natural lump charcoal looks like the wood it came from. It is nearly all carbon.

A 'furnace' made of a stack of car wheels filled with 25# of briquettes was blown out with a leaf blower throttled with some aluminum foil on the intake. I had about 8 lbs of coal left, a non melted mass of brass in a crucible and was disatisfied.

It is about volume and not pressure. Too much wind on a solid fuel will simply blow it out, and move the flame front back, becasue yo have to heat tooo much nitrogen to get your flame and that takes heat away, adding nothing to the flame front. The same with gas firing, overblowing will simply drive the flame out of the tube. gas and air flow must be balanced and leaned out for maximum heat.

2nd post, got a server error on #1

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Big Dave

Be Alert! The world needs more Lerts.

One other suggestion if using solid fuel like charcoal: the air needs to come in from the bottom through a diffuser, and pass through the fuel, with the item you're heating on the top. With the cold air coming in from the side you'll be getting too much cold air on the thing you're trying to heat, and not enough air supply to the fuel.

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Dan Watson
Building IC mikado in 1.5" scale

That is right also. Melting one side of a crucible and leaving the other cool black, will not be efficient or thrifty on equipment. A gas flame should never impinge on the outside of the vessel, it should be heated by the hot gas and incandescent liner of the furnace for best effect. A good two pass swirl around with the atmosphere is ideal.

Nobody that does founding uses solid fuel anymore, it is a throwback to earlier times. It can work, but has been replaced by more reliable methods. Stone hammers again, or are they rock? Big debate.

Induction furnaces have taken over the largest part of commercial founding, mainly because of precise alloy control, and stable temperature. Small foundries with low porduction rates will be fuel gas fired. New induction equipment is expensive. The economics of production must be supported.

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Big Dave

Be Alert! The world needs more Lerts.

To say nothing of the homogenous nature of the resulting metals. There's a serious stirring action created in the molten charge.

Harold

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Wise people talk because they have something to say. Fools talk because they have to say something.

That's probably true for commercial foundries, but is not accurate for home foundry types. I use charcoal for my infrequent founding sessions, and it works well for me. I fire up on wood (free), get everything heated up, then switch to charcoal. I keep several bags available so I don't run short. I can melt alum, bronze, or iron. No smoke, no running out of fuel in the middle of a melt, no complaints from the neighbors, no reason to change.

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Dan Watson
Building IC mikado in 1.5" scale