What lambda range do I want for a carburization furnace?

I'm planning on constructing a small natural gas carbonation furnace (about 2 or 3 cu feet of working space) with servo controlled gas and air valves and a wide band O2 sensor to control the mix. I'm guessing that at one end of the range, I'd want to run it at or near a stoich mix for heat treating stuff like O1 and 17-4 PH but what sort of mix do I want for case hardening mild steel? Are there charts available showing the relationship between time, temperature, lambda and depth of carbon penetration? What happens if I run it 'too rich'?

I'm planning on having second burn chamber to hopefully burn off any CO and carbon before I release the exhaust into my building's flue system. Does this sound like a good idea?

I hope I'm not cramming too many questions into one post, but also what sort of air supply will I want? Will a small blower work well, is it overkill, or will I need to feed it some O2? I know this last question will have some thinking I'm going for a Darwin award but I just realized last night how much better it would be to design my furnace around N.G. instead of electricity and I haven't had much time to research things yet.

Thanks for any help you can provide. I'll be sure to let you guys know how it eventually turns out.

-Jeff

I am not sure but you may be overcomplicating things. I do blacksmithing and use gas forges among others. A "rich" mixture will guarantee all oxygen is consumed in the chamber. You know your mixture is rich when the vent or door has a bit of flame coming out. We call this "dragons breath". The oxygen needs to be burned out of the chamber to reduce the formation of scale. (sometimes I will lean it out to create a "scale texture" for cosmetic reasons). If your flame is to rich it will cool the chamber. (indicated by a large amount of unburned gas igniting outside the chamber- you want just a little)
Lots of good info on heat treating furnaces and temp controlled forges is available.
Rob

Solo Inventor said:

I'm planning on having second burn chamber to hopefully burn off any CO and carbon before I release the exhaust into my building's flue system. Does this sound like a good idea?
-Jeff

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What are you saying here?

Sounds like you need to do more research on gas carburizing.

The gas for the heat and the gas for the atmosphere are different. The carburizing or endothermic gas is NG that has been run through a catalyst to enhance the chemisty.
You didn't mention the quench feature?
Pre-heat vestibule?
Yes, there are charts from places like Lindberg and Surface Combustion, but there's still some "black art" to carburizing. There are a lot of other factors that effect case depth and hardness, that's why you cut and section a part to check.
Here's 9 cubic foot furnace if you want some ideas: Heat Treat Equipment | Used Equipment | Surface Combustion IQ Furnace
JR

I think I would spend my energy and time building a vacuum furnace.

It would be more versatile.

Rob F. said:

What are you saying here?

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This is going in a small basement workshop. For safety I want the system as sealed as I can and vent everything up a chimney. I figure that to be extra safe I could use an extra burner to burn off any CO and unburnt fuel first. The burner would be in a separate chamber that would draw in both fresh air and the exhaust gases from the main chamber. This burner would also help establish a good draft up the flue.

Looks like I was wrong about ever wanting a stoich mix. From what I've just read it looks like the mix has to be matched to the type of steel or else carbon will always be leaving or entering the surface. Luckily I don't need things to be precise but it would be nice to have some sort of electronic control since it would be hard for me to see what is going on. If I had a big open space I'd make it a lot simpler and judge the mix by the amount of 'dragons breath' as Rob suggested. Also with the electronic controls I could just set it up and then focus on something else till its ready for quenching.

Solo Inventor said:

This is going in a small basement workshop. For safety I want the system as sealed as I can and vent everything up a chimney.

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I guess that you haven't heard, every now and then, the door will get blown off of a carburizing furnace. It's sure not something that I'd want to do in my basement! Then you've got the quench oil smell.
JR

I am sure that you really don't want to do what you are talking about in the basement of a house, not useless you build a fireproof room to put this stuff in and get an ok from your insurance company and local fire inspector. There are too many things that can and do go wrong. I know, I've had them happen.

Tom

digger doug said:

I think I would spend my energy and time building a vacuum furnace.

It would be more versatile.

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You may be right, Thanks

Maybe, the question should be asked; Have you done any heat treating?

Most small shops will start off using an air-hardening steel. Then all they need is a small box furnace with decent controls, and some stainless foil wrap. Carburizing is done for high volume applicaitons where the cost of using an air-hardening steel outweighs the cost of the furnace equipment.

I should also mention that the furnace control systems for carburization are not that simple. $50K is nothing for a furnace control for a carburizing furnace. http://www.supersystems.com/nitridingcontrolsystems.html

If you want a small box furnace, look at how the Lucifer is built. We've got a bunch of them. Lucifer Furnaces High Temperature Furnaces - Industrial Furnaces - Heat Treating Equipment Fired&name=Box&series=Blue Devil 7000
IMO, for home use, I'd look for a used electric. A lot smaller and cheaper than a gas.
JR

JRIowa said:

I guess that you haven't heard, every now and then, the door will get blown off of a carburizing furnace. It's sure not something that I'd want to do in my basement! Then you've got the quench oil smell.
JR

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Ooh Ooh Ooh.....old special heat treat oven for special exotic lamination steel, hydrogen atmosphere, burning
flame at top opening of furnace, sand seals at floor......woof it went every once in awhile.

Hot sand everywhere

Yep, we blew the 5' square door through the side of the building and 100' out in the parking lot after a power failure.
JR

Throw in the cost of a UPS and backup generator to the mix. Let's see, special room with ventilation, direct access to the outside for fire protection, equipment, backup power system, power and gas company hookups, gas generators, special assessment by the fire department, fire and liability insurance....up front costs including special permits $300,00 plus operating costs $30,000 a year. Cheap compared to having an outside heat treater do the work.

Am I low?

Tom

Thanks for the heads up guys. While I'm confident I know enough to add enough safeguards to prevent the natural gas itself from exploding, having a vessel full of red hot, unstable, carbon and hydrogen based molecules in my basement is probably not the best idea. I can see how a lot could go wrong. Especially during a sudden shut down.

BTW, while working on this idea, I found this site selling inexpensive interfaces for automotive type wide band O2 sensors at WBo2.com WIDEBAND AFR/LAMBDA (Tech Edge) They might be useful for monitoring the mix in a gas furnace but it might be hard to keep the sensor from overheating. I'm not sure how they monitored things with the old style O2 sensing elements, as they pretty much only tell you if it's either lean or rich, but the newer wide band sensors can give you an actual range of readings.