Heat Treating in a Controlled Atmosphere - Page 2
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  1. #21
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    Have you experimented with straightening after heat treat and draw?

    Our local heat treater offers a straightening service. It's my understanding that they reheat the finished workpiece back up to it's draw temp and do the deed while it's hot.

    Maybe a hot plate set to 400 deg would be sufficient to tweek them back to straight.

    Just a thought.

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    Quote Originally Posted by Thunderjet View Post
    Have you experimented with straightening after heat treat and draw?

    Our local heat treater offers a straightening service. It's my understanding that they reheat the finished workpiece back up to it's draw temp and do the deed while it's hot.

    Maybe a hot plate set to 400 deg would be sufficient to tweek them back to straight.

    Just a thought.
    Yes, but we do them cold. After machining they are straightened in an arbor press, then stress relieved, then straightened again, then heat treated, then ground flat. There's a little bit of bow that's permissible if it can pull down easily during the final grind as it will likewise pull flat in assembly. We're just trying to minimize the stored stress in the part.

    Perhaps hot straightening after stress relief would put less stress back into the part.

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    I think you can straighten them easily above 600f if you don't let them cool first.

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    Ok, Paragon got back to me with some documentation on the process.

    A couple key points are the flow rate of the gas, and time for it to cycle. They recommend that the gas be regulated to 20 psi, and the flow meter be set to the number of cubic feet in the furnace. Ours is 3.5 cubic feet, for example, so we would set it to 3.5 SCFH.

    The atmosphere will cycle 5 times per hour for every 1 SCFH at room temperature, but that happens faster at higher temperatures since the gas expands as it gets hotter, 50 times per hour at 1000 degrees F. The gases will mix the longer you keep positive pressure on the incoming gas and seep out of the door, which has a fiberglass seal. How 'pure' of an atmosphere depends on your needs. The less oxygen, the better, but that takes more gas and more time for it to saturate the atmosphere.

    I'm thinking my first step will be to let the gas flow for an hour before the first fire, keep it on until it gets up to temperature, then shut it off once the oven cools down to 1000 degrees or so, and see how that effects the scaling. If it's still too much, then I'll try letting ti flow for 2 hours prior to fire, then 3.

    Ultimately, I think it should work. We're not the only guys who have a furnace like this, but they apparently are not common. Wither or not it's cost effective I think will depend on the parts being treated. Are there better ovens, sure, and we'll be looking at them when we're back in the market for one, but If I can get a part to heat-treat without having to bag it and not have it scale, it has merit.

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    Sometimes, it's best to do this kind of heat treating in house to control warp age and such. Man, the learning curve seems to last for ever.
    Dad used to deal with a small heat treater just south of Bush Airport many years back. They only dealt in small parts. I recall their largest furnace load not being over 25 lbs. Later after he passed, I tried to find them and could not locate the place, don't even remember the name of the place. All I remember they were located next to a UPS depot up off of Christman Road just south of Aldine Bender.
    Ken

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    We put in a heat treat line in a shop I used to run.

    This thread made me cringe a little. A bottle bleeding CO into the furnace isn't a controlled atmosphere- it's just CO in the furnace.

    Proper atmosphere heat treating requires an atmosphere furnace and an endo generator so you can control the carbon potential. And positive pressure so no ambient air gets in, and a way to flare off the hydrogen...

    Heat treaters pay good money for atmosphere furnaces and endo generators, and have proper recording so they can go back and see what went on in the furnace. Seat of the pants- you can get good results one time, and crappy results the next time because you have no control of the gases.

    I'm in the "send it out" camp if you want carburizing and/or preventing decarb...

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    Jancollc,+5!

    Tom

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    Quote Originally Posted by M.B. Naegle View Post
    So we had bought a second oven a few years back, and we opted for one with the plumbing for a controlled atmosphere. Initially we thought about using Compressed C02 to carborize parts, but then we realized that the risk of asphyxiation wasn't worth it so we stuck with our usual procedures of treating O1, A2, D2 steels and using stainless bags to seal them.

    We're currently trying to improve the process for a D2 part that looks like a long skinny blade. It's not sharp but rather acts as a precise guide for another sliding part. These pieces tend to bow when treated, so we're looking for a way to make them that's less involved than the typical rough cut, stress relief, finish cut, stress relief, heat treat, grind, etc. until it's mostly within flatness tolerances.

    We've tried bolting a pair together, and bolting them to a "tombstone" fixture, with varied success, but one idea one of our guys had was to hang them vertically off a little rack. To do so we couldn't wrap them, so we're revisiting the atmosphere feature on the oven.

    Currently it is plumbed to Argon and is getting 5 lbs. positive pressure. I heat treated a drop of D2 last night and started running the Argon with a closed door as soon as I started a heat treating cycle, thinking the air would be forced out. The oven is 13"x13"x36". Went through a cycle and the piece scaled badly. I'm thinking that the air was trapped in there, so how is the oxygen supposed to escape and be replaced by Argon? There is only one inlet into the oven for the gas, but no outlets, so I'm thinking the door is the only escape? Argon is heavier than air too, so should I leave the door cracked on top?

    I'm confident with our heat treating procedures, but throwing in the atmosphere component is confusing things. Anyone have any tips or advise on the subject?

    I'll add too that IMO there's no way around the bowing issue with these parts other than lots of stress relief in between operations, but It would be nice to see the atmosphere feature put to use.
    Oven/furnace like you showed in picture is probably not that well sealed and most probably wont hold 5lbs(PSI??) positive pressure inside. Maybe 5 Inch h2o would be possible positive pressure.

    Air gets out when you "flush" or dilute the internal volume with enough argon. But the oven needs to be reasonably pressure tight without leaks all around the door.

    Large temperature differentials tend to induce strong convective air currents inside and outside of the oven so if any ambient air leaks inside it tends to mix. Not easy like backpurging TIG weld.

    Tube furnace would be lot easier to seal and purge during the warming-up. You might be able to improvise smaller inner cavity with gas purge to your oven.

  11. #29
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    Or a simple bell oven with a loose sand seal....this one should be easy enough to lift off by hand or a simple chain fall attached to the ceiling.


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