Heat Treating in a Controlled Atmosphere
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  1. #1
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    Default Heat Treating in a Controlled Atmosphere

    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.

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    Couple things come to mind:

    1) Replacement of atmospheric gas takes a while, and argon's not free. How about using the stainless foil wraps, but with a stainless tube running into the wrap so it just purges where the part is?

    2) Set the foil packs vertically, blocking them in with insulation blocks to constrain them.

    3) Use pottery furniture (ceramic shapes) to brace the parts? I have "half moon" strips of Al2O3 that are ~1" x 1/2" x 12", could they sandwich a piece of your D2 and be constrained with a stainless wire overwrap? If you think that could work I'd be happy to send you a couple pieces to test for postage costs.

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    Vacuum furnace.

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    Quote Originally Posted by digger doug View Post
    Vacuum furnace.
    Wut? You set fire to a Shop Vac??

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    Have you tried Nitrogen?

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    Quote Originally Posted by Milland View Post
    Wut? You set fire to a Shop Vac??
    "fire starter" they are....

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    Send the parts to a reliable heat treater.

    Tom

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    Quote Originally Posted by 4GSR View Post
    Have you tried Nitrogen?
    Not yet but I've read it's cheaper so I plan to pick up a bottle. We had a few Argon bottles already for welding.

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    Quote Originally Posted by TDegenhart View Post
    Send the parts to a reliable heat treater.

    Tom
    We've tried a few but none of them will do anything about the straightness issue. They'll harden the parts, but any fixturing involved with it is in our hands. We don't have issue hardening D2 in general, it's the specific elements of this part being narrow and flat that make's it troubling.

    I'll add too that we're trying to hit a price point with this piece. It's an expendable part on a machine that we can buy from an OEM manufacturer, but the cost is so high that our customers tend to avoid replacing it until the machines are trashed. We've successfully made them in the past, we're just trying to simplify the process.

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    Let me clarify that first, I'm trying to understand the controlled atmosphere heat treating process, and second how our oven can work with it.

    I'm trying to find where we bought the oven from to see what they say in the matter. No tags on it though other than for the control.
    hzaxknzg.jpg
    o6uxjwga.jpg
    Out of the box, the oven had a single gas inlet with a flow control regulator. We added some plumbing to connect it to a tank regulator.

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    from my personal experience tig welding things using back purge, argon is heavier that air, so to purge the air you'd want really slow flow from the bottom towards the top, high flow will make the argon mix with air and the purge will take forever, there has to be an opening at the top to allow the air to escape, for a furnace, I'd guess the opening could be closed after few minutes

    I've seen people use a stainless box with a lid and a stainless tube running from the box the outside (inlet for the gas), so they wouldn't waste more argon than necessary

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    Nitrogen should cost less. I know it's used in semi load qty's for basic shielding locally (explosion proof electricals, lead pot coverage, paint activator tanks, etc.)

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    Nitrogen has been discussed in the past here, with cautions about undesirable reactions to the nitrogen. Another problem is that unless you have a vacuum furnace, the refractory is saturated with air and moisture. Heating with drive those gases and vapors out but slowly as the heat penetrates the refractory. Most furnaces that are not vacuum enclosed the article to be HT'd in a coating or a sealed box. Depending on the size of the article, if you are going to do much HTing then it may be cheaper in the long run to get a vacuum furnace.

    Tom

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    I made a SS tube with a tube running to the outside of the furnace much as jz79 describes running nitrogen, didn't work , got scale no matter what the volume of gas flow. Back to the bags. If someone makes it work please tell us!

    Ed.

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    Quote Originally Posted by atex57 View Post
    I made a SS tube with a tube running to the outside of the furnace much as jz79 describes running nitrogen, didn't work , got scale no matter what the volume of gas flow. Back to the bags. If someone makes it work please tell us!

    Ed.
    "retort furnace" is I believe what they are called.

    Neighbor's weld shop made one 20' long, 18" wide and 12" high, with end flanges, with gas connections....all from Inconel....parts travel thru the tunnel, heat is from the outside.

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    Quote Originally Posted by TDegenhart View Post
    Nitrogen has been discussed in the past here, with cautions about undesirable reactions to the nitrogen. Another problem is that unless you have a vacuum furnace, the refractory is saturated with air and moisture. Heating with drive those gases and vapors out but slowly as the heat penetrates the refractory. Most furnaces that are not vacuum enclosed the article to be HT'd in a coating or a sealed box. Depending on the size of the article, if you are going to do much HTing then it may be cheaper in the long run to get a vacuum furnace.

    Tom
    Yeah, you can only get so good of an atmosphere with that sort of furnace purging. Maybe you could build a simple retort for a trial run? Like a stainless box with a couple of fittings for gas(and maybe a thermocouple)?

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    You were using CO2 for carburizing?
    The ones I ran did like that two on the end as it can literately suck the carbon out of the steel doing just the opposite of what you want.
    I hope you meant CO. For me any CO2 in the furnace meant a lot of scrap parts and an inquisition as to how I let this happen on my watch.
    Bob

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    Have you contacted the steel supplier?

    I did a search and came up with

    High Speed Steel | D2 Steel | D2 Technical Data

    for D2 steel

    If you call the supplier and ask they can provide technical data for what you are wanting to do. The temperatures shown in the graphs are not insignificant numbers. I believe it would be important to track their requirements precisely.

    I also like the idea of the part being in a sleeve so the gas can be held to correct concentration. The sleeve has to be able to allow the temperature of the atmosphere in the oven to transfer quickly so perhaps choose a material to allow excellent temperature coupling between the sleeve and the part.

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    Quote Originally Posted by CarbideBob View Post
    You were using CO2 for carburizing?
    The ones I ran did like that two on the end as it can literately suck the carbon out of the steel doing just the opposite of what you want.
    I hope you meant CO. For me any CO2 in the furnace meant a lot of scrap parts and an inquisition as to how I let this happen on my watch.
    Bob
    We never got to, but I think your right it was CO we were going to try. We might revisit that process but I think the standard oil quenching or packing parts in case-hardening compound would produce the best results.

    A lot of our parts that previously called for case hardened mild steel have done better by making them out of air hardened tool steels. It's been a while since we've needed to case harden something.

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    I found the oven's original manufacturer, Paragon in Mesquite, TX. I'll get in touch with them and see if they have any ideas.


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