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Anyone have experience preventing de-carb during HT?

torinwalker

torinwalker

Member
Acquired an electric muffle furnace for heat-treating and looking to shoot the breeze with anyone who has experience keeping A2 and O1 parts from de-carbing during the HT process. I just bought 50' of 321 stainless foil (arrives next week) but am also thinking enveloping in clay with a sprinkle of charcoal or kraft paper inside (demonstrated by Chris on his Clickspring videos.) Also considering Borax if I can contain it to prevent from destroying the lining of my furnace.

Should I have gone propane so to be able to create a carb-rich fuel mix?

What, in your experience, is the depth of decarburization? I've read a few papers (this is a good one) describing the depth to be anywhere from (~0.013") for 2hs@800C on 1075 steel to as much as 1.27mm (0.05") for 2hrs@1000C. For the stuff I'm doing (1" ~ 3" sections, I think I need only to bring it up to temperature and soak for 30 min + 5 minutes per extra inch beyond one. So one hour, say. That would reduce the decarb layer to have the values above, which can be easily fixed during the finishing grind.

If I can find a boat deep enough, immerse the part in borax... but I'm concerned with off-gassing and boiling (spitting of borax outward onto the chamber walls/floor.

I've also heard of a methanol drip - slow drip through needle creates carbon-rich environment akin to positive pressure from an inert gas feed

Anyone have experience with a particular method?

The interior of my furnace is 12 deep by 4.75 wide by 4" high, so not a lot of room in there for big, deep boats, so shooting for clay or stainless wrap. Could also be used to carburize mild-steel to add case-hardening?
 
Well all that is interesting. I’d love to see some photos and updates as you progress. I wish I could be of more help but somebody in here will be!
 
torinwalker I dont think the link you attached is the intended one. I'd like to read up on that just out of interest.
I don't know much about this other than I think the blade guys use sealed foil with inert gas, or their ovens have provisions to add inert purge gas in. One guy I talked to was using molten salt, which is on another level but I think it offers some advantages or they wouldn't bother.
 
The browser URL field was replaced with something else when I went back to copy&paste... not sure. Here is the correct link

https://www.forging.org/uploaded/content/members/public/PDF/Role of Stable Carbide Formers on Decarburization Depth.pdf

Looked at other postings where some suggest using charcoal can release carbon/hydrogen that have a deleterious effect on the elements. Think I'll stay away from that.

Boric acid and denatured alcohol (Clickspring - Making a D-Bit (2m02s)) are another possibility, but I'm still looking for the potential effects on the chamber and elements.

Otherwise, I can tolerate a certain amount of decarb providing it's within the depth that I leave for grinding. For that matter, I can calculate based on section thickness just how much time is required in the furnace, then leave a corresponding amount equal to the decarb depth, and use foil to reduce this depth as much as possible.


Torin...
 
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I use ATP-641 Anti-Scale Coating from Brownells when heat treating O1 parts. It is a ceramic that you brush on prior to heating. When you quench in oil it just falls off and leaves a clean un-oxidized surface. I have an electric HT oven so there is no way to adjust the atmosphere inside to mitigate oxidation. For A2 you should be fine with stainless foil wrap. I looked at using stainless foil for heat treating O1 but the work piece has to be removed from the foil before the oil quench and that seemed clumsy (or dangerous) with a red hot part.
 
I have only brushed it on but apparently you can also dip or spray. So, I think the thickness would depend on how it is applied. When brushing it on, the thickness would be ~1/32" or so (but that is a wild guess). As for drying, it doesn't take long - under an hour for sure. Of course that would depend on how thick the coating is.
 
I'm looking into ATP-304. Supposedly ATP-641 is for stainless and ATP-304 is for tool steel. Based on your statement above, they're probably all the same.

Duff.png


But I have to find a Canadian supplier for ATP-304, so working on that too.


Torin...
 
50' of Precision Brand 321 Too Wrap Foil was received yesterday. I also have some 641 and 304 on the way now, due to arrive Friday. The sales guy said 641 has a greater temperature range and suggested it, over the 304, for use with O1 but couldn't explain to me why. Now I have both to try out.

I bought some cones and "furniture" from the local Pottery Supply House company (psh.com) so I could sit parts off the furnace floor (actually, the furniture will sit on a little ceramic plate that came with the furnace), and fashioned a fork from two pieces of 3/8" square bar to lift my A2 cylindrical square from the furnace after heat-treat. Had to repair (clean) the pyro meter on the furnace because it was sticking at 200, 400, and 620 degrees, and will use the cones to make sure the meter is still working properly, along with a few other meters and thermocouples to corroborate what the furnace is reading.

I'd like to experiment with carburizing 1018 steel as well, so will be looking into getting some clay from PSH after all.

Last issue is controlling the heat. This furnace, being from the 40's, comes with a mechanical motor duty-cycle controller. I imagine inside it has a motor (I can hear the whir of something turning) driving contactors that cycle the power on and off at some period defined by the dial's rotation. My objective is to let the furnace get up to temperature, then at that moment set the duty-cycle to some estimated percentage (say, 950C for A2 out of a possible 1100C max temp = 86% setting on the dial) then let it sit for the recommended soak time.

muffle_furnace1.jpg


I'll have to remind myself to take pictures when all this happens. I have a tendency to get caught up in the moment and often forget to take pictures until it's too late.


Torin...
 
Brian Ross said:
For A2 you should be fine with stainless foil wrap. I looked at using stainless foil for heat treating O1 but the work piece has to be removed from the foil before the oil quench and that seemed clumsy (or dangerous) with a red hot part.
Click to expand...

I am no HT expert so might be way off kilter here. But some of this corresponds to my vague recollection of what one of the bladesmith guys told me. Hopefully I understood it correctly. In liquid quench steels (O,W) by the time you have the foil off, the part has cooled enough so it is entering the quench medium below proper temperature. So the bag did its job of slowing oxidization/carbon loss all right, but it doesn't matter because you didn't achieve proper hardness. So you have nice, scale free, unusable part. And apparently you cant (or shouldn't) over-temp the steel to try & game the temperature loss during foil removal so it enters quench at the right temperature. One fellow told me on small parts you can achieve decent torch results (with practice) but that has more to do with the part always being in the carburizing/neutral flame. So there is no foil & you can enter quench quickly. But this gets less reliable with bigger parts. More mass, hard to raise & maintain consistent temp & lots of fuel energy. That introduces the need for electric oven. Now we are back to square 1 with carbon loss over extended heating period without some kind of inert gas flood.

I think the molten salt bath is another technique that enters the picture. No foil, but no carbon loss, consistent heat & immediate quench. But a specialized setup & even a few more precautions. I think there is even a salt bath quench but now I'm really pulling fluff from my rear end.

Anyways, I think 'A'-type tool steels steels are different because the quench is so much slower in air. Again I am on uncharted waters, but I think they can remove the bag like to put it between ambient temp quenching plates for straightness. Or leave it on because it doesn't factor in the quench temperature falloff. That combination achieves both hardness & low scale.

On the carburizing front, I think that's what all those carbon enriched powders like Cherry Red & Kasenit are about. You will achieve a surface hardness, but its low penetration. Perfecty fine for certain applications, but its not like a fully heat treated (quenched & tempered) tool steel part. 1018 has something like .14-.20% carbon. You just cant get carbon saturation. That occurs back atthe foundry. 4140 has higher .38-.43% which is maybe why one can achieve 'a certain hardness'. Tool steels by comparison: O2 = 1.4-1.8%, A2 ~ 1.0%?
https://www.practicalmachinist.com/...erience-cherry-red-hardening-compound-244242/

One day I'm actually going to sit down & read this HT book I got. Maybe some of this will stick in my noodle. Anyways, keep us up to speed on your progress.
 
I don't claim to know a lot about heat treating but your assessment above is pretty much my understanding as well. I found the attached book to be really helpful. It contains tables of steel compositions, heat treat cycles, quenching times/temps, annealing times/temps, etc.

Torin,
If you find that the controls are not working as well as you'd like, you could look into replacing them with a digital temperature controller and solid state relays. They are both pretty cheep these days and you would end up with an HT oven that you don't have to babysit.
 

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Torin
What voltage is your furnace and how long does it take to heat up.
A heat treat furnace in on my to do list but I haven’t decided on 120v or 240v.
120 I can plug in anywhere but 240 should be a lot quicker.

Thanks
John
 
John, you might want to check out commercial ones only to see the corresponding amp requirement. I think its pretty straightforward watts math & you likely have suitable breakers/wiring. But just in case you were intending to run machines on the same circuit while its running - seems like even the little guys (1500W) pull 13A.
https://www.evenheat-kiln.com/knife-heat-treat-ovens?c=4&pg=category

Do you have a design you are following or something you are creating on your own?

Forgot to mention, looks like offer control kits too. I've heard this can be replicated with less expensive aftermarket parts but I am 'electrically challenged' on these matters.
https://www.evenheat-kiln.com/controls?c=4&pg=category
 
Peter
I’ve found a few designs on the net that I am basing mine on. https://ibuildit.ca/projects/heat-treatment-oven/
The size is largely determined by the size of the bricks and involves minimal cutting. The internal dimensions will be similar to Torin’s.

I believe I have everything necessary to complete the build. It just never seems to get to the top of the project list.
 
Torin
I was just watching a clickspring video where he makes and hardens a milling cutter. He uses borax to protect the work piece.
 
What about purging your oven with an inert gas, like Nitrogen?

The salt bath is a good option, but I have been told that they are very dangerous. Any contact with moisture and you will find yourself coated in molten salt. Yikes!

Cheers,
Chad
 
Crosche said:
What about purging your oven with an inert gas, like Nitrogen?

The salt bath is a good option, but I have been told that they are very dangerous. Any contact with moisture and you will find yourself coated in molten salt. Yikes!

Cheers,
Chad
Click to expand...

Yes I have read that also, very similar to wet lead being introduced to a molten lead pot...there is a contingent of ammo reloaders that are using a molten salt bath pot to anneal fired brass ( molten salt at 700 deg. F or so) and I did read of one fellow that had dried some cleaned brass but missed one and it still had moisture in it...caused an immediate eruption of salt splatters that burnt him in a hundred or more small spots immediately....be careful if you go a salt bath route.
 
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