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Lyesyuv

Heat treating D2 with a coal forge

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Hi all,

I need to HT a D2 blade (Nakiri) in a coal forge. The info I found calls for 1/2-1 hour of soaking at 1020c. It's very difficult to maintain steady temperature in a coal forge. Also - Isn"t there a problem of carbon loss at that atmosphere and time?

 

How much "performance" do I lose if I go with shorter time? what do you suggest?

 

Thanks

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Well, that 1.5 hours is per 2.54 cm of thickness, so you can divide to find the proper shorter time.  Also, if this blade was forged, you have already done some of the soak time.

Not enough time results in poor distribution of carbides.

And yes, D2 is prone to decarburization. Use a muffle pipe (steel tube with on end sealed) in the forge with a bit of coal, charcoal, or even wood pushed all the way to the closed end. This will provide a reducing atmosphere and prevent scale and decarb.

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Only part of the soak time is for thickness purposes.  The old (and still often used) industrial parameter is to hold for 1 hour per inch of thickness, with a 1 hour minimum.  Modern testing has shown that much less is needed, and 1 hour for the first inch with 30 minutes for every inch thereafter is plenty safe.  Things get weird when doing thinner things, because you have to worry much more about grain size (and therefore growth) and the whole parts comes up to temp very fast, but it still takes time to dissolve carbides and let the components diffuse properly.  

One solution to the dilemma is to get it quite a bit hotter for a shorter period of time to dissolve the carbides and allow for diffusion.  Then still air cool.  This will leave you with larger grains, but good carbide distribution otherwise.  Now you can do rather short soaks (like a minute) at just above austenization temp for a few normalization cycles to refine the grains without messing with the carbides.  If you don't get too hot or soak too long, then your carbides won't dissolve and you won't have to worry about them.  You do need to soak for a little bit, both on the normalization steps and the hardening, because you do want to dissolve some of the carbides so that you have some carbon in solution.  

And next time use a more solid-fuel friendly alloy.  ;)  

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Always a good idea to choose a steel that you have the equipment to heat treat.  The higher alloy steels pretty much need to have a regulated heat treating oven to get the most out of them.

 

Doug

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Thanks. First - you"re right about my poor steel choise. When I ordered the D2 I only "knew" it was air hardening, and thought that meens get it to temp and out of the fire. silly me. But now I"m stuck with it. Here in Israel, It's extremely diffucult to get any steel other than O1. We have to mail order it from abroad, and pay an arm and a leg for shipment. So D2 it is.

Jerrod - If I air quench, is there a difference between the normalizing and hardenind cycles (in your suggestion)? Do you mean all these are a minute at 1020c?

Also - Will plate quench be beneficial? The main reason I went for D2 was I hoped air quench will eliminate the worping I always get when I oil quench thin blades. Does anyone know how plate quenching is in that respect?

 

Thanks,

 

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Plate quenching should work for thin D2. And for the record, O1 is much easier to work with in all respects, and is a fine blade steel.  D2 makes a very tough blade, but you'll use twice the abrasives to finish it.

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I should have clarified.  A standard normalization isn't really a good option for D2, especially if it is very thin, as that is indeed a hardening process.  You'll want to heat it to about 1000 C, then only let it cool (in still air) to about 300 C (not below 250 though, as this will start to harden things).  What you are going for here is austenitizing then some stresses from the thermal expansion to get some refinement.  Really though, letting it get hard is also a great way for grain refinement.  If you aren't worried about cracking, I would say do that.  And if you are wanting to make it soft enough to shape prior to hardening then the key will be to do a sub-critical anneal.  If I were to be working D2 my process would be to do the carbide distribution cycle noted in my previous post.  Then let it harden fairly thick (see Alan's warning about going through abrasives), but the key would be to let it cool in a state that is not going to encourage cracking.  Then do a sub-critical anneal or two (or three).  Do all my grinding and sanding.  Harden, probably with plates depending on geometry.  Temper.  Then finish sanding.  But I never plan on using D2 because that sounds like a lot of work to me.  

 

If you haven't seen the TTT for D2, you have plenty of time for hardening.  You have nearly 5 minutes to get it below 700 C, and from there almost an hour until you start to form bainite.  The slower you cool, the less likely you are to develop warps.  So you can let it air cool over the fire a bit to be very slow for a while, then plate quench it or still air quench it from only like 400 C.  Keep in mind that plate quenching from hotter will help correct some warps, while going slow the whole time will help keep them from forming due to the cooling in the first place.  Warps can develop during, heating too.  

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I've been toying with the idea of playing around with D2 (I can get as much as I want from scrap tooling at work), but you're doing a good job of talking me out of it Jerrod.

Lyesyuv, I agree with Alan.  O1 makes a hell of a blade, even if you only have a simple heat treat setup.

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1 hour ago, Alex Middleton said:

I've been toying with the idea of playing around with D2 (I can get as much as I want from scrap tooling at work), but you're doing a good job of talking me out of it Jerrod.

 

No, by all means, give it a shot!  ;)  

Seriously, it should perform extremely well.  It is also one of those alloys that was developed in a time of need, and new alloys have been developed to replace it.  I wouldn't go out of my way to play with it, but if you have access to a bunch it could be fun (I mean that).  

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