Planning for D2 blade

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Hello All - So I have made quite a few leather skiving knives out of O1 steel and apart from the second mess of a knife, they all came out pretty good. What I would like to try now is D2 steel hoping that it will provide not only the razor sharpness of O1 but would also help to keep the edge longer.

I got myself a used laboratory electric muffle furnace capable of sustaining 2100 f (for free!). It’s a small furnace 4x4x6 but large enough for my small blades, and well, it was free.

Plan is to shape as usual then harden at 1825f for 30-45 mins in a stainless steel wrap. Pull out of oven and open bag. Let rest until about 50-60 degrees and then double temper @ 400f for two hours each cycle.

Does this sound reasonable? Is there anything that I should be aware of before jumping into it? I hear that some people preheat. Is this necessary? I would like to shoot for around 60 rockwell.

As always, thanks for your time guys.
 
Pre heat really is only needed in complicated cross sections, which knives are not. No real need to pre heat. I would do 1850F for 30 minutes, with you SS foil, and plate quench. After the plate quench and once able to handle bare handed (without delay), to reduce the significant amount of retained austenite D2 has in it post quench, the blade will either go into cryo (LN2), or at least a sub zero slurry (dry ice), to help eliminate the retained austenite. After that, then on to 400F tempers.

Traditionally retained austenite has been dealt with during the secondary hardening tempers (around 900f-1000f), but the secondary hardening brings it's own problems to a fine cutting, stable, edge. Cryo or sub zero is the preferred method of dealing with RA.
 
Hmmm. I didn’t account for a sub zero quench. I can get some liquid N2 but so don’t have a dewar. Dry Ice will probably be the easiest for me.

To make a slurry could I put the dry ice into glycerin or glycol? What is the typical “oil” or carrier people use?

Thanks for the reply Samurai!
 
You will get as many different answers as you do replies for the slurry mix. I use denatured alcohol, instead of acetone or anti freeze, etc. One gallon of denatured alcohol, and about 5 pounds of dry ice. Crush the dry ice in the bag with a hammer, the finer the better, but in all reality it probably doesn't matter. I like to get it crushed up well. Pour that into your metal container (I use a plastic igloo, it's only denatured alcohol). Then add your denatured alcohol.
Insert blade and soak (I have mine suspended from a wire). No need to soak long, as all we are doing is martensite conversion, which happens literally at the speed of sound, once the temperature has been reached. But, with that said, I find it convenient enough just to leave it overnight, that way the dry ice is all gone, and the alcohol can be poured right back into it's container. One hour would probably be plenty for the conversion we need, but I like to wait overnight so the dry ice has evaporated completely. If there is any DI left in the alcohol, and you pour it back into the metal can, it can blow it up if you screw the cap down!
 
Thanks for the very thorough answer. I’m excited to use a new steel though I am still only making different types of leatherworking tools and chisels.
Will probably try my hand at a honest hunting knife next.
 
If I may just ask as well, what is the critical temperature that needs to be reached with the cryo treatment?
 
" What is the critical temperature that needs to be reached with cryo treatment?" Well, technically that will depend upon the steel itself. For example, low alloy steels will have near 100% of their martensite conversion done at around 200F (well above room temp). That is why simple steels like 10xx series, O1, 52100, etc don't need any sub zero or cryo treatment. Their retained austenite amount is so low, there is no benefit. You can prove this yourself by hardness testing after quench then after sub zero or cryo. If you DO SEE an increase in hardness with those steels, you need to rethink your heat treatment!!!! The higher alloyed steels, I would say A2 and UP, you WILL notice an improvement in HRC after the sub zero or cryo. A2 after quench has more retained austenite than it "should" have for a fine cutting blade. D2 even more so. The higher the alloy, the higher the hardening temp, the more RA will be present in the steel. Cryo (LN2) will convert more retained austenite in steels over sub zero (dry ice slurry), but we're talking the higher alloyed steels. If your goal is ultimate D2 performance, and you will be working with D2 quite often, my recommendation is get set up with LN2. Sub zero is BY FAR better than nothing with D2, tho. DO IT.

OK back to the question. I am not exactly sure what the Mf temp of D2 is, as Mf temp is dependent upon not only the aus temp, but the cooling rate as well. I THINK it is about -120F, in which case a dry ice slurry will just about get most of it, and cryo will get it all. Technically there is never 100% conversion. I have a data paper on CPM M4. The martensite start temp is 375F, and martensite finish temp is -130F. That gives you an idea of D2...it will be similar.

One other benefit about cryo (LN2) that sub zero (dry ice) cannot do. Cryo baths (LN2 with a soak of 8 hours+) will allow the formation of "eta" carbides when the blade is tempered (do not use secondary hardening tempers here with cryo!!!!! Stay in that 300f-450f range!!!). These carbides are super small, like nano sized. These will form after an extended LN2 soak and then tempering in that 400F range. These eta carbides do 3 things that will benefit most all alloyed steel. 1. They increase overall toughness!!! 2. They increase overall wear resistance!!! 3. They give the matrix added cohesion between the aus grain and carbides!!!
 
Thanks for the comprehensive reply. It certainly helps me with getting better insight into the topic.
 
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