Cryogenic treatment for Knife steel

Jeff Conti

Jeff Conti

Well-Known Member
For this tutorial, I don't have any photos, but then they would be rather anticlimactic any way.

I recently read: Heat Treatment, Selection, and Application of Tool Steels [Paperback] by Bill Bryson (Author). This is an excellent easy to understand and follow source. In it MR Bryson describes why we would want to cryogenically treat tool steel (knife steel). I was sucked in and had to try it. I have to say, it was easy to do as it seemed when I read it.

The knives should already be hardened and drawn (tempered).

Materials: cheap Styrofoam cooler, K1 kerosene and Dry ice. Bill describes laying the steel in the bottom of the cooler and I wondered how it would circulate the cold properly so I cut 2 pieces of round stock to lay down first.

Then I laid my knives down on the rods, poured about 3 inches of kerosene in the bottom and then slit the plastic bag (holding the dry ice) open a little bit (as described by MR. Bryson) and placed the dry ice in the kerosene. I placed the lid on, punched a couple of holes in the top and walked away. I checked back on it every few hours to make sure the dry ice hadn't evaporated away to early (you want about 6-7 hours of cold).

The next day all the ice was gone and the kerosene was room temperature. Watch out, if you stick your fingers in the kerosene when there's dry ice in there, the temperature is reported to be minus (-)120 degrees F. The frozen flesh (burn) you will get will leave a lasting impression, scars and possibly amputation. As for the holes in the top, the dry ice evaporates and causes CO2 to build up in the chamber. We don't want a tight seal and or any unwanted releases on the floor. Personally, I placed the cooler in my "heat room" on the gravel floor.

Once you've retrieved your blades re-draw them (temper) them (again) about 25 deg below the original draw temperature. If D2 draws at 960 deg F, then draw them again at 935 deg F.

For those of you who have liquid Nitrogen, MR Bryson also has a process for that. As for me, this is simple and my customers/family/friends love the story of their fired and then frozen blades ;).
 
Jeff, thank you for that. At most I'll be an enthusiastic hobbyist, lacking the lifestyle to support a fully decked out shop, so DIY solutions are ideal for me.
 
Cryo has become fairly standard practice with highly-alloyed steels. It is actually more effective in converting RA to martensite if done directly after quench, although this may sound counter-intuitive to guys who are used to oil quenching steels. The RA will stabilize to some degree during the first temper cycle and should be completely stabilized by the time tempering is completed. When done after tempering, the only claimed benefit is long-term dimensional stability. If done before tempering it often gives a point or 2 of hardness over non-cryo-treated steel.
I use rubbing alcohol in place of kerosene or acetone, it is friendlier to handle and less flammable.
 
very interesting and informative..I've got a video put out by Benelli that shows their cryo-treatment of shotgun barrels..in the video it looked quite complicated but ur way is MUCH simpler and attainable...thanks much !!!
 
I would kind of doubt it, there's very little alloying in 52100, according to what I can find... <2% chromium and barely a half percent manganese.
 
Ed Fowler "cryotreats" his 52100 blades in the kitchen freezer and swears by the results. It would have to be something that I would have to test with a hardness tester before and after the quench to say one way or the other on, myself. Being that I don't think that I'm going to get a hardness tester any time soon, I think that I will do without that step until I see something more definitive on it. You could go ahead and try it if you want. It might not do any good but the worst it will do is make your steel cold.

Doug
 
I should listen to Kevin's last words of caution - but I'll offer my understandings anyway.

Dry ice is about -109F, so plenty cold enough for effective cryo. If there is a drawback, it is that it is pretty much all gone in 24 hours or less, so it's a one batch proposition. Liquid nitrogen lasts MUCH longer, but the dewar to store it in is not cheap.

I am told by knowledgeable metallurgists that cryo above Dry ice temperature is not useful. I haven't challenged that by testing, but I have compared room temperature quenches to full cryo quenches and I'm satisfied that there is significant conversion to high alloy steels that is desirable. I have also tested two blades with cryo after double tempering and found no RHC change. That doesn't necessarily mean no benefit, but it probably does suggest maximum benefit when cryo is a continuous part of the quench.

I have cryo'ed some tool and carbon steels after quench. Rockwell testing would suggest little if any positive benefit, but it generally meets the test of "do no harm". I have not had the courage to try cryo on 1095 because that stuff breaks way too easy without adding significant stresses.

In short, I believe all stainless blade steels and some high alloy tool steels benefit from cryo - either dry ice or liquid nitrogen. In answer to the 52100 questions, I would expect little gain, but no expected harm either.

Note, this is just my understanding - based in part on experience and testing - and in part on listening to those who know more than I do. Hope it helps some.

Rob!
 
The dewar's for LN are extremely expensive....$750 and up.

However, I found a local gas distributor that rents me one for $0.55/day. It's another option that can be persued if you want to cryo treat your blades.
 
Used Dewars can be found through AI operations and at many dairy farms. I bought mine for $200 - and while not up to it's new specs, still holds 20 litres through about five weeks of use several times a week. With a bit of hunting around, they can be had for quite a bit less than that. For the occasional user, it's probably not cost effective - but for someone HT'ing once a month or more, the payback would be pretty quick. The rental option is great if you can find it. Very 'cool' idea. :)
 
You must log in or register to reply here.
Back
Top