I've heard such varied opinions that I thought that I would ask all of you. How much does cryogenics actually help a knife blade? What steels does it affect the most and which ones don't really benefit from it? Let me know your opinions.

Gary Mulkey

what cryo does is convert retained austenite into martensite. It'll give you some extra points of hardness if you didn't cool the blade fast enough to get 100% martensite when you quenched it. Use it for stainless steel, or anything else you didn't quench right (sarcasm).

In that sense, it's good, but remember that if you're not very careful you can cause microfractures by chilling too fast (pretty unlikely, but a pre-chill isn't a bad idea), you can crack it by dropping the blade between cycles, or just plain rough handling, and be sure you temper afterwards.

I think it has it's place in air-quenched stainless blades, if you like that kind of thing, but for simple alloy (5160,52100, or any of the 10XX) you need a faster quench if your blades aren't getting full hard. Bringing blades up to temperature slowly, and letting them soak just below critical so that the core of the steel is at temperature will go a long way in a more thorough hardening and help to eliminate the need for the potentially damaging cryo treatment.

Heat treating is a very individual kind of thing, depending alot on shop practices, the batch of steel you have, and the way the planets are lined up that day. There are alot of different ways to get the same results, and alot of people who like to fight over which method is better. Experiment and find out which one makes the best blades with your processes.

Cryo has it's place in bladesmithing/heat treatment. You'll not only gain some Rc points, but from spectorgraph anaylsis, it will also reduce grain size even further than with a straight guench/temper type hardening.
Cryo will have some effect on any hardenable steel, but it really shines with those steels that possess small amounts of Chromium such as 5160 and 52100.
Must will depend on the individual method, but as I use it in my shop with a multiple, edge quench, it will give a 52100 blade approx. 15-20% more cutting ability.
Joe mentioned soaking, and that is OK if one has a very well controlled forge/furnace to heat treat with. This in often not the case with the vast majority of bladesmiths, and is why I never advise anyone to soak a blade when hardening.
Some might call cryo a compensating measure for less than optimum temp control when hardening, if so that's OK, as long as the end product performs to the standards it should. I guess what I'm saying is that there are as many ways to do something, as there are bladesmiths. For me cyro works on the steels I use, with the methods I use, which is what I'm always looking for.
I was sceptical about it when I first started learning, but the spectrographs proved that it does it's intented function very well.

I got to experiment with it this past summer. Three blades of O-1 and 1095. I was impressed enough to keep using it. I did learn a few things; temper 1095 once before freezing and twice after, raise the after freeze temper 25 degrees, and O-1 steel is greatly improved by this. The O-1 blade returned to 5 degrees off straight after a 90 degree bend. Hope this helps.

Dennis,

That's interesting that you double temper after cryo. I had always heard to cryo between the second & third tempers. How did you come across this technique?

Gary

I've had good success with the cryo both ways. There is a greater chance of the micro fractures if it is done immediately following hardening, but the results are greater too. Dennis rings true about the tempering, always run it 25F higher after the cryo treatment.

Gary, sorry to get back so late. Just as Ed said, simple carbon steels seem to be prone to cracking if not tempered once before freezing. I was fortunate enough to learn from a friends experiments. That's what so great about knife makers, we share info.

My only means of doing cryo is with dry ice. Does anyone know what the temperature of dry ice is? Just curious since I'm sure that it is cold enough.

Gary

Ed,

It is time for me to show my ignorance about this.

Are you using liquid Nitrogen to Cryo?

Do you try to slowly get it cold? I'm sure you don't just stick it in the LN as that would be pretty drastic temp change and could cause a crack. How do you go from ambient to submerged?

How do you store it? I believe there is a special container but is it just sitting in the shop? What keeps it from boiling away?

Where do you get Liquid Nitrogen? The only place I ever saw it was at the Doctor's office for burning warts off of people or at a breeding facility for animals.

Is it expensive?

Are there any requirements you need to meet to buy it? Is it considered hazardous?

I haven't tried it yet but the process as explained by Bruce Di Vita is to sandwich the blade between two layers of dry ice placed in a cooler for a minimum of 6 hours. I don't have access to any other methods.

Gary

Gary, I used dry ice to freeze my blades. It will get down to -125F degrees. Here's what I did: Take a cheap styrofoam ice chest that is tall enough for your blades and fill it with dry ice. I had to use the pellet type since that's all I could find. 20 lbs. of dry ice for 75 cents a lbs., not too expensive. I then took a steel square tube a little taller than the ice chest and welded a plate on one end. Put the square tube in the center of the chest and pour the dry ice pellets around it, then fill the tube with kerosene. Cut a hole in the lid of the chest so it will fit on top of the chest. WARNING : drill a 1/4" vent hole in the lid. Dry ice needs to vent !Put your blades in the kerosene overnight and they'll be very frozen tomorrow. The kerosene does not freeze, it stays liquid.If I left something out or if this is hard to understand let me know, I'll try it again.

Dennis,

What is the advantage of using the kerosine over placing the blades directly in the dry ice? Is it to avoid direct contact or a temperature varience that you're after? Also, as dry ice evaporates it gives off CO2. Does the vent hole in your cooler help keep the lid on tighter or is there another advantage to the vent?

Gary

Hey Bob!

I use Liquid Nirtogen, kept in a "dewer" tank (the tank used by vets to keep semen in for artificial ensimination). The tank I have holds 56 lbs of liquid nitrogen.
The tank is basically like a huge thermos bottle. I did have a hard time keeping it from boiling away until one day I had a brain storm. I built a plywood box with 1" foam insulation covering the inside, I then set the tank inside, and filled all the airspace with vermiculite. Now a bottle will last about 6 months. It is rather pricey to get the tank filled..........about $70. I get it at the local welding supply house. No need for any permits of such, just drive up with the tank and then taker it home.
I did have a problem early on trying to figure out how not to slowly bring a blade down to temp without shocking it. I found after several tense experiments that I really didn't need to worry about bringing the blade down slow, it's the warm up that will really screw things up!

I have never had a blade crack by submersing it in the Ni, but I did have a couple go "PING!" when I took them out and laid them on a rack to come back to room temp. After that I found two solutions that worked well. 1. Take the blade(s) out of the liquid Ni and quickly place it/them between two pieces of Ka-Wool and allow to warm to room temp (about 2 hrs.) , 2. (My favorite), Bring them out of the liquid Ni and stick them down in the vermiculite surrounding the tank. Again it take about two hours for them to get back to room temp. There are many different ways to run the "steps" of cyro, temper, etc., my preferred method is to temper three times, cryo, and then temper again at 25F higher than the previous tempers.

Gary, the kerosene is simply to have uniform temp. surounding the blade. The 1/4" hole is to let the CO2 escape. The big square hole is to allow the lid to fit down around the square tube. Dry ice doesn't last very long. This summer it was 100+ degrees and it lasted only 2 days. Now that it's cooler (in Calif. 40 degrees is cold) it may last longer.

Thanks,

I'm excited about giving it a try. I feel a lot more confident in the procedure now.

Gary

CO2 will get you down to -70C, if kerosene is used as a medium then not quite as far. Basically martensite transformation is instantenious so no dwell is needed beyond temperature equalisation. It is usefull on those alloy steels where alloying and/or carbon has partly stabilized austenite down to room temperature. It is not much fun to try it on something where the stabilization is intentional like AISI 304 (never seen a blade made out of that), it can easily take temperatures down to liquid N, I think that some of the cryo dewars are made out of that.

Brittleness in most cases follows from going below the steels critical temperature where it is brittle anyway, ofcourse the stresses from quench do not help at all, so against that doing one temper before going deep down might not be a bad idea.


TLM

Ed, Where did you get your dewer tank? Approx Price?

Thanks,

Bob Sigmon

I picke up my tank 3rd hand from one of the cattle ranches on the east slope of the rockies.
It ran me $300, later I check the prices of new ones and the same tank new is $850.
The hardest thing to find is one that has a large enough opening to get blade sized items into. The neck on mine is only 1 1/2", so anything larger just ain't getting cyro'ed!