Can anyone tell me how low a temperature to achieve, and for how long it's necessary, for effective cryogenic tempering?

It differs for each blade material, you should try to find someone with a set of tables for heat treating.

Thank you. I'll see what I can find...

If you use Liquid Nitrogen, and leave your blades in for 24 hours, you'll achieve better than 99.5% Martensitic transformation in ANY steel that has been properly heat treated and tempered once. Period. Yes, it is that simple!
Don't forget to do at least 2 tempers after allowing the blades to return to room temperature.


Stay Sharp,

RJ Martin

Thanks guys. I appreciate the help.

Been wondering for a while when people say that they keep it in the cold for a long time. Martensite transformation is somewhat fast, it takes practically no time and no diffusion proces works (practically) at -196 C. So why to keep it longer than what temp stabilization takes?

TLM

6 hours minimum is recommended by Crucible for some of the CPM alloys. What's the hurry??????

RJ Martin

So, where do you get Liquid Nitrogen, how much is it and how long will it last in that container (not sure what it is called)?

Ben wanting to play with it but it probably won't last long in the hot days in Texas.

Yea, What Bob Said........

Post some pics of your setups.......Please!

Thanks,

Bob Sigmon

Yes Yes Need more info please Gib

Guys: What you need is called a Dewars flask. It's just a big thermos with a metal lining and a pressure vent.
Taylor Wharton is a nationally known brand. You can buy (order) them from any good welding supply house.
I have a 10 Liter flask, with about a 2.5 Diameter opening.
Evaporation loss is faster with a bigger opening. You need a big enough opening to get your blade inside.

If memory serves, mine loses about .3 L per day, so, a fill will last 30 days, if I don't use it on knives. Of course, if you put knives in, it bubbles off and evaporates more quickly.

knives are strung up on string, and lowered slowly into the LN2, then, the plug is replaced and you wait. The string hangs out the top-there is a slot in the side of the foam plug so it doesn't bind on the string.

Oh-a flask will cost you $450 and up, and LN2 will cost between $1.50 and $3 a liter. remember, you need the blade FULLY immersed, so, a long blade in a half empty flask is a no-go. Taller, narrow flasks are therefore more desireable.

There is no danger of explosion, LN2 is inert. You do have to provide ventilation-asphyxiation is the only danger, aside from splashing. Wear eye protection and gloves, DUH!

Good Luck,

RJ Martin

Thanks for the info. It looks like a trip to the welding supply house is in order.

Luckily it only three blocks from where I work!

Thanks again,

Bob Sigmon

Thanks, everyone.

Great detailed info., R.J. and your safety tips are most important. You all know that that stuff is very harmful if it lands on your skin, fingers, eyes, etc.

Be careful regarding spillage.

Guys: Using a commercial flask that will accommodate your blades is key, IMO, to safety. There is no transferring of the LN2 into another container, which is where spillage could come into play.
If you put your strung up blades into your refridgerator's freezer for an hour or so, there is a lot less boiling off of the LN2. This means that you can immerse the blades into the LN2 a bit more quickly and with less risk of splashing. You do need to pay attention during this step!
The shape of the flask and the size of the opening is a bit like a volcano-if you lower your blades in too fast, that LN2 may boil right up and out. So, use a string about 4 feet long, so you can stand upright while you do this.
Overall, it's a lot safer than grinding, I think.
If you have a hardness tester, you can learn valuable information about your heat treating cycle based on the pre and post cryo hardness values.
More on this later.

Just remember. It's minus 320F. Almost too cold to comprehend.

RJ Martin

Very important tips, R.J. I hope that anyone trying this treatment will follow your safety precautions.

As i work in a physics dept. at Cornell, I work with niquid nitrogen alot and it will burn you if get some on your hands and you expecially dont want to get it in your eyes..so wear your gloves and goggles when working with this stuff...and if you get worts you can use it on those too! A professor here, when we broke ground for our microkelvin lab (for trying to reach absolute zero) actully gargled with it....only for trained professionals.

stan

Okaaaaaaaayyyyyyyyyy!

We'd better delete that "gargling" part!

I know one maker who'd take that as a challenge!

well, the professor in question works in low temp research and is a nobel prize winner.....definitly not for the everyday person to perform and highly unrecommended!

Thanks, Stan.

Safety is the word. Although common sense will dictate to a great degree how we handle this cold stuff.

I/we really apprecaite your input.

First, thanks for all the great information. Now, a question. I just spoke with the local liquid nitrogen supplier about dewars. He asked what I was using it for and suggested using an insulated thermos, like Stanley makes. He claims that other have used this method and that they will fill them there. It does seem very inexpensive. Has anyone tried this? I realize that is is inert, but am I potentially asking for trouble. For example, what if I dropped the thermos?

TIA,
Brett

The main problem with the thermos is the mouth opening size is fairly small.
Big blades won't go down inside.
I have a new DEWER I got from Jerry Hossom last year, and neither he nor I have ever used it.
They run a couple hundred bucks new,I will sell or trade for.
I traded some mammoth ivory handle materials for it.
Anyone interested, E mail me directly please.This is not a for sale forum, OK?
geneosborn@centercross.com

If you just want to do one blade at a time, and waste your time driving back and forth for each liter of LN2, fine. You won't be able to screw on the cap tightly, unless you want the thing to explode, and, the first time you fumble a blade, it's gonna break the thermos, assuming you use a glass one.

You need enough LN2 to keep the entire blade immersed, and, as Geno pointed out, an opening big enough for it to fit into.

There are some things that just shouldn't be done half-a**ed. This sure seems like one of them to me.


RJ Martin

Wow! I haven't been on the forum for a few days. Lots of good info. Thank you everyone!

That dewer is sold.
Thanks for all the responses.