cryogenic tempering

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.