Liquid Nitrogen Treatment Questions

I have searched around for cryo and LN answers, and haven't found what I was looking for. I am going to utilize this process during the HT of my blades. I will only be doing one blade at a time right now. Just getting started. My questions are:

1. How much LN do you usually buy and how long does it last? 10L, 20L, etc.

2. What type of container do you use to treat your knives with the LN? (do you just use the container the LN is in, or do you have a separate container you use for when treating a blade)

3. How are the blades sitting in the LN? Suspended, etc..

4. How long are you treating the blade? I have seen overnight, and as quick as 30 min. I have CPM-3V at .190 thick.

5. Do you treat it more than once? I have seen multiple and just once.

Thanks guys!!!
 
[video=youtube;b9Q_AobEfWE]https://www.youtube.com/watch?v=b9Q_AobEfWE[/video]

Here is a video from USA Knifemaker that talks about LN, fast forward to about 47 minutes or so.
 
see the red replies below...

I have searched around for cryo and LN answers, and haven't found what I was looking for. I am going to utilize this process during the HT of my blades. I will only be doing one blade at a time right now. Just getting started. My questions are:

1. How much LN do you usually buy and how long does it last? 10L, 20L, etc.

We have a 30l dewar I bought used on eBay for around $200. It was used for veterinary applications. It's pretty much like this one: http://www.ebay.com/itm/UNION-CARBI...hash=item3efd60b48a:m:mVyWR8CX7Rv_By5PLqCrFLQ
mine did not come with a lid. I rolled up some mattress foam (mold foam for kydex will work) and made a cork out of that.
I also built a box out of plywood on wheels and insulated it with 1" insulating panel. 20liters lasts about 6 to 8 weeks depending on how much we use it. If you don't use an insulating box, it might last 3-4 weeks so it's worth building one.


2. What type of container do you use to treat your knives with the LN? (do you just use the container the LN is in, or do you have a separate container you use for when treating a blade)

Use the dewar. You will boil away too much otherwise.

3. How are the blades sitting in the LN? Suspended, etc..

Hang them from a wire. In many dewars there are baffles built into the bottom that a blade can hang up on. Lower it in slowly and enjoy the fog show that comes out of the dewar.

4. How long are you treating the blade? I have seen overnight, and as quick as 30 min. I have CPM-3V at .190 thick.

I use 8 hours as that is what I found doing my research on cryo. I have seen some use 3 hours but the charts I've seen tend to show there is more to be gained at 8 hours. I just put a blade in overnight and it's done in the morning. 30 minutes may not be enough time to fully convert available austinite to martinsite. The stuff is around -280F so the atoms take time to get in line.

5. Do you treat it more than once? I have seen multiple and just once.

Just soak it once.

We buy it at the local welding gas supply, Praxair. They can't seem to get a full 30l into the dewar as it boils, bubbles and farts when you fill the container. We just filled ours last week and it was around $50. This was a bargain as I have never paid the same price twice for getting it filled.

Thanks guys!!!


Have fun.
If you have a hardness tester, take a reading before heat, after the quench, after the cryo and after temper. This is good data to know.

After cryo, it's pretty brittle so careful where you take your reading. Too close to the edge and it could crack. I've done it. Let it get back to room temp and then get it into temper right away. It is so brittle and stressed after cryo it can crack just sitting on the bench overnight. Something else I've also seen.

Cyro tends to add around 1.5RC to as much as 3RC in hardness. I see an average of 1.5-1.75 increase. Some say that if you properly heat treat and quench in the first place, you don't need to cryo. I agree when it comes to carbon steel but my experience has been different on stainless alloys. Also most steel manufactures recommend cryo for stainless and those guys have been in business for a couple hundred years and have expense savings down to a fine science. They wouldn't suggest it if it wasn't worth doing.

There are two schools of thought when to cryo: 1) cryo after the first temper or after a "snap" temper (quick 1 hour at 350F) to avoid stress cracking. 2) cryo after the quench when the blade is back to room temp -- which is what I do. I've tested both ways and see a bigger change in RC with the cryo after the quench, before the first temper. I have never had a blade crack or chip when dropping it into the juice. Maybe I've been lucky but I wouldn't worry too much about that.
t
 
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You are spot on Tracy. Some folks get confused between retained austenite mechanisms in carbon vs. stainless, or even highly alloyed, steels and if they say that cryo is not necessary to combat retained austenite in those richer alloys, they would be wrong and a quick review of the basic metallurgy behind it would clarify things.

In something like 1095 the only thing interfering with the austenite’s transformation to martensite is the carbon levels in solution, so careful control of temperatures and limiting the amount of carbon in solution easily defeats retained austenite. But with richly alloyed steel generous amounts of large substitutional alloy atoms (Ni, Cr etc… ) reinforce the iron matrix and makes the austenite much more resistant to transformation and pushes the point at which the quenching process is complete to below room temperature. This would be the case no matter how much carbon you did, or did not, put into solution, it is inherent to the alloy, so just being careful in hardening is still not enough, and that is why industry recommends the deep cold treatments, to complete the quenching process.

I hope I have never left the impression that highly alloyed steels would not benefit from liquid nitrogen. When I have heard people mention that they got a couple more points HRC from 1095 by putting in the kitchen freezer, I have strongly urged them to review their hardening temperatures, but that is a whole different story from zapping that retained austenite in stainless or highly alloyed steel.
 
Ok, Kevin, a point of clarification. When you refer to substitutional alloys are you referring to atoms of these alloys replacing iron atoms in the iron matrix of steel? This is something that doesn't appear in any of my reference books, few that they are. I also take it that this is totally different than carbides of these elements forming at the boundaries of the iron crystals.

Doug
 
Doug, I specify substitutional atoms to differentiate them from interstitial alloy atoms, like carbon. Textbooks that will get more in-depth regarding modes of alloying within the iron matrix will cover these differences but average heat treating literature may not. The difference is in where the atoms will reside within the iron matrix, in the spaces between the iron atoms (interstitial) or occupying a space where there was an iron atom (substitutional). Interstitial carbon atoms are very mobile whenever the matrix stacking changes to allow it, but substitutional atoms move at a glacial pace since they need to find a way to exchange positions with adjacent iron atoms. This is why you get equalization in damascus carbon content while the other chemistry stays within the original layers even at welding heats.

The reason substitutional atoms are substitutional is that they are too large to be interstitial, a chromium or nickel atom is huge. This creates a distortion within the planes of the iron matrix that makes slip movement of those planes more difficult. The austenite to martensite transformation is shear based (deformation via slip) so these elements tend to stabilize the austenite by allowing it to resist the strain forces necessary to make martensite, and more cooling than normal is required. With enough big, fat Ni atoms you stabilize the austenite enough that you get austenitic stainless steel.

Carbides are compounds of iron or alloying elements with carbon that is out of solution, so they don’t directly count in this discussion since austenite is the solution made from dissolving those carbides.
 
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Thanks for taking the time to replay. I'll read through it a few times and I think that I will get a better handle on what you are saying. Just another point of clarification to make sure that I'm reading this right. You state "...so these elements tend to stabilize the austenite by allowing it to resist the strain forces necessary to make austenite..." Is that correct or did you type in another austenite when you meant to type in something else?

Doug
 
Its great to read these posts.. I always learn when Kevin starts talking. Also appreciate Doug's questions and posts which often help solidify things. "The more ya know"

Thanks to Boss!! your input is often appreciated. ;)


Randy
 
... You state "...so these elements tend to stabilize the austenite by allowing it to resist the strain forces necessary to make austenite..." Is that correct or did you type in another austenite when you meant to type in something else?

Doug

Good catch Doug, due to some quirk of the interface here I have to type my responses in a word processor and then convert it into simple text before pasting it here. My spell checker has been trained for metallurgy but for some reason will not accept "martensite" and so it autocorrects to austenite if I don't catch it.
 
Good catch Doug, due to some quirk of the interface here I have to type my responses in a word processor and then convert it into simple text before pasting it here. My spell checker has been trained for metallurgy but for some reason will not accept "martensite" and so it autocorrects to austenite if I don't catch it.
....and when it does happen, that's the famous retained austenite LOL ;)
 
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