Cryo: opinions on proper length of time to soak

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I have recently been working with steels that are supposed to perform better with a cryo treatment. S35VN, Nitro V, etc. Up until recently I have only been using CPM154, PSF27 and various carbon steels for my trade so I never really considered cryo. I am fortunate that I have a welding supply a mile from my house that sells liquid nitrogen so I’m in business so to speak.

In everything I’ve read, soak times vary from 30 minutes to overnight, with some even soaking 24 hours. There seems to be no rhyme or reason to it. My first batch of S35VN I soaked for 30 minutes immediately after heat treat but obviously before tempering. I wasn’t very happy with the results, as it didn’t really increase hardness at all per my hardness tester.

Most data sheets for a particular steel will only reference a “cryo treatment “, never mentioning a specific time.

Im interested in what your preferred soak times are. Thanks in advance.
 

Stacy E. Apelt - Bladesmith

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The normal time is 30 minutes minimum to 6 hours maximum. The changes that occur at cryo temps take a little time. I don't know that there is any definitive exact length of time.

What is important is the time delay to get the blade into the liquid nitrogen. This must be as quickly as possible after the blade reaches room temperature. Once below 100°C/200°F the blade should be cooled in water to drop to room temp and then placed in the cryo. 10 minutes is the normal time frame to do this. After one hour it may be of no use to do the cryo treatment. This is due to stabilization of te retaiuned austenite.
 
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Oops. I’ve only done cryo once now and I didn’t cool the blade to room temperature. I quenched it between my plates and cooled it with air until it was comfortable to hold by hand, tied a wire to it and went straight to the liquid nitrogen. It didn’t crack or warp, so hopefully I’m ok. I left it in the quench for about 3 hours.
 

scott kozub

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Oops. I’ve only done cryo once now and I didn’t cool the blade to room temperature. I quenched it between my plates and cooled it with air until it was comfortable to hold by hand, tied a wire to it and went straight to the liquid nitrogen. It didn’t crack or warp, so hopefully I’m ok. I left it in the quench for about 3 hours.
You want to dunk it in water to get it as cool as possible so you don't boil off as much LN. just make sure its dry or it'll form a layer of ice
 

Larrin

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The transformation is instantaneous and controlled by temperature. That is why only an hour is necessary. If you know that it didn’t increase hardness that means that you didn’t go immediately into cryo. No need to measure hardness first. You can find journal articles claiming that an extended soak is required for enhanced wear resistance but those papers are wrong. It’s the biggest scam in materials science.
 
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The transformation is instantaneous and controlled by temperature. That is why only an hour is necessary. If you know that it didn’t increase hardness that means that you didn’t go immediately into cryo. No need to measure hardness first. You can find journal articles claiming that an extended soak is required for enhanced wear resistance but those papers are wrong. It’s the biggest scam in materials science.
What about the data that suggests longer soaks in LN, such as overnight, forms more and different carbides? I haven’t looked at it all that closely, that’s why I’m asking about it. I’m definitely not arguing with you, just trying to learn.
 

Larrin

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What about the data that suggests longer soaks in LN, such as overnight, forms more and different carbides? I haven’t looked at it all that closely, that’s why I’m asking about it. I’m definitely not arguing with you, just trying to learn.
https://knifesteelnerds.com/2018/12/17/cryogenic-processing-of-steel-part-3/

I also did my own study where I compared edge retention of 36 hour D2 to no cryo D2: https://knifesteelnerds.com/2020/05/01/testing-the-edge-retention-of-48-knife-steels/
 
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imill3567

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I was just wondering about this same thing as I prepare to start using z wear. Thanks for starting the thread and those that have replied. My gut feeling was that it should be a quick conversion but I am no metallurgist.
 
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I was just wondering about this same thing as I prepare to start using z wear. Thanks for starting the thread and those that have replied. My gut feeling was that it should be a quick conversion but I am no metallurgist.
Have fun grinding :D
 

Stacy E. Apelt - Bladesmith

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Blackdirtcowboy - You did nothing wrong. The cooling curve needs to be fast and continuous. No long stops or rests. Your method was fine. It probably used a bit more LN, but there really isn't a problem dropping a 200°F blade directly in the LN.
Most folks get it to the Mf between 150° and 200°F and then water cool before dropping it in the LN tank. The faster this hap[pens ( within reason), the more the RA is converted.

Larrin's studies and other more recent articles have taught me that I bought into the eta carbide wear resistance thing "back in the day".
It seems that even if they form, they will convert back anyway once warmed to room temp. There is a good reason they are called transition carbides.

It is a shame really, It was such fun to watch people's eyes glass over when you would talk about beta carbide transitioning into eta carbide by precipitation at 77.36 Kelvin, and then transforming into theta carbide when warmed back to 298.7 Kelvin.
 

A.McPherson

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Hah! When my girl is having trouble falling asleep she asks me to talk about "things I don't care about" to help her fall asleep. Guess what I talk about?
Phase transitions.
 
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