Liquid nitrogen question

[JTknives]

I guess I will pipe up.

In regards to the original question, no you will not see any difference or improvements on hardness. But this is assuming a proper heat treat to start with. During all the heat treating I have not seen any difference in hardness between Cryo and sub zero (<-100°f). We do notice less bowing and warping with subzero and it allows you to clamp blade between plates where that’s just about impossible with cryo.

As for price you have to realistically look at the number of blades you do per week. If your only doing a few blades a month then LN is going to cost you more. And if you do t get a good quality dewar it’s going to get expensive to keep full.

 

[DeadboxHero]

RA% will be lower with LN.

Hardness testing doesn't tell the full story about the microstructure.


Also, if one waits + 6 minutes at Room temp AQ before Cold treatment, the RA will stabilize and DI will not be cold enough to convert like the LN still can.

So LN gives peace of mind that variables are being ruled out. For in house HT and sending out for HT service.

Personally, I want the highest performance cutting knives possible with the highest edge stability I can give them.

LN is a small component of that.

There are other things to worry about.

 

[HSC ///]

I guess I will pipe up.

In regards to the original question, no you will not see any difference or improvements on hardness. But this is assuming a proper heat treat to start with. During all the heat treating I have not seen any difference in hardness between Cryo and sub zero (<-100°f). We do notice less bowing and warping with subzero and it allows you to clamp blade between plates where that’s just about impossible with cryo.

As for price you have to realistically look at the number of blades you do per week. If your only doing a few blades a month then LN is going to cost you more. And if you do t get a good quality dewar it’s going to get expensive to keep full.

Here’s a way to clamp blades that are going into a dewar.

https://www.instagram.com/p/CAZsRD0jHbU/?igshid=62l3rptbrhut

 

[JTknives]

RA% will be lower with LN.

Hardness testing doesn't tell the full story about the microstructure.


Also, if one waits + 6 minutes at Room temp AQ before Cold treatment, the RA will stabilize and DI will not be cold enough to convert like the LN still can.

So LN gives peace of mind that variables are being ruled out. For in house HT and sending out for HT service.

Personally, I want the highest performance cutting knives possible with the highest edge stability I can give them.

LN is a small component of that.

There are other things to worry about.

this is why I said “proper heat treat”. All our blades are continually cooled down to temp. Thy go from the oven into the plates. Then straight from there into the freezer. We also keep LN on hand for any picky steels that might need it. The steel shown above is T1 which is not what the average knife maker uses, at least not what gets sent to us. I would love to see that chart showing steels like AEBL. That is by far the most common steel we see.

 

[JTknives]

Here’s a way to clamp blades that are going into a dewar.

https://www.instagram.com/p/CAZsRD0jHbU/?igshid=62l3rptbrhut

if you do that with AEBL the outside blades will bow outwards so hard that the screws will embed into the surface of the steel. I found this out the hard way.

 

[JTknives]

The other question I have is this. In a knife would a difference of 4% austenite even detectable. And I would expect that extra 4% to lower skittle after a few temper cycles.

 

[Ken H>]

Those are neat clamps and should work good. I'm "assuming" those clamps are for clamping blades between two flat bars similar to using C-clamps. The screws shown would never touch the blade itself. The bars would be as long (or longer) than the blade with those clamps holding the bars to prevent warp.

I used something similar to clamp thin blades between aluminum bars so the bars would go between my 50mm dewar opening. Only used it once on a couple of .062" AEB-L blades but it did seem to work nicely with no warp.

 

[HSC ///]

Those are neat clamps and should work good. I'm "assuming" those clamps are for clamping blades between two flat bars similar to using C-clamps. The screws shown would never touch the blade itself. The bars would be as long (or longer) than the blade with those clamps holding the bars to prevent warp.

I used something similar to clamp thin blades between aluminum bars so the bars would go between my 50mm dewar opening. Only used it once on a couple of .062" AEB-L blades but it did seem to work nicely with no warp.

No. The screws touch the blade and there’s no flat bars

 

[HSC ///]

if you do that with AEBL the outside blades will bow outwards so hard that the screws will embed into the surface of the steel. I found this out the hard way.

OK if you say so but I find it hard to believe when the AEBL steel is 62 Rockwell and the screws are maybe 40

 

[DeadboxHero]

It's about best practices and the sum of all parts.

Using LN rules out the difference needing to be observed in the first place.

One can argue anything of any difference having any value.


"Meh Real world" could be used to marginalize anything that anyone disagrees with.


The fact is LN is more effective.

The other question I have is this. In a knife would a difference of 4% austenite even detectable. And I would expect that extra 4% to lower skittle after a few temper cycles.

 

[Ken H>]

If the screws touch the blade with no flat bars to hold blade flat, how would that prevent warps?

I envisioned something like small quench plates clamped to hold blades flat while in LN. That's what I've done for dry ice or freezer. Works like a champ. I've done something similar using 1/4" X 1.5" aluminum flat bar for LN dewar once and it seemed to work.

 

[JTknives]

OK if you say so but I find it hard to believe when the AEBL steel is 62 Rockwell and the screws are maybe 40

I have already proven that aebl is soft before the first temper, there is a chart floating around here. Up until the first temper the blades can easily be bent by hand. Even blades out of cryo can be bent/tweaked. AEBL is very soft and very prone to warping and any uneven cooling will cause the blade to bow that direction. If you cut a blade out of its foil packet and set it on a cooler surface you can watch the blade bow and rise up in the center. I have nothing to gain by telling incorrect information to my friends here.

 

[JTknives]

It's about best practices and the sum of all parts.

Using LN rules out the difference needing to be observed in the first place.

One can argue anything of any difference having any value.


"Meh Real world" could be used to marginalize anything that anyone disagrees with.


The fact is LN is more effective.

This is also assuming less retained austenite is better. I have done a decent amount of reading on the topic. What’s clear is that the experts have differing opinions on the subject it seams and that there is tons more learning to do. I did come across a really interesting paper on the topic and I felt since we where on the topic I would post a few charts. Everyone loves charts lol. What I have found in my research is how retained austenite affects impact strength. When steel is impacted the impact force causes the retained austenite to change into martensite which helps to prevent fracture propagation. Or at least that’s how I understood what thy where saying.

 

[Willie71]

AEB-l martensite finish is at about -95f, so dry ice/acetone getting to -100f is perfectly fine. Other steels have martensite finish in the -135-145f range (3v, V4e, 4v). This of course depends on austenitizing temp, but not to put extra variables in, LN does a better job in steels that have the lower martensite finish.

 

[DeadboxHero]

Well it's convenient when the RA converts during impact testing, it's detrimental when it converts over time during use and one is left with untempered martensite just hanging out and now more brittle if tested.

Everything is explained beautifully and in more detail here

https://knifesteelnerds.com/2018/12/10/cryogenic-processing-of-steel-part-2/

From Larrin's article, it's explained how the RA can reduce crack propagation by converting to untempered martensite. He also explains in text that the RA is also a ductile itself.

Conversely however, he also shows that the RA can convert under lower stress not just high stress that causes cracking. So when it converts one is left with untempered Martensite which is more brittle and without the neat transformation mechanism to squeeze the crack together.

We can also see that converted untempered martensite can reduce the impact toughness

Most importantly is this.

RA drops the YS.
Hardness correlates with YS but more strongly with UTS.
Hardness doesn't measure the YS directly and edge stability is more sensitive to YS.

This is also assuming less retained austenite is better. I have done a decent amount of reading on the topic. What’s clear is that the experts have differing opinions on the subject it seams and that there is tons more learning to do. I did come across a really interesting paper on the topic and I felt since we where on the topic I would post a few charts. Everyone loves charts lol. What I have found in my research is how retained austenite affects impact strength. When steel is impacted the impact force causes the retained austenite to change into martensite which helps to prevent fracture propagation. Or at least that’s how I understood what thy where saying.

 

[JTknives]

Well it's convenient when the RA converts during impact testing, it's detrimental when it converts over time during use and one is left with untempered martensite just hanging out and now more brittle if tested.

Everything is explained beautifully and in more detail here

https://knifesteelnerds.com/2018/12/10/cryogenic-processing-of-steel-part-2/

From Larrin's article, it's explained how the RA can reduce crack propagation by converting to untempered martensite. He also explains in text that the RA is also a ductile itself.

Conversely however, he also shows that the RA can convert under lower stress not just high stress that causes cracking. So when it converts one is left with untempered Martensite which is more brittle and without the neat transformation mechanism to squeeze the crack together.

We can also see that converted untempered martensite can reduce the impact toughness

Most importantly is this.

RA drops the YS.
Hardness correlates with YS but more strongly with UTS.
Hardness doesn't measure the YS directly and edge stability is more sensitive to YS.

what I’m getting at is what you just showed in your last clip. We are talking about very low levels of retained austenite already. That last graph/screenshot you posted shows very little reduction in yield strength between 8% & 12% retained austenite. If we where talking about 20% or more like all the other quotes you inserted then I would agree with you in that retained austenite is bad.

 

[JTknives]

AEB-l martensite finish is at about -95f, so dry ice/acetone getting to -100f is perfectly fine. Other steels have martensite finish in the -135-145f range (3v, V4e, 4v). This of course depends on austenitizing temp, but not to put extra variables in, LN does a better job in steels that have the lower martensite finish.

This is why we switched and bought a lab grade freezer. Everything gets a -86°C (-180°F to -190°F) overnight soak. I was not trying to say dry ice was the best for everything.

 

[DeadboxHero]

Everything gets a -86°C (-180°F to -190°F) overnight soak.

What is the cooling rate? 12hrs to -86°C?

 

[Willie71]

This is why we switched and bought a lab grade freezer. Everything gets a -86°C (-180°F to -190°F) overnight soak. I was not trying to say dry ice was the best for everything.

I know you are doing things right. Just clarifying the difference. If the steels I listed were heat treated at lower temps, there would be less RA, and martensite finish would be higher, so going below -100f would not be needed. You will get a fine blade with those steels below probably Rc62 with dry ice, but higher aust temps will benefit from going colder. Even then, thete is a point where no cooling will get rid of most RA.

 

[JTknives]

What is the cooling rate? 12hrs to -86°C?

Probably faster then liquid nitrogen. We keep sets of steel plates in the freezer ready to clamp each blade. Each set is 2 bars of 1/2”x3” wide steel that takes bolts and c clamps. If you want to know what the cooling time is for a bare blade I can chart that with my thermocouple.