Cryo, and sourcing LN2

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I haven't seen many discussions here about cryo treatments. I know what I've learned from larrins articles, and videos, and some things I've picked up here, and there. But I thought if anyone here would be willing to put any pertinent information forward to the fairly new makers like me. It would be really helpful.

To start, I just look at cryo in general, like an extended quench. Instead of taking the steel to room temperature rapidly (or whatever speed is needed for the alloy), you continue the quench, to whatever the temperature of the cooling bath is. Turning whatever austinite is remaining into martensite, considering you get into the cooling bath fast enough, and that the bottom temperature is low enough for the given case. Going off of the information given in Larrins video and article on the topic, it shows how critical speed of getting the steel cold can be however (especially using my dry ice slurry). So I tend to plate quench and get the steel out of the tool wrap and into the bath as fast as humanly possible. I've got an idea on how to go even faster from around hand hot to at least slightly below freezing even quicker but havent tried it yet.

I've also heard (and I do this with what I currently have) that some remaining austenite, that makes it through the original quench, cryo, and 1st temper cycle, could be converted to martensite by doing a second cryo before the 2nd tempering cycle. This is where things seem to come into debate more, but with what I currently use (dry ice slurry), I do a second treatment, just because what could it possibly hurt? Whatever the case may be. I've always heard it recommended to follow any cold treatment with a following temper cycle, because whatever martensite is formed with be unhampered and brittle.

I'm sure there are guys here that have a lot more information, and a better understanding of this, than myself so I would love to hear from them.


Also, I'm planning to get a dewar at some point in the near future. If there is anyone around the houston area, that knows a good source of LN2, I would love to hear about it. I've looked into it a bit, and I'm not sure how promising the results I'm finding for buying it in small quantities (10L or less) is, with what I've found.

 

[Skiller1nc]

I haven't seen many discussions here about cryo treatments. I know what I've learned from larrins articles, and videos, and some things I've picked up here, and there. But I thought if anyone here would be willing to put any pertinent information forward to the fairly new makers like me. It would be really helpful.

To start, I just look at cryo in general, like an extended quench. Instead of taking the steel to room temperature rapidly (or whatever speed is needed for the alloy), you continue the quench, to whatever the temperature of the cooling bath is. Turning whatever austinite is remaining into martensite, considering you get into the cooling bath fast enough, and that the bottom temperature is low enough for the given case. Going off of the information given in Larrins video and article on the topic, it shows how critical speed of getting the steel cold can be however (especially using my dry ice slurry). So I tend to plate quench and get the steel out of the tool wrap and into the bath as fast as humanly possible. I've got an idea on how to go even faster from around hand hot to at least slightly below freezing even quicker but havent tried it yet.

I've also heard (and I do this with what I currently have) that some remaining austenite, that makes it through the original quench, cryo, and 1st temper cycle, could be converted to martensite by doing a second cryo before the 2nd tempering cycle. This is where things seem to come into debate more, but with what I currently use (dry ice slurry), I do a second treatment, just because what could it possibly hurt? Whatever the case may be. I've always heard it recommended to follow any cold treatment with a following temper cycle, because whatever martensite is formed with be unhampered and brittle.

I'm sure there are guys here that have a lot more information, and a better understanding of this, than myself so I would love to hear from them.


Also, I'm planning to get a dewar at some point in the near future. If there is anyone around the houston area, that knows a good source of LN2, I would love to hear about it. I've looked into it a bit, and I'm not sure how promising the results I'm finding for buying it in small quantities (10L or less) is, with what I've found.

so from the second paragraph, it seems like you have the basic understanding down pat. As for paragraph 3m you are right, it isn't going to hurt but is it worth the time and effort, i don't know? maybe Larrin or someone else can comment with actual fact. Also afaik the temper after any cryo step is necessary. Getting LN isn't too hard any welding supplier will have it, companies like Airgas will have it and normally sell small amounts to the public but if you can find a mom-and-pop style smaller or local shop you will likely get a better price, just call and ask.

 

[Ken H>]

About all there is to know about cyro treating is covered in Larrin's 3 part series here:

Cryogenic Processing of Steel Part 1 - Maximizing Hardness - Knife Steel Nerds

What happens during cryogenic processing of steel? How does it lead to increased hardness? How does retained austenite play into this?

knifesteelnerds.com

Cryogenic Processing of Steel Part 2 - Toughness and Strength - Knife Steel Nerds

The effect of cold treatments on toughness, strength, and fatigue of knife steels. Does the soft, ductile retained austenite improve toughness?

knifesteelnerds.com

Cryogenic Processing of Steel Part 3 - Wear Resistance and Edge Retention - Knife Steel Nerds

Cryo improves wear resistance by increasing hardness. Some claim that cryo also improves wear resistance by other mechanisms. Does it?

knifesteelnerds.com

Welding supply houses seems to be one of the best sources around for LN. Check prices in the area - they vary a lot. Here the price ranges from $5 to $65 to fill a 10 liter dewar. Those are the two extremes I found, with most in the area in the $40 range for the same 10 liter dewar.

 

[Blankblank]

so from the second paragraph, it seems like you have the basic understanding down pat. As for paragraph 3m you are right, it isn't going to hurt but is it worth the time and effort, i don't know? maybe Larrin or someone else can comment with actual fact. Also afaik the temper after any cryo step is necessary. Getting LN isn't too hard any welding supplier will have it, companies like Airgas will have it and normally sell small amounts to the public but if you can find a mom-and-pop style smaller or local shop you will likely get a better price, just call and ask.

That's one thing, cryo is so quick, it doesn't really take any real time or effort to do it a second time. Once it's down to the temperature of whatever method you're using, take it out. So, its only an extra minute or so for the second cryo.

I honestly leave the first one longer, to really be absolutely sure it's gotten as cold as it is going to get, and I would rather leave it a little longer than I know I need to, than take it out before it hits the bottom temp.

The one good arguement I can see for the cryo after the first temper cycle though, is the same one, I can see for recommendations of a 3rd temper cycle, on certain alloys. Usually, as far as I understanding during tempering, and then bringing the steel down to room temperature, there is going to be some retained austinite that gets turned into untempered martensite, that needs to be then tempered in another cycle. The thought i have, is that it could aid in turned more retained austenite into martensite after the first temper. Depending what kind of austenizing temp, and the alloy, are what I think would really be the variables, on whether it ends up making any kind of noticeable difference.

But, yeah its probably not going to do a whole heck of a lot I suppose, I tend to just be a safe rather than sorry kind of guy, when its something that takes barely any time.

 

[Skiller1nc]

J

About all there is to know about cyro treating is covered in Larrin's 3 part series here:

Cryogenic Processing of Steel Part 1 - Maximizing Hardness - Knife Steel Nerds

What happens during cryogenic processing of steel? How does it lead to increased hardness? How does retained austenite play into this?

knifesteelnerds.com

Cryogenic Processing of Steel Part 2 - Toughness and Strength - Knife Steel Nerds

The effect of cold treatments on toughness, strength, and fatigue of knife steels. Does the soft, ductile retained austenite improve toughness?

knifesteelnerds.com

Cryogenic Processing of Steel Part 3 - Wear Resistance and Edge Retention - Knife Steel Nerds

Cryo improves wear resistance by increasing hardness. Some claim that cryo also improves wear resistance by other mechanisms. Does it?

knifesteelnerds.com

Welding supply houses seems to be one of the best sources around for LN. Check prices in the area - they vary a lot. Here the price ranges from $5 to $65 to fill a 10 liter dewar. Those are the two extremes I found, with most in the area in the $40 range for the same 10 liter dewar.

Just to piggyback off this b/c i remembered it. Im in western NC and go to a local chain (like 5 total stores) welding shop and I pay $90 for my dewar. it's $2-3/ liter and doesn't scale by how much you buy, so in houstin idk how useful that is but anyway

 

[Lorien]

I just snagged a 10 litre dewar, and it has a pretty standard 50mm opening, so it's a little limiting when it comes to the width of blade I want to put in there. I'm working on a knife right now which is too big to fit, so I stuck it in the freezer like I've been doing.
For me, the big deal about going the cryogenic route is adjusting my procedure so that I have a bunch of knives to go through heat treatment at around the same time. I'm not sure how long that LN will stick around for, so my plan is to make sure that my window is kept small and that there are knives ready to go when it gets filled

 

[Scaniaman]

Yes definetly get a wide mouth dewar, and 20 or above litres size so you can fit long blades.
Mine is 80 mm and 20 L and can take something like 39 cm blades.
Also, unless you do heat treat very often, you can just go and get a splash of LN2 when you need it. I just did a batch the other night, and picked up 2-3 litres in the morning. Of course this depends on your access to LN2.

 

[Blankblank]

Oh one thing I've heard, is how long the LN2 lasts can depend on the size of the dewar. Larger ones can last over a month, smaller ones about a week. I have no idea the validity of that, but that's what I have heard.

 

[Skiller1nc]

Oh one thing I've heard, is how long the LN2 lasts can depend on the size of the dewar. Larger ones can last over a month, smaller ones about a week. I have no idea the validity of that, but that's what I have heard.

That's correct, there are other factors like airflow around it, shop temperature, etc. I have a 30 or 40L one with like a 4" mouth. It'll hold for around 90 days

 

[Scaniaman]

Oh one thing I've heard, is how long the LN2 lasts can depend on the size of the dewar. Larger ones can last over a month, smaller ones about a week. I have no idea the validity of that, but that's what I have heard.

How wide the mouth is, is a major factor in evaporation rate. On my 20 litre, there is a 48% difference between 50 and 80 mm mouth (0.088 vs 0.13 litres per 24 hrs)

There's no difference in evaporation rate between 3 and 20 litre size though, looking at 50 mm models.

 

[Blankblank]

How wide the mouth is, is a major factor in evaporation rate. On my 20 litre, there is a 48% difference between 50 and 80 mm mouth (0.088 vs 0.13 litres per 24 hrs)

There's no difference in evaporation rate between 3 and 20 litre size though, looking at 50 mm models.

Hmm. I would have thought thermal mass, something something science would have to do with it. But I'm no physicist, so I just need to defer to others experience here.

 

[Scaniaman]

Hmm. I would have thought thermal mass, something something science would have to do with it. But I'm no physicist, so I just need to defer to others experience here.

Me neither. I just looked at the brochure from the place where I got mine.

 

[Larrin]

A 10L dewar can last three months or so but the more you use it the less time it takes for the LN2 to evaporate away.

 

[Ken H>]

I've read of building a nice insulated box for the dewar to allow it to last longer. My 10 liter dewar with a 50mm opening usually lasts around 6 to 8 weeks depending on how often I use it. I suspect if never opened from fill it might last darn close to 3 months.

 

[DeepCutsCutlery]

I'm on the north edge of Houston and get my 10L filled for $30 out the door at Professional Welding Supply in Tomball

 

[Blankblank]

I'm on the north edge of Houston and get my 10L filled for $30 out the door at Professional Welding Supply in Tomball

Thank you! I'll check them out. I've filled out a form with airgas, before I made this thread, so I'm expecting to hear something back from them some time soon. Hopefully I get an answer. If not, it's good to know I can get out to Tomball and get some.

It's also good to hear it can last this long. Honestly going on some things I've read, i was thinking it was only going to last a week. Which is part of why I didn't worry to much about trying to upgrade from dry ice. Because I can get that to last a week in the right conditions.

 

[fitzo]

Study enough to understand Mf, aka, martensite finish. It's the temp at which you'll get no more conversion to martensite. It varies based on alloy content. That way, you'll learn if your steel actually needs -108F (dry ice) or -378F (LN).

And, a note about Airgas: the local jagoffs won't fill my Dewar and when I asked the home office if this was a local or "global" practice, they didn't deign to answer. So, I just bought my welder and accessories elsewhere and I'll get my argon from the other welding place that said they'd fill my Dewar no problem and thanks for the business.

 

[Skiller1nc]

Study enough to understand Mf, aka, martensite finish. It's the temp at which you'll get no more conversion to martensite. It varies based on alloy content. That way, you'll learn if your steel actually needs -108F (dry ice) or -378F (LN).

And, a note about Airgas: the local jagoffs won't fill my Dewar and when I asked the home office if this was a local or "global" practice, they didn't deign to answer. So, I just bought my welder and accessories elsewhere and I'll get my argon from the other welding place that said they'd fill my Dewar no problem and thanks for the business.

Yeah airgas tends to treat smaller customers like that, we are not worth their time.

 

[SBuzek]

Oh one thing I've heard, is how long the LN2 lasts can depend on the size of the dewar. Larger ones can last over a month, smaller ones about a week. I have no idea the validity of that, but that's what I have heard.

I have a 25 litre dewar with a 1-1/2" opening. My shop is temp controlled, depending upon how much h/t I do mine will last about 90 days.

 

[Blankblank]

Study enough to understand Mf, aka, martensite finish. It's the temp at which you'll get no more conversion to martensite. It varies based on alloy content. That way, you'll learn if your steel actually needs -108F (dry ice) or -378F (LN).

I mean, martensite finish varies so much between the different alloys, but more importantly the different austenizing temps used. Especially if I'm interested in reaching higher hardnesses without going into the secondary hardening range. It can just be difficult to find the information charted out for some steels giving the mf for the different austenizing temps. Well, if it isn't difficult, I just don't know where to look to easily find them.