Ive been around for a while now in lurking or active form and havent seen a serious thread on this subject. What does cryo do, how cold is cold enough, is it cheap to do, etc. As usual, I have my own ideas about what it does and how, but I would like to hear from the people who have used knives treated this way, or those who do the treating. Technical info is also welcome. Cliff, I'll get the chart we talked about on as soon as I find it.
Cryotreatments?
- Thread starter Elwin
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It's actually been discussed a lot on the Shop Talk forum. I would consider it best to improve grain structure on stainless. Given time it seems like dry ice and alcohol would work. It is quicker to do it with liquid nitrogen. I think you do it after your first annealing cycle. You want to do another anneal afterwards.
Do a search in the Shop Talk forum for "cryo" and you'll find out what the experts do.
Do a search in the Shop Talk forum for "cryo" and you'll find out what the experts do.
Texas Knife Supply does Cryo... cant vouch for it, but just FYI.
Their site is www.texasknife.com.
But yes, do search in the Shop Talk forum... that'll get you the best information.
Their site is www.texasknife.com.
But yes, do search in the Shop Talk forum... that'll get you the best information.
After quench in some steels (alloy and SS -steels) there is still retained austenite in martensite matrix (up to 20% austenite). Cryo treatment completes quench and converts this retained austenite to martensite. This is the main benefit. Lovered stress levels etc. are additional benefits. Tim Zovanda wrote a good article about this in Knife Illustrated (1999 or 2000).
Temperature depends on steel. If my memory serves me -100 C (about -150 F) is enough for all steels.
Temperature depends on steel. If my memory serves me -100 C (about -150 F) is enough for all steels.
Cliff Stamp
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The best way to think of cryo is that it is just an extension of the quench. When you rapidly cool a steel after it was raised above the critical point (the soak), you get martensite formed which is the structure you want for high strength and toughness. The amount of martensite formed increases with decreasing temperature, and this continues below room temperature. Deep cryo (liquid nitrogen) is not necessary for full martensite transformation and grain refinement and a dry ice (201 K) bath will give you close to 100%. Thus from a basic point of view liquid nitrogen is overkill.
However there are those that argue that the extra temperature produces other benefits such as martensite decomposition and wear enhanced carbide formation. This doesn't happen according to them if you stop at the higher dry ice temperature. Nu-bit for example has done numerous studies (published articles) showing the vastly increased edge retention on wear based work with liquid nitrogen treated blades. They also argue that multiple tempers will degrade performance (they have tested that as well), and that you should only use a single temper post deep cryo.
However there are others that argue that all of that is just hype and you can get the exact same results with multiple tempers which will also promote grain refinement and increase martensite transformation (this is well proven). I have however not seen any commentary about the type of carbide formation on their part, which is Nu-bits critical point, nor actual test studies on the performance properties of steels as compared to cryo treated blades.
Phil Wilson has done a lot of cryo testing, as well as experimented with the procedure that Nu-Bit uses which has very long soak times. He concluded that he could see no benefit for long soaks for knife blades, and now he just does a very rapid quench in liquid nitrogen by just jamming the blades in a dewar.
There is a lot of misinformation on cryo concerning knife blades, mainly because its not used correctly. For example doing a cryo with no post temper results in a very brittle blade as you have untempered martenite which is very hard. As well doing cryo after tempering achives next to nothing as the austenite will stabilize and resist further transformation and decomposition as well as carbide enhancement.
One I the things I have planned later this year is to see if I can't get Phil to make me two blades, one in multiple tempered D2 and another with cryo and see if there is a difference in edge holding and felxibility. I picked D2 as its one of the worst steels in regards to grain structure and has huge segregated carbides, so it responds very strongly to cryo. Which is another point, some steels won't respond much at all to cryo (1084) as they have nothing really left to do after a simple quench. In general the more complex the steel, the more performance can be gained.
-Cliff
However there are those that argue that the extra temperature produces other benefits such as martensite decomposition and wear enhanced carbide formation. This doesn't happen according to them if you stop at the higher dry ice temperature. Nu-bit for example has done numerous studies (published articles) showing the vastly increased edge retention on wear based work with liquid nitrogen treated blades. They also argue that multiple tempers will degrade performance (they have tested that as well), and that you should only use a single temper post deep cryo.
However there are others that argue that all of that is just hype and you can get the exact same results with multiple tempers which will also promote grain refinement and increase martensite transformation (this is well proven). I have however not seen any commentary about the type of carbide formation on their part, which is Nu-bits critical point, nor actual test studies on the performance properties of steels as compared to cryo treated blades.
Phil Wilson has done a lot of cryo testing, as well as experimented with the procedure that Nu-Bit uses which has very long soak times. He concluded that he could see no benefit for long soaks for knife blades, and now he just does a very rapid quench in liquid nitrogen by just jamming the blades in a dewar.
There is a lot of misinformation on cryo concerning knife blades, mainly because its not used correctly. For example doing a cryo with no post temper results in a very brittle blade as you have untempered martenite which is very hard. As well doing cryo after tempering achives next to nothing as the austenite will stabilize and resist further transformation and decomposition as well as carbide enhancement.
One I the things I have planned later this year is to see if I can't get Phil to make me two blades, one in multiple tempered D2 and another with cryo and see if there is a difference in edge holding and felxibility. I picked D2 as its one of the worst steels in regards to grain structure and has huge segregated carbides, so it responds very strongly to cryo. Which is another point, some steels won't respond much at all to cryo (1084) as they have nothing really left to do after a simple quench. In general the more complex the steel, the more performance can be gained.
-Cliff
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ok i will try to answer some of these qeutions but i will say this i am not going to get in a pissing match over this thats why i dont answer very many of these you can get some benifit of -100 but not the full benifit of -320 lets say on 440 you get a 100 persent more rope cutting after cryo dendritic d2 forty percent allso ats34 reg d2 some these are the only steels that i use and heattreat for other people you do the cryo after hardening and do a double temper on all of them this is being done this way with advice from a metallergist
I found the article. It was in KI magazine and was a while ago. It was an attempt to show that knives that were cryoed were tougher. IMHO, it failed. The only sample cryotreated was also marquenched. Several other samples were marquenched or oil quenched, but only one sample was freeze treated. None of the oil quenched blades were cryo treated. The samples were also non standard, so numerical data cannot be compared to other published data.
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Cliff, I would be very interested in your findings if/when you do this test. I know that cryo treatments do improve grain structure when done properly, but I would love to know exactly what properly is. Do you only do a single quench then do the cryo? How many tempers should be done after? How much does the steel you are using determine if the cryo treatment will be effective?
Cliff Stamp
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Elwin :
The only strong statement I have seen regarding toughness was made by Nu-Bit who commented that they found no significant change in impact toughness, but did see an increase in the amount of work necessary to reach tensile failure, which is another kind of toughness.
From basic principles you would think that the decrease in grain size, and increase in carbide dispersion would act to increase the impact toughness, however the increase in martensite transformation should act to decrease it - so the net effect appears to be nothing.
KWM, all of those are interesting questions, and to my knowledge there is still a lot of debate on the answers both in the materials industry and specific to cutlery. I have discussed the subject with knifemakers and have recieved the perspective that deep cryo is (a) very positive, (b) does nothing, (c) is very negative. In support for deep cryo, I have found that the makers with the most extensive controlled research tend to support it. When someone is vague about the work they have to to reach this conclusion, I give little weight to their conclusions.
It is also very steel dependent, so someone working with D2 will have a very different perspective than someone working with 1095. As well, the main thing that it does is increase wear resistance. Thus you won't see much of a benefit in areas of performance that are not strongly correlated to the wear resistance of a steel. For example many types of edge retention are just dependent on RC, and even extreme changes in wear resistance have little to no effect on the lifetime of the edge.
For journals, there are many that feature metallurgy based articles. The best way to find such information is to do a search at a local library. Net searches will help as well as often people put up their published work in their online cv.
-Cliff
The only strong statement I have seen regarding toughness was made by Nu-Bit who commented that they found no significant change in impact toughness, but did see an increase in the amount of work necessary to reach tensile failure, which is another kind of toughness.
From basic principles you would think that the decrease in grain size, and increase in carbide dispersion would act to increase the impact toughness, however the increase in martensite transformation should act to decrease it - so the net effect appears to be nothing.
KWM, all of those are interesting questions, and to my knowledge there is still a lot of debate on the answers both in the materials industry and specific to cutlery. I have discussed the subject with knifemakers and have recieved the perspective that deep cryo is (a) very positive, (b) does nothing, (c) is very negative. In support for deep cryo, I have found that the makers with the most extensive controlled research tend to support it. When someone is vague about the work they have to to reach this conclusion, I give little weight to their conclusions.
It is also very steel dependent, so someone working with D2 will have a very different perspective than someone working with 1095. As well, the main thing that it does is increase wear resistance. Thus you won't see much of a benefit in areas of performance that are not strongly correlated to the wear resistance of a steel. For example many types of edge retention are just dependent on RC, and even extreme changes in wear resistance have little to no effect on the lifetime of the edge.
For journals, there are many that feature metallurgy based articles. The best way to find such information is to do a search at a local library. Net searches will help as well as often people put up their published work in their online cv.
-Cliff
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Cliff, thanks for your posts here... contributed to my knowledge of cryo for sure. (good enough to cut/paste into a Word doc for safekeeping).
If you could offer a few more sentences on what you believe is going on with multiple tempers and how that contributes to grain refinement and increased martensite transformation, that would again be very educational for me.
I can see how the deep quench furthers the percentage of martensite, but am uncertain how multiple tempers would do this. I have this idea in my head (since I'm not a maker) that tempering is followed by air cooling, then back up to temper. Maybe it's a temper/quench, then again temper/quench that does it. (I can see how the rapid quench would cause more martensite formation, but not a temper, slow cool cycle).
Anyway, info appreciated.
Hello All
The reason that multiple tempering works is as follows.
Upon quenching you do not get 100% transformatiom to martinsite.The first temper step tempers this martinsite.After this step the steel is still changing and more untempered martinsite forms .Although not very much it still is brittle and the second tempering releaves this.
This process of martinsite formatiom continues but at a rapidly dimminishing rate. I have read a study rhat was done on some swords made over 200 years ago that still are transforming untempered martinsite . This effect was extremly small but was measurable.I don't know the exact numbers but would guess 90% transformation after your final quench,temper this and wait for more to form and temper this and so on.This is the bases for cyro treatment. The extreme cold drives this process to completion. NOTE it is nearly impossible to get 100% transformation .
Thanks
Rex
The reason that multiple tempering works is as follows.
Upon quenching you do not get 100% transformatiom to martinsite.The first temper step tempers this martinsite.After this step the steel is still changing and more untempered martinsite forms .Although not very much it still is brittle and the second tempering releaves this.
This process of martinsite formatiom continues but at a rapidly dimminishing rate. I have read a study rhat was done on some swords made over 200 years ago that still are transforming untempered martinsite . This effect was extremly small but was measurable.I don't know the exact numbers but would guess 90% transformation after your final quench,temper this and wait for more to form and temper this and so on.This is the bases for cyro treatment. The extreme cold drives this process to completion. NOTE it is nearly impossible to get 100% transformation .
Thanks
Rex
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Thanks Cliff, as always I have learned from reading something posted by you. You have to somehow write a/some book/books about your experiences with steel. I would be thrilled to have such a great reference of the data you have learned from all the tests you have done.
Rex, it is great to have you join us over in this forum as well. Youe knowledge about steel and your willingness to pass on that knowledge is a great benefit to Bladeforums. I know you are busy, but hope to see much more from you in the future.
Edited because I am not to bright some times.
Rex, it is great to have you join us over in this forum as well. Youe knowledge about steel and your willingness to pass on that knowledge is a great benefit to Bladeforums. I know you are busy, but hope to see much more from you in the future.
Edited because I am not to bright some times.
Cliff Stamp
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rdangerer :
[multiple tempers]
First off, there is a difference in multiple tempers after a cryo and multiple tempers after just a quench to room temperature. If you stop at room temperature then some steels can have unresolved austenite up to 20+%, it is not just a small amount. Thus there is a huge difference in the initial conditions of the two versions of multiple tempers. For cryo, there are those that argue that only one temper is necessary because the maximum amount of martensite has transformed and once this has been tempered, further tempering just adds energy that can't resolve any further martensite transformation and simply induces grain growth and a degredation in steel structure.
Without cryo, I have never seen anyone argue that multiple tempers will degrade the structure. Why do multiple tempers do anything if you don't use cryo? Why can't you just do everything in one go - why not just do one long temper? Well for one thing, it is more productive to do multiple tempers at certain time intervals than tie up the oven for the whole period of time, thus multiple tempers need not be better than one long temper, just be able to get the same result and they would still be preferred. Anyway, it could be a number of factors. The austensite to martensite transformation probably takes place faster at room temperature, this seems likely. And maybe the induced air quenching as you noted speeds up the process. Also maybe the steel "settles" one it goes back to room temperature which induces some stress which is relieved in the next tempering.
In regards to grain refinement, grains are just the small bits of steel, they can differ in composition or orientation from each other. During the heat treat process, the crystal structure of the steel changes forms and thus new "grains" are born. These will continue to grow if the energy is there which is why you can blow a steel if you oversoak it because you will get very large grains formed. Grain size reduction is caused by stopping the growth of the grains that formed. The easiest way to do this is to cause a lot of them to form at the same time.
To understand this, picture yourself in an empty room and walk until you hit something, this length is your "grain size". Now put another person in the room and have both of you do the same thing. As you add more and more people, each person has a much smaller chance of walking a large distance because there are people there for them to smack into, thus the average "grain size" is strongly correlated to the amount of people. For steels this "number of people" is called the amount of nucleation sites, which is just a fancy term for the amount of places the transformation will take place.
This also by the way is why forging a steel can induce a smaller grain size in the steel once it has been heat treated. When you forge a steel you are altering the crystal structure, forcing it into messed up orientations. The fancy term for this is inducing dislocations. These disloaction will be nucleation sites for the transformations in the heat treat and the more of them the finer the grain, as noted in the above. This can also be seen by using the people example. If all the people in the room are facing the same direction, then they can all walk a very long way before they hit something and thus they have a large "grain size". If you went around and randomally turned them about, when they started walking then would quickly hit the people that were turned differently than they were - thus they have a small "grain size".
Back to multiple tempers, austenite has a very large grain size and thus if your multiple tempers are increasing the austenite->martensite transformation, and doing it quickly (high number of transformation sites) you will get the optimum grain size.
-Cliff
[multiple tempers]
First off, there is a difference in multiple tempers after a cryo and multiple tempers after just a quench to room temperature. If you stop at room temperature then some steels can have unresolved austenite up to 20+%, it is not just a small amount. Thus there is a huge difference in the initial conditions of the two versions of multiple tempers. For cryo, there are those that argue that only one temper is necessary because the maximum amount of martensite has transformed and once this has been tempered, further tempering just adds energy that can't resolve any further martensite transformation and simply induces grain growth and a degredation in steel structure.
Without cryo, I have never seen anyone argue that multiple tempers will degrade the structure. Why do multiple tempers do anything if you don't use cryo? Why can't you just do everything in one go - why not just do one long temper? Well for one thing, it is more productive to do multiple tempers at certain time intervals than tie up the oven for the whole period of time, thus multiple tempers need not be better than one long temper, just be able to get the same result and they would still be preferred. Anyway, it could be a number of factors. The austensite to martensite transformation probably takes place faster at room temperature, this seems likely. And maybe the induced air quenching as you noted speeds up the process. Also maybe the steel "settles" one it goes back to room temperature which induces some stress which is relieved in the next tempering.
In regards to grain refinement, grains are just the small bits of steel, they can differ in composition or orientation from each other. During the heat treat process, the crystal structure of the steel changes forms and thus new "grains" are born. These will continue to grow if the energy is there which is why you can blow a steel if you oversoak it because you will get very large grains formed. Grain size reduction is caused by stopping the growth of the grains that formed. The easiest way to do this is to cause a lot of them to form at the same time.
To understand this, picture yourself in an empty room and walk until you hit something, this length is your "grain size". Now put another person in the room and have both of you do the same thing. As you add more and more people, each person has a much smaller chance of walking a large distance because there are people there for them to smack into, thus the average "grain size" is strongly correlated to the amount of people. For steels this "number of people" is called the amount of nucleation sites, which is just a fancy term for the amount of places the transformation will take place.
This also by the way is why forging a steel can induce a smaller grain size in the steel once it has been heat treated. When you forge a steel you are altering the crystal structure, forcing it into messed up orientations. The fancy term for this is inducing dislocations. These disloaction will be nucleation sites for the transformations in the heat treat and the more of them the finer the grain, as noted in the above. This can also be seen by using the people example. If all the people in the room are facing the same direction, then they can all walk a very long way before they hit something and thus they have a large "grain size". If you went around and randomally turned them about, when they started walking then would quickly hit the people that were turned differently than they were - thus they have a small "grain size".
Back to multiple tempers, austenite has a very large grain size and thus if your multiple tempers are increasing the austenite->martensite transformation, and doing it quickly (high number of transformation sites) you will get the optimum grain size.
-Cliff