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Cryogenics

Posted by CliffStamp 
Cryogenics
January 01, 2015 08:21PM
The use of liquid nitrogen is one of the curious parts about metallurgy which is both rooted in science and steeped in mysticism. The two main effects are :

-minimization of retain austenite (and thus increases martensite)

-precipitation of carbides

In general this :

-increases hardness/strength and wear resistance

-has mixed effects on toughness (really depends on exactly what is being measured)

The issue with toughness is that retained austenite can increase toughness on large scale values such as charpy tests, but it can have adverse influence on an edge and it can transform to martensite and then because that is untempered can create brittleness in extended work.

Ref : www.ijreat.org/Papers%202013/Volume1/IJREATV1I1178.pdf

"Effect Of Cryogenic Treatment On Wear Resistance Of. Astm A387 Alloy Steel. M. Karthikeyan"

A little background on the steel :

"Both SA387 and A387 are described as a "Standard Specification for Pressure Vessel Plates, Alloy Steel, Chromium-Molybdenum". The specification covers alloy steel plates intended primarily for welded boilers and pressure vessels designed for elevated temperature use."

I checked this paper and can't find the reference to the grade of steel used, but in any case this is a low alloy structural steel (hence the ASTM specifications/grade). This paper is very sparce and has little analysis and detail and mainly compares the steel with and without cryogenics using a pin on disk test for abrasive wear. This basically is done similar to how a record player works. A pin scrapes a disk under a specific load and distance and speed and the parts are weighed to determine wear. In short :

-cryogenics improves the wear resistance by 60% and reduces friction

The paper obviously isn't written in English as a first language and there are some interesting phrases, a really curious one is "Cryogenic treatment creates denser molecular structure of cutting tools ...".
me2
Some Cryogenic Information
April 04, 2012 04:25PM
Just a couple links with some information on Cryogenic and cold treatments. This one is recent and has a lot of information. I'm still looking for one from Ireland published in the 90's that was also quite good. I have it in print, but can't find the PDF on line.

[locusdesigngroup.com]
me2
Re: Some Cryogenic Information
April 05, 2012 03:29PM
I still haven't found the link to the one from Ireland. From my print version, the title is "Cryogenic Treatment of Tool Steels" by David N. Collins, National Heat Treatment Centre, University College Dublin, Ireland. It was published in the 12/98 issue of Advanced Materials & Processes. This article attempts, successfully to me, to address what appear to be contradicting claims in cryo/cold treatment of steels. It addresses contradictory reports of change/no change in hardness. It also addresses the non-time dependent change of retained austenite to martensite vs. the specification of long hold times at low temperatures, sometimes exceeding 24 hours. It also clearly defines cold treatment (dry ice temps ~ -100 F) and cryogenic temps (liquid nitrogen ~-300 F). It also offers general procedures for optimizing strength (hardness) or wear resistance, depending on what is desired in the final product.
Re: Some Cryogenic Information
April 09, 2012 09:08AM
Quote
me2
Just a couple links with some information on Cryogenic and cold treatments.

It would be interesting to see :

-edge stability
-corrosion resistance (or just free chromium)
-abrasive (not adhesive) wear
-impact toughness

for the processes. Unless I am reading something wrong there is a mistake in Fig. 3 as the text notes that process 4 has the highest Yield, which would make sense, but the Figure shows it is process 1.

I had discussion about HSS with Roman awhile ago, he has a pretty interesting take on it and basically has no interest in HSS for knives for these reasons :

-there is no benefit from the HS part at all for knives
-the heat treatment is very complicated/costly
-you can get the wear from the cold work steels (things like Super Blue, etc.)

He concluded with something like (paraphrase) "if I was going to go to all that trouble to heat treat I would at least make it stainless".

Interesting comment about the carbon diffusing to the dislocations after cry during warming and this acting as a crack mitigating agent.
Re: Some Cryogenic Information
April 09, 2012 06:26PM
Another paper on deep cryo : [www.cryogenicinstitute.com]

A few highlights :

-24 hours soak in liquid nitrogen shows highest wear resistence
-all liquid nitrogen treatments severely reduce toughness

The second point is a bit not really relevant to blades because of how it works as it is simply because retained austenite elevates charpy type toughness as the austenite absorbs the crack energy in transformation, but with micro edge damage the reverse would happen.
me2
Re: Some Cryogenic Information
April 10, 2012 09:05AM
Did I read that right in that they oil quenched all the samples?
Re: Some Cryogenic Information
April 10, 2012 12:18PM
That's the procedure noted, Roman in general recommends faster than air even for air hardening steels, mainly to avoid the precipitation I would assume.
me2
Re: Some Cryogenic Information
April 10, 2012 02:55PM
Don't let it out that Roman recommends that. He'll be laughed out of town. It's interesting they use A2 and note it has the highest reported improvement in wear from cryogenic treatment. Any guess at what might be different if the steel was actually air quenched? I'm guessing slightly lower toughness, possibly slightly higher wear, and slightly lower hardness, 2 points maybe. I really need to finish my A2 kitchen knife, just to have an idea of what A2 will do. I've never used a knife with A2 steel. Now if I can just convince my wife it is worth it to pay $25 for the heat treating, when she could get 5 knives for that from the local Target/Walmart/etc. Of course one trip throught he dishwasher and all that advantage goes out the window.
me2
Re: Some Cryogenic Information
April 10, 2012 02:59PM
On a side note, I've read of makers getting 64-65 HRC as quenched on very thin (< 1/16"winking smiley O1 blades when using chilled aluminum plate quenches with 1" plates.
Re: Some Cryogenic Information
April 10, 2012 05:45PM
Plate quenching is becoming more common as people don't want to mess with fluids. The only real concern is that this is obvious a single blade quench and if you don't keep the plates cool the rate of quenching will decrease rapidly. You also have to take care to make contact with the entire blade during the quench.
me2
Re: Some Cryogenic Information
April 10, 2012 06:28PM
It does seem a little better suited to the home maker, who grinds blades after HT, though that brings in a whole new set of issues. Were I to heat treat my own air hardening steels, I'd use it, what with my blistering production rate of 1 knife per year.
Re: Cryogenics
January 13, 2015 10:23AM
here are some Cryo articles
[www.industrialheating.com] interesting here is that they did see a small performance increase but felt it was not worth the time and money
[www.industrialheating.com] sortof an overview main point here is that putting a blade in a container full of LN2 is not "Deep Cryo"
scott
Re: Cryogenics
January 13, 2015 04:12PM
There are lots of parts of that article which in general are not well understood, for example look at the massive difference in the Ms point of D2 as it relates to the soak temperature :

Quote

The amount of retained austenite increases with higher austenitizing temperature due to a greater saturation of austenite with carbon and alloying additions that lower the martensitic-start (MS) temperature.[1] Moreover, exceeding the optimal austenitizing temperature means that MS falls well under subzero. For example, the MS temperature of X153CrVMo12 (1.2379, AISI D2) is 175˚C (347˚F) for an austenitizing temperature of 1050˚C (1922˚F) but -100˚C (-148˚F) for an austenitizing temperature of 1200˚C (2192˚F).[1]

This is interesting and contradicts Bolher's own data :

Quote

The hardness of CT samples (-196˚C/-230.8˚F for 4 and 10 hours) is about 2.5 HRC lower than that of VANADIS 6 with no CT and is the same as after deep cooling to -90˚C (-130˚F) regardless of the subzero holding time.

This is the reason :

Quote

The retained austenite transformation during cooling from tempering temperature, which corresponds with the secondary-hardness peak, causes the increase in hardness.

If a cooling treatment is used, and a hot temper is used and the blade is allowed to cool after the very hot tempering then this softens it significantly. The question to ask then is what happens if you quench the blade after tempering.

The paper does show however that a simple statement such as an extended quench is useful isn't always the case, in particular if you are using hot tempering and counting on secondary hardening to prevent softening.
Re: Cryogenics
January 13, 2015 04:25PM
I looked up the Vanadis 6 paper and it notes that after a cooling the hardness does increase -but- the steel without the extended cooling has a high hardness after the very hot tempering.
Re: Cryogenics
February 19, 2015 09:20PM
I am looking up cryogenics now on HSS, the results are not as straightforward as you might think. The increase in wear resistance is very common :

- [article.sciencepublishinggroup.com]

- [www.ijser.org]

- [www.technicaljournalsonline.com]

- [www.wseas.us]

- [www.theijes.com]

- [ijmerr.com]

- [globaljournals.org]

- [www.ijsr.net]

- [www.academia.edu]

- [www.amse.org.cn]

And well explained by the reduction in retained austenite and precipitation of carbides.

The problem comes in the regards to toughness, which is contradictory. I am trying to sort that out now.
Re: Cryogenics
February 19, 2015 10:52PM
I found a lot of seemingly contradictory info as well, but since reading some of Landes' posts I realized that in some of the experiments I was looking at, the pieces were not tempered after cryo, and so would be expected to have un-tempered martensite replacing the retained austenite, reducing toughness. There's also that pesky "snap" temper to deal with in some cryo treatments.

On top of that (for us) is the whole issue of retained austenite increasing toughness for a single-strike test (such as a Charpy) which does not take into account the multiple impacts a chopper will suffer, as impacts may themselves convert retained austenite to un-tempered martensite...

Almost makes me want to revisit edge packing to convert RA... not really



Edited 1 time(s). Last edit at 02/20/2015 11:21AM by wnease.
Re: Cryogenics
February 20, 2015 12:30PM
The issue of when the cold treatment is done is critical, I don't think anyone seriously advocates post-temper cryogenics now for two reasons :

-the austenite is stabilized
-it produces untempered martensite

The only time that is normally done is on actual made tools and there is a lot of data showing very long increases in tooling life - but again, tooling life is NONlinear and that is never well discussed. Because of this, it takes extreme precision to show a significant difference and without this precision, well, all of those large values can be quite meaningless.

The issues I have are with a few things, the data can be scattered in regards to the effect of cryogenics on toughness due to claims about the martensite being produced being of a very fine grain (usually ten times as fine). This produces an increase in toughness in some research, in others it produces nothing, in others it produces a lower toughness (explained due to less retained austenite). Some examples :

-[www.comat.cz]

Deep cryo is done post quench, 24 hr, pre-temper and then there is a single temper, 550 C (M2), this produces :

-a much finer martensite structure
-a loss of one point in hardness
-increased toughness (but not statistically significant)
-wear resistance is higher under low load with cryogenic, but lower under a high load (3-roller cone)
-actual drill life is reduced with cryogenics (but again not statistically significant)

In short, if anything this shows cryogenics on M2 isn't of benefit. Here is where it gets interesting, note the loss in hardness. As there is less retained austenite, when the steel is tempered hot there is no transformation to martensite and thus increased softening. Thus an obvious question is well what about if you cryogenic and then temper hot but at a reduced temperature (but still hot) :

"Deep cryogenic treatment of AISI M2 high-speed steel, M. Pellizzari et. al"

-cryogenics before and after temper, a small increase in fracture toughness with cryogenics but minor
-a large increase in wear resistance when the tempering temperature is adjusted down with cryogenics

-[www.asminternational.org]

This again shows that cryo pre-temper reduces the hardness in M2, they don't look at adjusting the tempering temperature down. They don't look at the material properties but they look at very detailed changes to the sub-structure and predict the material properties and argue that cryogenics could increase toughness by causing the precipitation of very fine carbon clusters (because of the high dislocation density) and these could act as crack arrestors. But again, the materials data isn't actually measured.

-[www.aimnet.it]

This points out to one reason why and discusses the conflicting data on cryogenics. Cobalt has many benefits in HSS, it keeps ferrite out of the final form, it destabilizes austenite, it allows very high soak temperatures as it raises the solidus temperature and it increases hot hardness/strength. But it decreases toughness, and cobalt HSS are shown to react negatively to cryogenic treatment.

But the real breakthrough is in this paper :

[www.hindawi.com]

This looks at M2 and cryogenics but specifically the time of cryogenics from 4 to 50 hours :



This scatter in the behavior comes up in all of the properties and this kind of physical behavior tends to point to one thing - there are at least two things happening which have different time dependencies and have different effects. They argue that one is the precipitation of carbides and the other is the oswald ripening of the carbides. The latter is just a fancy way of saying the carbides can grow/coarsen by consuming smaller ones.

Now if you look at that graph and think about the different times used in cryogenics, and how different size pieces and different steels will have similar curves but the maximums will be shifted around, then it can readily explain the reason why some people report higher toughness and others lower.

--

What does this mean for the knife maker/manufacturer? Well it means you have to do some tests. You have to look at what is practical for you (likely not a 50 hour soak unless you are Ed Fowler) so you take your steel and you run a few basic checks :

-conventional
-cryogenics + temper
-cryogenics + temper (adjusted to take into account lower retained austenite)

and you see if there is a significant difference in your blades. Because the simple fact is that there is far too much going on, especially in HSS, to know what is really optimal. Now if you are not using HSS, and you are using A2 or similar, then it is simpler but still not absolutely trivial.
Re: Cryogenics
February 23, 2015 08:59PM
HSS summary : [www.cliffstamp.com]
Re: Cryogenics
March 26, 2015 10:01AM
two interesting links:

[www.cryogenictreatmentdatabase.org] an interesting article, note the date it was first published.

[www.cryogenictreatmentdatabase.org] seems to be a clearing house of any articles published about cryo. most are available to download free

scott
me2
Re: Cryogenics
March 27, 2015 03:48AM
I read the article from the Czech company. I have no idea why this article seems controversial on KD. The lower hardness for the cryo'd samples just means the secondary hardening out weighs the hardness increase during cryo. Again, Jim has suggested there is some nefarious goings on that allow such results, and that changes that allow lower cryo'd hardness have been hidden. That Moly is in there for a reason.
Re: Cryogenics
March 27, 2015 07:14AM
Quote
me2
I read the article from the Czech company. I have no idea why this article seems controversial on KD. The lower hardness for the cryo'd samples just means the secondary hardening out weighs the hardness increase during cryo. Again, Jim has suggested there is some nefarious goings on that allow such results, and that changes that allow lower cryo'd hardness have been hidden. That Moly is in there for a reason.

you see, i am the person in the crowd who doesn't make knives and found the only paper in existence that shows a slight decrease in hardness after CT. i have posted three times that the folks who wrote the article run a precision heat treat facility, why would they fudge data to make CT "look bad"? Jim has not posted any references to support his post of "LN2 for at least 24hours" and is leading the group questioning of method and intent without reading the article.
none seems to have read another reference i posted "Machine & Tool Blue Book" January 1955, pp. 124-134. it looks at sub-zero and cryo. the benefits listed are the same ones listed today
scott
Re: Cryogenics
March 27, 2015 07:21AM
Chris, it is obvious to you because you understand what is causing the changes in the microstructure.

In cryogenic hold, there are multiple changes happening to the micro-structure at the same time, just like any hold at temperature. It used to be thought that the main effect, in fact all that was significant was that the steel was passing the Mf point and thus martensite could be maximized. However it was shortly discovered that unique carbide precipitation forms as well. It was then noted that the type of martensite that formed was different. Further it was revealed that while carbides are precipitation, some are coarsening and this process is correlated and thus you can max/min behavior in properties. All of this is known in the literature.

Talking about it in idle speculation without referring to the literature and maxing up conspiracies is not not rational, at this point the discussion has degenerated into what happens if you talk to people who advocate the moon landing was a hoax. They don't live in reality, they have their own world which is much more exciting for them that the rubes like us simply can't see. It isn't like you can learn about gravitational theory by talking to someone who believes the earth is flat.
Re: Cryogenics
March 27, 2015 08:00AM
The decrease in hardness is well known, explained and verified in multiple independent and peer reviewed papers, I discussed it here :

- [www.cliffstamp.com]

Again, arguing against this is on the level of a flat-earth. If you are willing to see conspiracies against things as mundane as people faking carbide volumes in AEB-L, then obviously cryogenics is filled with them.

Ironically this is the group where I was threatened with banning because I noted peer reviewed research and the above nonsense about cryogenics (not what you are posting, but what you are responding to).

It is like bizarro-metallurgy.
Re: Cryogenics
August 04, 2015 04:23PM
There are any benefit in cryo any blade years after was produced, like Spyderco Vg-10 or Buck S30V or any other steel from production knives?

In steel, transformation is dependant of time and temperature, but we could see any improvement in 24 hours soak in liquid nitrogen of a production blade even after years that the knife was produced?

I always had this doubt and since I have a container for liquid nitrogen for frozen cattle semen, if the soak can improve some good property of the steel is easy to try.
Re: Cryogenics
August 04, 2015 06:16PM
There is a lot of data in industry where tools just soaked in liquid nitrogen show improved life. If you do a simple search then scads of reports show up. However, and this is a big however, they are often done by people who sell cryogenic equipment, the results are not peer reviewed and it isn't always clear how they are even measuring the improved tool life.

Austenite will stabilize in general which is why snap tempers are known to reduce austenite transformation but they don't stop it completely. I have not seen a lot of papers which investigate in detail mainly because it would take a long time. You would have to HT steel, wait say 1 year and then continue the experiment and then compare this as a base line with say immediate and then say 2 year cryogenics.
Re: Cryogenics
August 05, 2015 08:42AM
Thank you Cliff for your help to clear this subject with the usual sober and technical approach.

In middle 90s just for fun I soaked a blade of a Tramontina clone of Buck 110 ( probably in 440C ) in liquid nitrogen for about 48 hours but at that time I didn't mind to check things like edge retention or hardness. I still have this blade and it performs very poorly in comparison to Ganzo 9Cr18MoV ( 440C chinese version ) in terms of edge retention, but it's probably do to the low hardness that Tramontina run their blades at that time.
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