Sub Zero, huh?

[Quenchcrack]

Well, I was all ready to post a nice response to this question but several others have done an admirable job and I agree with their posts. In summary: 1. Higher carbon steels respond best. 2. The low temperature causes the retained austenite to transform to martensite. Being a diffusionless shear transformation, it relies on temperature, not so much on time. Overnight in the bottle is plenty. 3. A very light temper is appropriate. Any tempering will cause the austenite to transform to other strucures (like bainite) that are less desireable. Remember that hardness is only one measure of abrasion resistance and a slight increase in hardness may represent a much larger resistance to abrasion if it is the result of more martensite. Don't forget to temper AFTER the cryo treatment!

[tmickley]

Originally Posted by mete

Robert, you don't have to cryo but it doesn't make sense to me to use the best steels such as S30V and not get the best properties . On that point remember some of the variables ,as tmickley found ,quench speed is important . In addition for those that snap temper before cryo - a snap temper of 400F will stabilize the austenite preventing the cryo from transforming it !! Snap temper at 300-350F.

Mete or Quenchcrack, couple questions please.
Snap temper has always confused me. Crucible suggests going straight to cryo from heat but offers a snap temper can be done. They don't give a reason.
q1: a snap temper of 400F will stabilize...Just be stupidly sure, you're saying that if you snap temper at 400F, you will lose any (or at least most) of the benefit from doing a cryo.

q2:I've read some where the pro's snap temper for production reasons, ie to avoid cracking a customers blade and that the blanks could go straight to cryo with the understanding there may be a higher failure rate due to shock.

q3: what happens during a snap temper, I can't find reference to it in my meager library.

[RJ Martin]

Guys: Sorry I have not been more atentive to this post. Cryo is for real. The main benefit is transforming retained Austenite to Martensite, but, there are other benefits such as carbide precipitation, which acts to lower the average grain size and improve toughness. These are incredibly small carbides-I liken them to small rocks, carefully packed into the spaces between rocks in a rock wall.

The results from cryo treating have been documented in everything from engine wear to rifle barrel accuracy and ease of cleaning. Cryo tends to equilize/reduce stresses in a part, which helps dimensional accuracy over time, and over temperature change.

Cryo also helps correct over soaked parts-that is, parts that were soaked too long ot at slightly too high a temperature-both of these heat treating mistakes tend to reduce the transformation to Martensite. I have withessed first hand of an A2 blade, heat treated at 1950 that gained 2 points of hardness from an overnite soak in a household freezer!

colder is better, with Ln2 generally being considered as cold as is necessary. Slow cooling in a 24 hour period is really only necessary for large parts or those with varying cross sections to prevent cracking from the thermal stress. For knife blades, a dunk and soak approach is fine.

Lastly, it's not all about hardness increase. If your initial heat treat is right on, you may not notice a dramatic increase in hardness. This varies steel by steel-some steels you see a point, some, a half a point. Some, more. But, you are getting a benefit regardless!

Hope this helps.

[navajas]

Tracy, when you plate quench do you blow are in between the plates and do you leave the wrap on, or remove it before the plate quench?. T.I.A.
Roland

[navajas]

I mean air not are. :0)

[RJ Martin]

Tracey: The reason Crucible recommends a snap temper is to prevent parts from shattering. It can happen, even with knife blades. Not often, but, enough that a 300 or 350 degree temper for an hour is recommended as a precaution.

A snap temper is a quick, low temperature temper. It stabilizes a fraction of the retained Austenite, but, not all of it. It also may start to temper the Martensite, which reduces stress and helps prevent cracking. I don't agree with the previous statement that a 400F temper negates the subsequent effects of cryo treatment. It may result in a blade with a higher percentage of retained Austenite, but, in some cases that may be desireable.

[tmickley]

Originally Posted by navajas

Tracy, when you plate quench do you blow are in between the plates and do you leave the wrap on, or remove it before the plate quench?. T.I.A.
Roland

Roland,
nope, just lay the blade between the plates it in still in the foil. RJ Martin has been preaching plate quench for some time now, I just finally got around to trying it. These plates were (are) scrap (the guys at the yard will correct you and call it surplus, not scrap, which of course makes me call it scrap ) that are roughly 1" thick and 10" long by 4". These just happen to be those dimensions because that is what they guy pulled out of the box and I said I'd take them. They work fine. One of the guys here shoots compressed air in between the plates which sounds like a great idea, I just haven't done it. The blades are cool to the touch in less than a minute when plate quenched. Cooling in a wind tunnel I made with a fan took several minutes. That was proof enough for me.
The TTT nose is something like 3 seconds with S30V, a hair longer for ATS34 and I can't get the foil off quick enough for a fast air quench so this plate quench still in the foil works very well.

[Quenchcrack]

A quick note of clarification: Austenite is metastable at temperatures below about 1333F. Nickel will stabilize it at room temperature (as in Austenitic Stainless Steel) because nickel and austenite have the same crystal shape, face-centered cubic. Temperature does not usually stabilize austenite, it causes it to transform to bainite or troostite (and very rarely martensite), neither of which is as good as martensite for a blade. Grain size can refer to the prior austenitic grain size which influences hardenability, or ferritic grain size, which is what forms from the austenite upon slow cooling. Ferritic grain size can influence toughness. Carbides can prevent grain growth upon heating only up to the temperature at which they dissolve. Carbides precipitate during tempering, not quenching or cryo treating. In medium carbon alloy steels, higher temperatures and short times cause very fine carbides to precipitate. In high carbon tool steel, very fine carbides will precipitate at temperatures in the 250F-400F range. Soooooooooooooo...a fast quench misses the pearlite nose, a light temper WILL precipitate some fine carbides but improve the shock resistance of the blade, the cryo transforms most of the austenite that does not transform at the temperature of the quenchant, a second temper, 50-100 degrees hotter than the snap or light temper will precipitate more carbides in martensite that formed in the quench AND in the cryo. Whew.... 8o

[tmickley]

Quenchcrack,


I have read this three times. I suspect I will have to read it another couple to even ask some more questions.

[tmickley]

Lets start here, I took shop instead of physics in high school and in college too now that I think about it.

I had to look up metastable:
SYLLABICATION: met?a?sta?ble
ADJECTIVE: Of, relating to, or being an unstable and transient but relatively long-lived state of a chemical or physical system, as of a supersaturated solution or an excited atom.


From what I understand, Austinite is not a state steel wants to be in naturally and it will turn to bainite or pearlite over time or a little push with the right temperature. Correct? (Troosite I have rarely heard or read about. Isn't it soft and 'blotchy' chunks of various structures?)

"Carbides precipitate during tempering, not quenching or cryo treating."
I was thinking carbides precipitated during quench and helped form grain nuclei so that with more available carbides a steel had, the finer grain that could be achieved?


"In high carbon tool steel, very fine carbides will precipitate at temperatures in the 250F-400F range."
Then this steel is still mostly martinsitic and we want the remaining austenitic steel to convert to martinsitic but before doing that via cryo, we want to precipitate some carbides out to increase toughness. Correct? Also, aren't carbides what actually does the cutting in a knife edge?

"the cryo transforms most of the austenite that does not transform at the temperature of the quenchant, a second temper, 50-100 degrees hotter than the snap or light temper will precipitate more carbides in martensite that formed in the quench AND in the cryo. "
When you precipitate carbides out of martinsite, there must be some point at which you go too far and the steel is no longer martinsitic?


Oh yeah, Merry Christmas!

[RJ Martin]

Quenchcrack: Have you read any of the published work on Eta-carbides precipitating out during cryogenic treatment?

Stay Sharp,

RJ Martin

[mete]

RJ Martin, I have read the paper that is often quoted in people who are selling cryo.It is apaper dealing with a specific tool steel and they relate it to things like rifle barrels !! Give me a break. Rifle barrels are made of 4140 which you would hardly expect any retained austenite or a lot of other things found in tool steel. We have to separate the facts from the hype. While some barrel makers have their own cryo equipment and ar e making many claims , others , NRA, other barrel makers find no benefit for cryoing barrels.....tmickley, once we get over the eutectoid [ .80%C] we have large carbides that never were dissolved in the austenite . So a quench and tempered hypereutectoid steel has large carbides , martensite, and small carbides which come from the tempering of the martensite....A snap temper is to reduce some of the high stresses found in the as quenched steel. While it is not always necessary it can be useful with complex shapes......Btw carbides are a metastable state - the stable state for carbon is graphite !!....The finest carbides strengthen the matrix , the largest ones are more for wear resistance.....as we temper at higher and higher temperatures we precipitate more and more carbides . The martensite crystal is an elongated cube .We can measure it's dimensions with X-ray diffraction techniques . As the tempering temperature increases the long dimension of the crystal decreases until it becomes a cube - ferrite......BTW cryo does NOT reduce grain size.

[DaveL]

This has turned out to be a really fine posting and there is a lot to digest. I assume, unlike heat treating, there is no chart of how long, what method for different steels. Plenty of folks use O1, D2, A2 and such as well as the ATS crowd and the super steels too. Is the cryo the same? Is there any charts on "how to?" I am reallly enthused over the response and just think for the average Joe Sixpack at home in his shop may be looking for the simple answer.

[RJ Martin]

Dave: 24 hours in liquid Nitrogen will get the job done for any steel you'd care to cryo.
If you don't have liquid Nitrogen, 24 hours in dry ice and kerosene will do.

Stay Sharp,

RJ Martin

[Quenchcrack]

Ray, No I have not read anything about eta carbides precipitating during cryo. Perhaps you could post it or elaborate on it.

TMickley, I think you got most of it right. Austenite is a reluctant structure at room temperature but nickel, and high carbon, can stabilize it. One reason austenite is best transformed by cryo is that it can also be transformed by mechanical impact. Any transformation of austenite to martensite that is not followed by tempering can leave the structure brittle. Martensite can have micro-cracks if not properly transformed. The cracks can propagate into larger cracks. Troostite is an obsolete term that refers to a pearlitic structure that has round carbides instead of long finger-like carbides. Being old and obsolete myself, I still use the term. Most carbides precipitate in tempering, however, RJMartin mentioned the precipitation of eta carbides during cryo. I am not sure how this happens since carbide precipitation is a diffusion process that would be EXTREMELY sluggish at -300F. Yes, carbides do provide much of the abrasion resistance in a cutting edge. Grades that have massive carbides, like D2 are very abrasion resistant although not extremely tough. Finally, if you temper martensite to the extreme, you get ferrite with carbides, or cementite, precipitated in it. As Mete said, martensite is body center tetragonal and ferrite is body center cubic. Sufficient time and temperature will reduce the tetragon to a cube.
There are recommened procedures for heat treating any alloy but most of them focus on quenching speed down to room temperature. I have not seen any tables of optimum cryogenic temperatures for any specific alloy. It is possible to calculate the Martensite Finish Temperature (Mf) based upon the chemistry and this would let you know the minimum temperature you must cryo treat it to. At this point, I think -300F is used because it is achievable with LN and it does not appear to hurt anything if you go colder than you have to.