Cryo Question

rolynd

Active Member
I have a Question concerning Cryo/deep Freeze Treatment.

When I use Stainless I work with Sandvik Steels like AEB-L, 13C26, 12C27. I do a Dry Ice/Acetone treatment immediately after HT.
According to the Sandvik HT manual there is no holding time at cryo temperature necessary, you only have to make sure the whole blade reaches the deep freeze temp.

AFAIK the process of converting the retained Austenite is instantaneous once the Freeze Temperature is reached.

Sometimes I read about prolonged holding time in cryo (about up to 6h). Why? Is there another benefit from longer holding time?

Other than sandvik HT Regime of AEBL I have states a 3x cryo treatment with intermediate snap temper


AEB L
For normal HT this steel
1. Preheat stage 1 550 -600C soak for 3-5min
2. Preheat stage 2 900- 1000C soak for 3-5 min
3. Austenizing 1050-1080°C equalize for 3-5 min,, soak 8 min
4. Quench in preheated in Oil 60-80°C (For Vak. Furnace min 4Bar N2)
5. Cryo minimum -70°C or more soak for 1h (Cyo. has to be done immediately after Quench to be efficient, maximum time for delay is 2h)
6. Temper 1 150°C for 2h
7. Quench in Cold Water
8. Cryo minimum -70°C or more soak for 1h immediately after Quench
9. Temper 2 150°C for 2h
10.Quench in Cold Water
11.Cryo minimum -70°C or more soak for 1h immediately after Quench
12.Final temper 150°C-200°C for 2h depending on the job that has to be done you select the final temper temp.
This will lead you in the range between 63 - 59HRC

Why 3x cryo? I know the reason for multiple tempering cycles, - after the first temper , some retained austenite converts to some fresh untempered martensite which is then tempered in the following temper cycle - but whats the benefit of the multiple cryo cycles?

Is it because not all the retained austenite converted in the first Cryo cycle? tempering first and then cryo is usually not recommended because tempering more or less stabilizes the retained austenite.


The blades I have made so far are good and I am satisfied but I do not want to simply follow some instructions but understand the reason behind it.

Best Regards
rolynd
 
Rolynd, you are actually dealing with two separate processes here, cold treatments and deep cryo. Although with the availability of liquid nitrogen the line between the two gets rather blurred and gray, beyond just temperature levels.

A standard cold treatment is well defined and well understood for its benefits. The idea here is to complete the quenching process by converting any stubborn austenite that did not get sufficient motivation to do so in the standard quench (be it air or liquid). The people at Sandvik know their stuff as the austenite to martensite transformation actually occurs at the speed of sound, so it is pretty much instantaneous. Martensite transformation is also shear driven and has no diffusion involved, this means that it doesn’t give a flip about time all it cares about is the driving strain brought about by decreasing temperature, so yes simply obtaining the necessary temperature throughout is all that is required.

Deep cryo is another thing. It involves a lot of things that are still theoretical and in the process of being fully understood, but the idea here is beyond basic austenite conversion and into the precipitation of special carbides and homogenization of lattice defects, things that are affected by time. I am very careful about making any definitive statements about these concepts because there is still a lot left unexplained here.

But if all you are looking for is the extra hardness by zapping some of that 6-20% retained austenite that can occur due to alloying just getting it really cold is enough to do the trick.

Edited to add- sorry I missed a couple of your questions. The 3 times thing is about tempering back the new martensite but also destabilizing any remaining austenite so that the next cycle can zap it; once again, if the process was time dependent all you would have to do is soak it at the low temp, but the cycling provides the strain to complete the process.
 
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Thanks a lot Mr.Cashen for answering my Questions!
Since I dont have acess to liquid nitrogen and a simple cold treatment is enough to achieve hat I want I will stay with it. The reason for the 3x cryo is now also clear.
As said, the results I get are satisfying, the blades are fine and hold up well in use.

Would a cold treatment to -78°C be enough to get rid of the retained austenite in other SS steels as well or is the transformation temperature somewhat related to the composition of the steel? I am thinking for example of Bohler N690 which is a popular steel in my country.
I remember reading that the process starts at -70°c but somewhere else read about a transformation maximum at -100°C . I did no pay great attention at that time because it was not a steel I use.

Best Regards
Rolynd
 
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A related question/issue: I think I remember Kevin saying once that for simpler carbon steels (10xx), even a night spent in the freezer would help somewhat to "zap" the retained austenite. I expect that whatever effect this has, it would not be as dramatic as what would be found either with a true cryo treatment on the same steel or cryo on high allow steels. Even so, what effects might we see from such a cold treatment, and is there any real benefit to be gained? I assume it won't hurt anything, at the very least.
 
I have heard Kevin say the opposite, that if a home freezer makes a significant change, it might be time to re-evaluate the HT procedure. On the other hand, it won't hurt most likely, so have at it. Cryogenic treatment is rife with odd tales and sound science at the same time. A good search will show many good published papers on it and cold treatment, but one must be careful to compare apples and apples. Many of the seemingly contradictory statements come from different sources that aren't even comparing the same thing.
 
I should clarify and say that I would expect no changes at all for anything other than simple carbon steels, and very little even then, if any. I agree with Kevin's statement that you mention above. I am fairly comfortable with my HT procedure(s) for 10xx steels, and, though I've not noticed any material difference between the frozen blades and the unfrozen, if there is ANY benefit to freezing them I might as well do it. It doesn't cost anything except time and an insignificant and unquantifiable amount of electricity. My question is mostly theoretical.
 
Good thread - and caught my interest because of the Sandvik steel used. I use 12C27 and 14C28 and like them quite well. I'd like to ask the folks who know their comments on Sandviks comments about a -5ºF freezing. I talked to one of their engineers who confirmed the info on the website that even -5ºF freeze would add about 1 point of Rockwell hardness to the steel. I've tested a couple of blades right off quench plates, then after an hour or so in home freezer at -10ºF and it does seem to pick up 1/2 to 1 point of hardness before tempering at 345ºF for 60Rc. Does anything think multi freezing/tempering cycles would benefit toughness of finished blade any? 60Rc is plenty hard, and even 59Rc holds an edge very good.

Link to Sandvik's info on single blade heat treating 13C27 with freezer to -5ºF: http://tinyurl.com/ombo3lr

Ken H>
 
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A related question/issue: I think I remember Kevin saying once that for simpler carbon steels (10xx), even a night spent in the freezer would help somewhat to "zap" the retained austenite. I expect that whatever effect this has, it would not be as dramatic as what would be found either with a true cryo treatment on the same steel or cryo on high allow steels. Even so, what effects might we see from such a cold treatment, and is there any real benefit to be gained? I assume it won't hurt anything, at the very least.

It all depends on where Mf is for the steel. I know the term Mf has lost favor to percentages in martensite but for this discussion it is applicable. Mf, or the maximum conversion from austenite to martensite is going to be a function of the chemistry of the steel, and while you can’t really change the alloying with you heat treatment (unless you burn the stuff out), you can add or subtract for the available carbon in solution and carbon plays a very large role in where Ms and Mf are as it tends to stabilize the austenite to resist the strain required for the transformation.

If you follow the whole mess so far you will start to see that how high you heat the steel will affect how low you will have to cool it to get all of the martensite. The cold treatments come into play when alloying or oversoaking pushes Ms below room temperature. If the steel is a simple one that really only has carbon as the austenite stabilizer then the answer is to simply avoid overheating before the quench, but if you do you still shouldn’t have to go super cold to notice some hardness gain. But if you are getting noticeable gains from just some time in the kitchen freezer with 10XX series, W1, W2, 5160, 52100 and other basic steels the obvious answer is to not overheat it in the first place.
 
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