Super duper CPM-s3v heat treatment recipe?

Some people are still way behind the curve ! It takes a while to understand that ordinary martensite and carbides are different from eta carbides and their formation.
Best example of the differences are that all our heat treatable steels can get martensite and with enough carbon , get carbides. But this has nothing to do with eta ! The greater wear resistant is due to eta not further changes in RA ! ...Read that over 100 times ! LOL
 
So to summarize this discussion(or my questions/confusion anyway), and the papers I've been able to find and read online, with the fewest acronyms possible:

  • Sub-Zero (-100F) treatment is required to eliminate retained austenite (RA) and finish converting it to martensite (Martensite Finish temperature, Mf) in high alloy steels. This has little effect on wear resistance compared to tempering without Sub-Zero treatment
  • Cryogenic (-300F) treatment is required to precipitate "eta carbides" which enhance wear resistance. (Can someone define "eta" I cannot find this anywhere? Just Fe2C?)
  • We don't know which alloys will precipitate eta carbides, it's assumed they're created in higher alloyed steels but some steels with higher alloy like AEB-L do not seem to form them? (Performance based evaluation or some other means of determining existence or lack?)
  • The book is still out on whether cryogenic treatment and eta carbide precipitation lowers toughness when compared to High Temper Temp (HTT) alone? (referencing a post Larrin made in 2016 and a paper on carbide formation in carburized steel) Or is this more alloy dependent?

Nothing I've read here or elsewhere has given me any confidence that cryogenic treatment is always desirable, or, if not always, when it is desirable, outside of known/tested heat treat procedures such as the 3V Low Temper Temp procedure.

My ultimate two questions being: Is it beneficial to simple steels (1095) and if it is, at what cost if any, and by what mechanism? And 2, would cryogenic treatment of S-7 increase it's wear resistance without lowering it's impact toughness?

Both of these I would test on my own if I were set up for cryo. But the answers to the questions impact my decision of whether to invest in cryo or not. And while I previously adhered to JT's posts about "everything in cryo" if only because it's easier/cheaper than doing both cryo and sub-zero one's self, now I'm questioning whether everything in cryo has some negative impact on some alloys in regard to impact toughness.
 
Again, all speculation here Kuraki, but I gather that the eta carbide precip will not only increase wear resistance (I am dubious as to the claims being made, however, of 400% and more in some steels), but they also will increase impact toughness, because the bond between the eta carbides and the martensite matrix is said to be enhanced by this eta carbide precip, by some of the tech papers available online, I think in part to their extremely small size and distribution/cohesion with the martensite during tempering.

Just guessing here on my part, White steel would not benefit a whole heck of a lot from an LN2 soak (assuming HT was correct to begin with). Blue steel on the other hand, might indeed benefit, due to the eta carbide precip that happens because of present alloys in the mix , like Cr, W, and V. That is a complete assumption.

Does 1095 precip eta carbides? I have no idea. I don't think you would gain anything as far as Mf is concerned, either, if your HT was done correctly. In quick layman's terms on that...you would not see an increase in as quenched hardness (again assuming you weren't quenching from above 1475-1500F). If you did quench 1095 hotter than this, it would have more RA than if quenched from 1475F, a lower as quenched hardness (say 65 instead of 66+), and at least sub zero would be needed for that excessive RA.

I would expect S7 to have increased wear resistance, and some small amount of increased impact toughness.
 
Eta carbides are a type of "transition carbide." They form in all steels, particularly with low temperature tempering (<300°F) or at the early stages of higher temperature tempering. The claims of cryogenic processing is that there is an increase in the density of these transition carbides. As to a greater density of transition carbides increasing toughness, I highly doubt it, and at least one paper I can think of showed the opposite.
 
eta carbides form preferentially in certain locations in the matrix. Not surprising ! How and if they grow I haven't seen mention.
With eta we have to think micro rather than macro. With the ordinary steels fracture usually goes from carbide to carbide , where with the 'powder steels' fracture goes often between carbides .Remember that the smaller the carbide the more likely there is cohesion which adds to toughness.
If you come across good photos of eta carbides and info of cohesion forces please let us know.
 
If they form in the early stage of HTT do they disappear later during the tempering? Or might one be able to soak at 300F then ramp to 1000F with something like Z-Wear to form these carbides and also gain the benefit of secondary hardening from the HTT sans cryo?

Larrin said:
Eta carbides are a type of "transition carbide." They form in all steels, particularly with low temperature tempering (<300°F) or at the early stages of higher temperature tempering. The claims of cryogenic processing is that there is an increase in the density of these transition carbides. As to a greater density of transition carbides increasing toughness, I highly doubt it, and at least one paper I can think of showed the opposite.
Click to expand...
 
What I have deduced from what I have read is probably a mix of right and wrong. Take the rest of this as my feelings based on reading articles.

What makes AEB-L different is that it is a hypo-eutectiod steel (67% C) with lots of chromium (13%) .... It has no free carbon to form cementite in the martensite matrix that forms upon the supercooling of the austenite and its conversion to martensite ( I probably said that sort of wrong). It is basically 1070 with 13% free chromium. The chromium alloying is merely for stainless properties, and doesn't make chromium carbides. Thus, AEB-L doesn't form eta carbides because it doesn't have any available carbides to change. A few seconds at -100 is all it needs to reach Mf, so an 8 hour soak at -300 is just a waste of resources and time.

I think I understand that the epsilon-carbide is something between Fe2C and Fc3C ... Fe2-3C. It changes into eta-carbides the elements get really cold and motion slows down.
Then there is a Fe5C Hagg carbide that forms as the eta and/or epsilon carbides warns up from cryo, and then precipitates in the temper cycle. I think this is actually what we are dealing with in wear resistance.

The long and short of what I have read is that eta-carbides are a big deal in specialty metals used in aerospace and high pressure industries. It is of most concern with other metal's carbides like chromium, tungsten, and vanadium, but not so much with iron carbides.

Do they make some difference in knife steel - yes.
Is it worth staying awake late into the night figuring them out - NO!
Without at least $1,000,000 in lab equipment will any of us even be able to tell if they are there and if they make a difference in our knife - absolutely not.

I have tried to study the many types of carbides and it is really difficult to understand. To make that worse, a lot of the subject is not fully understood yet.
I have to poke fun at myself and tell you guys that I had read about n-carbides ( pronounced in my head like the letter N) in several papers before it suddenly struck me that it was eta-carbides (the greek eta looks like an n). It gets even harder to keep them straight when you realize that the epsilon carbide is written as e-carbide.

You hve to realize that non of this existed when I was younger, so don't take my explanation as gospel - I still refer to Troosite in hamon discussions.
 
some more food for thought
http://www.industrialheating.com/ar...ogenic-treatment-on-properties-of-tool-steels some of the conclusions:
"Generally, there are no significant advantages that would prolong the lifetime of tools by providing higher wear resistance and toughness. We can perceive certain improvements, but CT does not make sense considering the economics. Real experiments with tools made from VANADIS 6 did not show a longer life cycle."
"4. The differences that have been proved so far between CT and no-CT materials are not too practically important considering CT costs.
5. Investigators are not unified in the opinion of CT benefits, although research has been going on for more than 50 years. General conclusions and achieved results do not differ much steel by steel." my underline
http://www.industrialheating.com/articles/91311-the-deep-cryogenic-treatment-question?
"Another distinguishing characteristic of DCT is the timing of the treatment, where the object being treated has its temperature slowly reduced into the cryogenic range. This reduction in temperature is on the order of four to 10 hours. The temperature is then held at a low cryogenic temperature for a period of time – about four to eight hours. This may be followed by a tempering cycle, depending on what material is being treated."
"Many researchers try to circumvent the timing of the process by immersing materials in liquid nitrogen or liquid helium. At best, there is no change in the performance of the material; at worst, there is catastrophic failure. Unfortunately, the results of these experiments are usually cited as proof that DCT does not work." Deep cryo(temperatures less than -300F) requires equipment few if any of us could afford.
https://www.cryogenictreatmentdatabase.org/ a free information source provided by Cryogenic Society of America.
here is a link to an article written about this dated January 1955 https://www.cryogenictreatmentdatab...chilling_toughens_metals_increases_tool_life/. It is interesting that almost every article not written as a sales pitch ends with "More research required."
 
kuraki said:
If they form in the early stage of HTT do they disappear later during the tempering? Or might one be able to soak at 300F then ramp to 1000F with something like Z-Wear to form these carbides and also gain the benefit of secondary hardening from the HTT sans cryo?
Click to expand...
They are replaced by cementite, hence the "transition." I'm not aware of studies that compare the wear resistance contribution of transition carbides vs secondary hardening carbides. The transition carbides may simply dissolve and be replaced by the secondary hardening carbides, similar to the process of how the transition carbides are replaced by cementite. Thermodynamically, the transition carbides would not maintain their stability at high temperature.
 
1955 ? Then they didn't blame it all on climate change !!
"Types of carbide " One bit of research I did to learn about carbides just got me more confused . Carbides and their bonding --some have metallic bonding, some have molecular and some have both ! Sorry Stacy !

I wish some of you would put out of your minds the subject of secondary hardening. I'm sure that if you take eta carbides and heat to 900F the eta carbides and their cohesion field would disappear .Stick to 300 or 400 F.
 
Stacy - I agree, all this HT'ing theory makes my head hurt also. I keep thinking "all I want is a simple "cook book" type recipe for HT..... then I find myself reading threads with all the theory, chasing links with more info.... when I all I really want is a recipe - but do wish to have a basic understanding of the theory.

Now, on the study of cyro treatment, do papers from the 1950's, or even 1980's for that matter have any real bearing on current knowledge since the knowledge of cyro treatment is growing so extensively?

Ken H> (still trying to prove "Old Dogs can still learn")
 
kuraki said:
So to summarize this discussion(or my questions/confusion anyway), and the papers I've been able to find and read online, with the fewest acronyms possible:

  • Sub-Zero (-100F) treatment is required to eliminate retained austenite (RA) and finish converting it to martensite (Martensite Finish temperature, Mf) in high alloy steels. This has little effect on wear resistance compared to tempering without Sub-Zero treatment
  • Cryogenic (-300F) treatment is required to precipitate "eta carbides" which enhance wear resistance. (Can someone define "eta" I cannot find this anywhere? Just Fe2C?)
  • We don't know which alloys will precipitate eta carbides, it's assumed they're created in higher alloyed steels but some steels with higher alloy like AEB-L do not seem to form them? (Performance based evaluation or some other means of determining existence or lack?)
  • The book is still out on whether cryogenic treatment and eta carbide precipitation lowers toughness when compared to High Temper Temp (HTT) alone? (referencing a post Larrin made in 2016 and a paper on carbide formation in carburized steel) Or is this more alloy dependent?

Nothing I've read here or elsewhere has given me any confidence that cryogenic treatment is always desirable, or, if not always, when it is desirable, outside of known/tested heat treat procedures such as the 3V Low Temper Temp procedure.

My ultimate two questions being: Is it beneficial to simple steels (1095) and if it is, at what cost if any, and by what mechanism? And 2, would cryogenic treatment of S-7 increase it's wear resistance without lowering it's impact toughness?

Both of these I would test on my own if I were set up for cryo. But the answers to the questions impact my decision of whether to invest in cryo or not. And while I previously adhered to JT's posts about "everything in cryo" if only because it's easier/cheaper than doing both cryo and sub-zero one's self, now I'm questioning whether everything in cryo has some negative impact on some alloys in regard to impact toughness.
Click to expand...


With the simple steels like 1095, I would be worried about a quenched blade going into a cold chamber for 10h without a temper. With something like 52100, I would think a AAA quench would produce less stress than a parks50, maybe letting it survive.
 
Those of us not currently doing cryo can't afford to put it out of our minds. Hence the question on the impact to transitional carbides.

mete said:
1955 ? Then they didn't blame it all on climate change !!
"Types of carbide " One bit of research I did to learn about carbides just got me more confused . Carbides and their bonding --some have metallic bonding, some have molecular and some have both ! Sorry Stacy !

I wish some of you would put out of your minds the subject of secondary hardening. I'm sure that if you take eta carbides and heat to 900F the eta carbides and their cohesion field would disappear .Stick to 300 or 400 F.
Click to expand...
 
Why? If the austentite->martensite conversion is being completed, where is the residual stress coming from that would cause it to crack? The Linde article you posted also indicates they don't predict it to be useful for simple carbon steels anyway. It is very interesting that sub-zero treated D2 was found to lose toughness, while cryo D2 was found to have increased toughness. That would lend towards the idea of cryo all the things and avoid sub-zero.

Willie71 said:
With the simple steels like 1095, I would be worried about a quenched blade going into a cold chamber for 10h without a temper. With something like 52100, I would think a AAA quench would produce less stress than a parks50, maybe letting it survive.
Click to expand...
 
I think any knifemaker considering this should read this article first http://www.industrialheating.com/articles/91311-the-deep-cryogenic-treatment-question?. they describe a process that takes 20 hours or more, a controlled 30 minute ramp down to -300F, a 6 to 40 hour soak, followed by a 4 to 10 hour ramp up to room temperature.
http://ctpcryogenics.com/ makes this sort of equipment. read thru their site, especially their FAQ. a selection
Q: Why can’t I just dip a part in Liquid Nitrogen?

A: Research indicates that dipping in liquid nitrogen is successful about 10% of the time. Think of it this way. If you dip a part into liquid nitrogen the surface wants to contract to the size it would be when it is -320F. The inside of the part is still at room temperature and therefore at the size it would be at room temperature. This creates stresses, and the stresses at the surface are there at temperatures where the metal is brittle. This is an invitation to disaster. Our system of slow cooling eliminates the extreme temperature differentials created by dipping the part or by having a nitrogen spray hit the surface.

There are other reasons to avoid dipping. Some the the things that are happening in the metal take time and happen within certain temperature ranges. Blasting through these temperature ranges by quick cooling or dipping does not allow enough time for these changes to take place.
 
During secondary hardening in the 900F range , new and different carbides are formed ! Do you expect eta carbides and their cohesion would survive that ?
 
kuraki said:
Why? If the austentite->martensite conversion is being completed, where is the residual stress coming from that would cause it to crack? The Linde article you posted also indicates they don't predict it to be useful for simple carbon steels anyway. It is very interesting that sub-zero treated D2 was found to lose toughness, while cryo D2 was found to have increased toughness. That would lend towards the idea of cryo all the things and avoid sub-zero.
Click to expand...
Any claims that cryo is increasing toughness on a steel like D2 are dubious, in my opinion.
 
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