Liquid Nitrogen vs Dry Ice

MBB said:
Larrin,

Thanks a lot for posting this. I am a relative newbie (although I really enjoy the science side of knife making) and this has left me with several questions.

1) So looking at this table, 1095 austenitized to 1500 F has essentially all of its carbon (0.94) in solution versus austenitization at say, 1450 F, where ~90% of its carbon is in solution (0.86)? Once it is initially quenched and then treated with liquid nitrogen to have a final retained austenite of ~3% (down from 32%), how does the presumed increased carbon (associated with the 1500 F temperature) in solution at the time of quench affect the steel relative to quenching from lower temperatures (e.g. 1450 F)? I would suspect that this would increase hardness by ~10% but does the toughness suffer from loss of RA?

2) Essentially, the increased Rockwell hardness seen at lower temperature heat treating (say 1460 F vs 1500 F) for some steels is primarily due to decreased retained austenite? Or is it also related to carbon percentage in solution?

3) Related: How much RA is just right for maximal steel performance? Is that empiric/steel specific or are there general guidelines? Can cryo be a negative rather than a positive?

4) So based on carbon in solution at their austenitization temperatures, functionally this chart suggests that 3V and AEB-L are highly alloyed medium carbon steels (and thus explains their toughness)?

Thanks a lot in advance and please pardon any typos from my weary brain.

Mike
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1) If held for an infinite amount of time at those temperatures that is what the model predicts the carbon content will be in austenite prior to quenching. The toughness would go down by transforming RA. As carbon content of the martensite goes up lath martensite is replaced by plate martensite which also has lower toughness.

2) lower austenitizing temperature leading to greater hardness is usually because of less retained austenite with the lower temperature.

3) Sometimes recommendations are given by steel companies, such as 15% retained austenite for D2. Sometimes these recommendations are to minimize size change, sometimes for balance of toughness, and sometimes for both. Typically knife makers have not tested for optimum RA content because 1. Cryo is ubiquitous in high end knives and is perceived to be a positive and 2. Fears that austenite will transform during use leading to potential brittle fracture points. RA can be quite stable and it is definitely an area good for testing to determine if an optimum percentage of RA can be found for a given steel and application.

4) The relatively high toughness for 3V and AEB-L is primarily due to the low volume of carbides that they have. I had a whole thread where I laid out the effect of carbide volume on toughness but I am too lazy to find it.
 
To check the calculations I compared against Bohler-Uddeholm datasheets for A2, D2, and Vanadis 4 Extra. A2 and D2 are very close with slight overestimates for the calculated values, and somewhat underestimating for vanadis 4 extra. Overall I think the calculations are working well.
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Hello, Larrin Larrin , according to Fredrik Haakonsen, one of my steel gurus (and everything knife and steel related guru), to remove retained austenite the temperature should be around-85C (-121F). He explains it here: https://edgematters.uk/thread/2948-...?postID=30433&highlight=metallurgen#post30433
He also states that it’s not clearly proven what cryogenic treatment does to steel. And that “The optimum hardening temperature for a steel without the use of deep cooling will be the one were you don't get an increase in hardness if you deep cool.”, being it deep cool (-85C) or cryo (-190C).

What’s your opinion about it?
 
hugofeynman said:
Hello, Larrin Larrin , according to Fredrik Haakonsen, one of my steel gurus (and everything knife and steel related guru), to remove retained austenite the temperature should be around-85C (-121F). He explains it here: https://edgematters.uk/thread/2948-...?postID=30433&highlight=metallurgen#post30433
He also states that it’s not clearly proven what cryogenic treatment does to steel. And that “The optimum hardening temperature for a steel without the use of deep cooling will be the one were you don't get an increase in hardness if you deep cool.”, being it deep cool (-85C) or cryo (-190C).

What’s your opinion about it?
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He seems to primarily be responding to mete's recommendation of liquid nitrogen for the purpose of eta carbide formation. I agree with him the evidence doesn't seem convincing that liquid nitrogen temperatures would lead to increased eta carbide formation during tempering, but there are systematic reviews that disagree with me. However, my calculations that I included in this thread show that liquid nitrogen can very well lead to more retained austenite transformation than dry ice temperatures. Here is a paper that compares a "cold treatment" with a "subzero" treatment, and shows a further reduction in retained austenite with the subzero: https://www.jstage.jst.go.jp/article/isijinternational1989/34/2/34_2_205/_pdf/-char/en

They also promote the theory that increased eta carbide formation leads to superior wear resistance, if you want to read some more about that.
 
Thank you, Larrin. Allways a pleasure to read useful information regarding steel heat treatment and the science behind it.
 
Do you have studies that correlates cryo treatment and toughness? Mainly in low alloy steels. In high alloy, I already know that cryo eliminates the need of high tempers, promoting an increase in toughness. If you already mentioned that in another thread, Larrin Larrin , I apologize.
 
Larrin said:
He seems to primarily be responding to mete's recommendation of liquid nitrogen for the purpose of eta carbide formation. I agree with him the evidence doesn't seem convincing that liquid nitrogen temperatures would lead to increased eta carbide formation during tempering, but there are systematic reviews that disagree with me. However, my calculations that I included in this thread show that liquid nitrogen can very well lead to more retained austenite transformation than dry ice temperatures. Here is a paper that compares a "cold treatment" with a "subzero" treatment, and shows a further reduction in retained austenite with the subzero: https://www.jstage.jst.go.jp/article/isijinternational1989/34/2/34_2_205/_pdf/-char/en

They also promote the theory that increased eta carbide formation leads to superior wear resistance, if you want to read some more about that.
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Thank you. I’ve been trying to get straight answers on this topic, and I keep reading conflicting reports on eta carbides. All I know is eta carbides are good for marketing purposes. Customers seem to like that my process of full cryo can result in them.
 
hugofeynman said:
Do you have studies that correlates cryo treatment and toughness? Mainly in low alloy steels. In high alloy, I already know that cryo eliminates the need of high tempers, promoting an increase in toughness. If you already mentioned that in another thread, Larrin Larrin , I apologize.
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I don't know of any off the top of my head on low alloy steels. However, it is the wild differences in results from cryo studies that makes it hard to take them seriously. One study might show an improvement in toughness as well as a 10x the wear resistance when using 24h cryo and another might show a decrease in toughness and a marginal improvement in wear resistance. It depends on the material studied, the type of testing performed, and the bias of the researchers.
 
Larrin, THANK YOU for post #23. I would LOVE to see data for several other steels just like that - perhaps 1084, 80CRV2, AEB-L. Interestly it confirms my thinking and info from Sandvik - while a freezer at -5F isn't the best, it does actually do something. I see the freezer drops about the same amount of retained austenite as from freezer to dry ice. Your chart shows that all the austenite doesn't get converted until LN stage. I'd read austenite was converted by -95F, but the advantage of LN was the ETA carbides?

BTW, what is the last column B-U RT in post #23?

Thank you again for all the knowledge you've shared with the rest of us.

Ken H>
 
So dry ice is much better than nothing! But it looks like I still need to buy a deware!
 
Larrin said:
That is the experimentally reported value from Bohler-Uddeholm datasheets.
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Thanks, I should have related that column to the charts below and figured that out myself. I'd make the relationship on the different temperatures and charts, but missed the B-U relationship. Open me eyes!
 
So when Ed Fowler claimed putting it in the freezer showed improvement, he was correct?
I wonder if some apologies are not in order? :)
 
AVigil said:
So when Ed Fowler claimed putting it in the freezer showed improvement, he was correct?
I wonder if some apologies are not in order? :)
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That depends on what kind of "improvement" he saw, how he used the freezer, whether he still recommends using a freezer, and who it is you think should apologize and for what.
 
AVigil said:
That is very true.
Back in the day there were some pretty good discussion and take a look at this. You would have a far better knowledge about it and maybe give some feedback on their discussion?

https://www.bladeforums.com/threads/time-from-quench-to-temper.744224/#post-8251341
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The only comment that I have on that thread is that there is a difficult dichotomy between the need to temper to stress relieve as-quenched martensite and also the need to do cryo as soon as possible to maximize the conversion of retained austenite. Tempering helps stabilize retained austenite and makes cryo less effective. The solution often used/proposed is to do a "snap temper" that is relatively short and at low temperature, such as 300°F for 1h prior to cryo. The snap temper likely still reduces the effectiveness of cryo but if cracking in liquid nitrogen is a problem then that is the usual solution.
 
Larrin Larrin so would putting a 52100 blade in the freezer have any gains in Rockwell hardness as explained by Fowler?
 
AVigil said:
Larrin Larrin so would putting a 52100 blade in the freezer have any gains in Rockwell hardness as explained by Fowler?
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It depends. :) Hypereutectoid low alloys steels have quite high carbon in solution even with relatively low austenitizing temperatures, which leads to Mf below room temperature, despite the common perception about low alloy steels. However, up to 15% or so retained austenite there isn't a huge increase in hardness from cryo, maybe 1 Rc. I haven't done the experiment so I can't say for sure.
 
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