Liquid Nitrogen vs Dry Ice

MBB said:
I'm still curious of what impact maximizing carbon in solution at austenitization has on performance for low alloyed steels, assuming that the increased RA will be removed with cryo thereafter. Seems like it would be an interesting experiment with a couple of simple steels like 1095, 52100, O1, W2, etc. Maybe 3 samples of each steel with progressively higher austenitizing temperatures for each different sample to maximize carbon in solution, then all get cryo, and finally hardness testing and charpy testing to determine the effects. I realize there is a grain size component as well as free carbon component so it would not be entirely easy to tease out results, but it would still be interesting to see.
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Can't we predict with some confidence that it will result in lower toughness due to higher amounts of carbides and increased grain size?
 
MBB said:
I'm still curious of what impact maximizing carbon in solution at austenitization has on performance for low alloyed steels, assuming that the increased RA will be removed with cryo thereafter. Seems like it would be an interesting experiment with a couple of simple steels like 1095, 52100, O1, W2, etc. Maybe 3 samples of each steel with progressively higher austenitizing temperatures for each different sample to maximize carbon in solution, then all get cryo, and finally hardness testing and charpy testing to determine the effects. I realize there is a grain size component as well as free carbon component so it would not be entirely easy to tease out results, but it would still be interesting to see.
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In the 52100 study I posted earlier, the grain size was consistent across all three of the austenitizing temperatures used (1500, 1550, and 1600°F). They found that toughness was greater for an equal hardness if the austenitizing temperature was lower, due to the effect of carbon on the toughness of martensite. This has been found in a diverse group of different steels, though often times grain size is a confounding factor. In general, lower austenitizing temperature is better for toughness.
 
kuraki said:
Can't we predict with some confidence that it will result in lower toughness due to higher amounts of carbides and increased grain size?
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Higher austenitizing temperatures usually leads to a reduction in carbides. Grain size and carbon in solution seem to be more important in general for a given steel. I wouldn't rule out the effect of carbides in describing different types of behavior of course, but carbide size and volume comparisons seem to be most relevant between different steels rather than different heat treatments when it comes to toughness. There are some exceptions, such as studies where steels have been overaustenitized to reduce carbide volume and the re-austenitized at a lower temperature to keep the grain size and carbon in solution down.
 
bluntcut said:
RA is a symptom is dislocation/stress in matrix. Cold treatment alleviates RA symptom (conversion likely lead to increase dislocation). Temper alleviates dislocation.

Dislocation = more brittle.

Dealing with dislocation:

i. Low aust temperature - in general - produces less dislocation however with risk of un-even distribution of carbon in solution.
ii. Long cryo soak - setup for eta carbides following activation (temper). Almost a voodoo/dark process because poorly understanding on this mechanism.
iii. Aust & Mart tempering - hahaha most likely to increase dislocation (RA% increase).
iv. Finer grain - most likely to increase RA due to more grain boundaries surface/volume, more crystal misalignment,etc.. Actually increase dislocation.
...

So look like i. probably could yield a net-positive gain. Control the outcomes, more optimal options are available.
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Almost nothing you wrote in that post is true. Retained austenite is not related to dislocations.
 
Larrin , my apologies, I never looked , though I don't remember many posts.
My interesting application was with an air hardening steel , brine quenched .With other HTs it wouldn't work !
As for carbides photos are out there that show fractures with large carbides are normally carbide to carbide .With smaller carbides only sometimes is the fracture carbide to carbide .Yes I like PM steels ! Small carbides especially eta carbides also usually have cohesion fields around the carbide.
Let's keep looking and learning !
 
Larrin, I really appreciate your input as an expert. Hope you don't mind all of my questions.

I get the loss of toughness, I was curious if there was a concomitant increase in hardness with greater carbon availability/higher temps followed by cryo.

So, what's the mechanism for decreasing carbides with increasing temperature? Decarburization? Or do you mean decreased alloy carbides with increased iron carbides?

Thanks,

Mike
 
MBB said:
Larrin, I really appreciate your input as an expert. Hope you don't mind all of my questions.

I get the loss of toughness, I was curious if there was a concomitant increase in hardness with greater carbon availability/higher temps followed by cryo.

So, what's the mechanism for decreasing carbides with increasing temperature? Decarburization? Or do you mean decreased alloy carbides with increased iron carbides?

Thanks,

Mike
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I will try my best to fulfill the title of expert and I don't mind questions.

Higher temperatures for more carbon in solution followed by cryo will lead to higher hardness.

In the annealed condition there is a combination of ferrite and carbides, whether alloy carbides, iron carbides, pearlite, etc. On heating the ferrite transforms to austenite and carbides dissolve so that carbon and alloy from the prior carbides enriches the austenite. As the carbides dissolve there is a reduction in carbide volume.
 
Larrin said:
I was able to find some reliable retained austenite numbers on 52100, included in Appendix A, page 9b: https://dspace.mit.edu/bitstream/handle/1721.1/70610/07354787-MIT.pdf?sequence=2

Each was austenitized for 25 min. 1500°F - 8%, 1550°F - 15%, 1600°F - 18%. So the presence of retained austenite is confirmed, and this is not a surprise except to those who still want to hold to the idea that low alloy steels won't have retained austenite. Using a tempering temperature of 450°F led to the elimination of retained austenite but unfortunately this also correlated with reduced toughness due to tempered martensite embrittlement. Lower tempering temperatures did not eliminate retained austenite.
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I would like to have this put into "real-world-guy-using-a-knife" context.

- There was a time I would be sure to water quench after tempering, because someone smarter than me said it would reduce extreme cold embrittlement and had data to back it up.
- I stopped tempering back the spine on my larger choppers due to the threat of TME (tempered martensite embrittlement) that someone smarter than me warned about and had the data to back it up.
- I used dry ice and acetone to reduce the threat of RA in 1095, 52100 and L6 because someone smarter than me told me that every little bit counted and had data to back it up.

I can get into enough detail about the benefits of these "value added" steps and sound like I know what I'm talking about. There is real metallurgical evidence to back this stuff up. BUT what I can't do is SHOW anyone the practical difference between a blade I took the "extra steps" on, compared to a blade with a conservative(still appropriate) heat treat regime. So, I had to ask myself WHY is was doing all these added steps... Was it for the end user?.. Was it for me? Was it to keep up with the Jones's?

I don't know... maybe, it is because I depend on this craft to make a living. This isn't a hobby for me. Yes, I like to express myself and I LOVE MAKING KNIVES for folks who love knives... lol. I wish I could give them away for free... but I can't. In the end, everything I do in my shop has to prove it's worth in the hand of the user, not under a microscope. Don't get me wrong... I wouldn't be where I am if not for riding the coattails of the guys in the white lab coats. Metallurgy ROCKS! ... I just need to know if it is a 7% performance gain or a .07% performance gain, before before I start adding steps and cost to my processes.

It kind of reminds me of my career as a working drummer. I worked my butt off to get my chops up. Other drummers would be intimidated if I was in the room. I was fast and could play crazy complex rhythms in most genres. But the truth was that the other drummers didn't pay me for the gig... and I know for a fact that I lost gigs for being a showboat. Laying down a simple groove with a large pocket makes everybody sound better and band leaders call you back in. The ability to play at breakneck speeds doesn't translate well when you are the background music at an Art Gala.

Anyway... I'm tired and in pain(blew out my back) so take my comments with a grain of salt. Great thread, Larrin.
 
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You may or may not want to eliminate retained austenite. It's up to you with the testing of your knives to determine whether that is a positive or a negative. It might depend on the knife, the customer, the application, etc. For some customers it might not matter which is better. If you make art knives then I don't recommend you spend much time learning about retained austenite.

For some more context, I spend 40 hours a week trying to develop steels and heat treatment cycles to maximize retained austenite. I would never say that retained austenite is universally bad.
 
That is what I'm asking, Larrin. What about for customers who DO want to know if it matters? I make performance knives. Do I show them some micrographs and say, trust me, I know what I am talking about? I find it embarrassing when I can only substantiate a claim by referencing data, rather than showing real world performance.

At this point, its as if I'm being convinced to buy a car because the data shows it can run 10% longer at max rpm. Who will ever see that benefit outside of a lab?
 
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Rick Marchand said:
That is what I'm asking, Larrin. What about for customers who DO want to know if it matters? I make performance knives. Do I show them some micrographs and say, trust me, I know what I am talking about? I find it embarrassing when I can only substantiate a claim by referencing data, rather than showing real world performance.
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Measuring real world performance is data. You just need to generate it. I haven't seen many knife enthusiasts that are concerned about the details of your cryo treatment, or even heat treatment details in general. Is that a common occurrence for you? If it were me I would focus more on big picture things to promote knives. Base your heat treatment on good principles, and compare your heat treatment to a baseline. Compare a knife with both the baseline and your "improved" heat treatment and determine if there is really an improvement. The difficult part is designing a quantitative experiment that is not affected by your own bias.
 
Most of the research is done for tooling. They count the number of cycles for stamping, the number of holes drilled, the number of feet cut, and other things that determine length of tool life.

With knives, it’s harder to determine. Prep-chefs gauge a knife by how many shifts before resharpening, hunting guides count the number of animals skinned before resharpening.

Cutting cardboard or rope will give some idea of how a knife will hold up. With toughness, most knives don’t require the ability to be bent 90 degrees.

The more heat treating steps = the bigger the story = the higher the price.

Hoss
 
So for someone like me, who’s just learning, and building knives for hunters, and heavy duty outdoor applications, some RA is good? If I’m looking for ultimate toughness and edge stability, I’ve heard some RA is good, some people like Nathan Carothers says you want as little RA as possible, and we’ve seen his tests on edge stability! He’s a big believer in cryo. This can get confusing if we over think it! I finally got access to liquid nitrogen! So some testing is definitely in order. If one of my customers can skin out an elk or bear without a touch up using cryo, vs not, then I’m all for it!
 
I think we are on the same page, Larrin. I am interested in making a knife that performs beyond my customer's expectations. I test my knives. I was an engineer and a machinist for 15 years and enjoy creating test conditions that are repeatable and eliminate bias. I know that data is collected from real world tests but they are composed of Charpy tests, Rockwell, Metallography and such. Like Hoss, made comment to... my customers gauge by deer skinned, fuzz sticks made and tomatoes sliced. How many extra potato fries can a 7% reduction of RA net me before I have to do a couple swipes on the ceramic rod?... lol... just messin' I do appreciate your work and generosity.
 
rodriguez7 said:
So for someone like me, who’s just learning, and building knives for hunters, and heavy duty outdoor applications, some RA is good? If I’m looking for ultimate toughness and edge stability, I’ve heard some RA is good, some people like Nathan Carothers says you want as little RA as possible, and we’ve seen his tests on edge stability! He’s a big believer in cryo. This can get confusing if we over think it! I finally got access to liquid nitrogen! So some testing is definitely in order. If one of my customers can skin out an elk or bear without a touch up using cryo, vs not, then I’m all for it!
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I haven’t read Nathan’s tests on edge stability. I know that in Roman’s testing that retained austenite lowered edge stability due to the localized soft spots.
 
Larrin said:
I haven’t read Nathan’s tests on edge stability. I know that in Roman’s testing that retained austenite lowered edge stability due to the localized soft spots.
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That’s what I was gathering from Nathan! Makes sense. My tests won’t be as intricate as yours, but when I get my equipment, I plan on doing plenty of tests, with various steels. It’ll be years before I know anywhere near what you guys do, but I’m trying to learn! Thanks Larrin.
 
I've tried to keep up with this interesting topic and forgive if I've missed it during such BUT........

How long does a cyro or sub zero treament need to be? I've read 10 minutes, 1 hour and 24 hour?

I've actually read one knife makers web page articles that stated cyro was definitely needed but ALSO BETWEEN TEMPERS?
 
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