Talk to me about no cryo heat treat.

To chip in with my experience with one steel, not to make this too long. Steel 420HC, throwing knife. I was going for 60HRC but never achieved it without cold treatment. I was satisfied with 58HRC at the time. But to know what's going on after a bunch of throws I did some hard testing. Throwing in hardwood targets and puling them out sideways to see how the tip behaves. After some time of nice results, I decided to throw very hard. Tip bent. Wanted to straighten it while it's still stuck. After some straightening, a new bend occurred above the first bend. What happened? With force, RA turns into martensite and hardens the part. After more abuse that part broke. It was very hard to straighten. I tried to bend one more and it was hard but not nearly as straightening. Conclusion was that there is way more RA than anticipated. Ergo, I'm not selling what I say I'm selling. Tried with the freezer and dry ice. Freezer helped somewhat but dry ice made it stronger, as it should be. Now I'm hitting 60-61HRC with ease and after throwing in hard concrete, only scratches occur. Tried the difference JoeX style, trying to brake a rock with them. Iced one had minor dents on the edge while cold absent one had more dents and splinters. Yes, splinters. So small it cut effortlessly thru my finger. Noticed the blood, not the cut itself. RA has many disadvantages. To be avoided at any cost. For me now, cold treatment is as important as choosing quench temp. It's a unity in the end. Without dry ice, I have same results with 4x cooling and heating (-23°C and 100°C, hour each), it just takes longer. Any other stainless needs at least dry ice, with LN highly advisable.
My 2 cents.
 
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"any other stainless needs at least dry ice, with LN highly advisable" - I assume this applies IF your goal is the highest possible HRC ? what if your goal is NOT the highest possible HRC - what if you desire a lower HRC (for any number of reasons)
 
Did a 27" blade, 37" overall knife/sword in MagnaCut just over a year ago. Absorbed some of Larrin's info on MagnaCut. Had Peter's skip cryo. Chopped logs in half with it. Wouldn't skip cryo on a knife sized knife.
 
from the magnacut data sheet:

When tempering at 300-750°F (150-400°C), freezing treatments may be
necessary to minimize retained austenite

what I get from this, is that I think I'd use the higher tempering temperature range and forego a cold treatment for a big blade/sword. It's still a highly corrosion resistant material anyway
 
victoroni said:
"any other stainless needs at least dry ice, with LN highly advisable" - I assume this applies IF your goal is the highest possible HRC ? what if your goal is NOT the highest possible HRC - what if you desire a lower HRC (for any number of reasons)
Click to expand...
Choose a different steel. In my example if I want more toughness and not sacrifice stainlessness, 420 would be a good choice.
Rulz of knifemaking:
1. geometry
2. steel
3. HT
Keeping the steel in the optimum boundaries is always best practice.
I tried with D2 without cold treatment and discovered in some papers that 950°C would keep RA under 2%. But 60HRC with the freezer endured more abuse. Softer (57-58HRC) bent and got dents instead of scratches. Tho manageable, it was not in optimum condition. Purpose of custom making is to get things in optimum range. Otherwise, there is no difference from mass production.
 
Lorien said:
from the magnacut data sheet:

When tempering at 300-750°F (150-400°C), freezing treatments may be
necessary to minimize retained austenite

what I get from this, is that I think I'd use the higher tempering temperature range and forego a cold treatment for a big blade/sword. It's still a highly corrosion resistant material anyway
Click to expand...
While steel is tougher that way, impact toughness drops with softer matrix and more carbides. Stainlessness is questionable as Larrin made it with just enough Cr for full stainlessness and without Cr in carbides. High temper would pull Cr from the matrix and make more carbides. Something to be measured. Larrin Larrin ?
 
lots of variables involved in "optimum" condition - maximum hardness would be achieved in a "as quenched" condition but you certainly would not leave it un-tempered - so optimum condition is to be determined by the maker - just referencing Larrins work on AEB-L tempering one can see the trade-offs with different types of cryo as compared to no cryo - so if one is not concerned with achieving an RC of 63 he may just skip the cryo to achieve an RC of 60 because thats what he is trying to achieve - and thus that criteria is his "optimum" condition
 
D DevinT
"Makers that don’t do cryo tell their customers that it isn’t necessary."
HC9yd5V.jpeg

"Makers that do cryo tell their customers that it’s a necessary step in the process."
s8ronv6.jpeg
 
DeadboxHero DeadboxHero I really like the etches on those samples. Excellent depth and 3-D.

I hope you don't mind a question or two:
1) What is used to etch these samples? Is it the old Nital or other?
2) Just curiosity, but what type of metallograph? What type of lighting? My experience (1971-1972) was with WCs and I used an ancient carbon-arc lamp, inverted stage, oil lens unit whose name I forget but not the fact it worked wonderfully. I've always wished I could be around one again.

Thanks for any reply and your time!
 
fitzo said:
DeadboxHero DeadboxHero I really like the etches on those samples. Excellent depth and 3-D.
Click to expand...

Thanks you

fitzo said:
I hope you don't mind a question or two:
1) What is used to etch these samples? Is it the old Nital or other?
Click to expand...

A mixture of trinitrophenol, hydrochloric and ethanol for that one.

(Doesn't taste very good)

fitzo said:
2) Just curiosity, but what type of metallograph? What type of lighting?
Click to expand...
Halogen

fitzo said:
My experience (1971-1972) was with WCs and I used an ancient carbon-arc lamp, inverted stage, oil lens unit whose name I forget but not the fact it worked wonderfully. I've always wished I could be around one again.

Thanks for any reply and your time!
Click to expand...

Nice, I use dry objectives but they have a high NA and are designed for metallography.
 
DeadboxHero DeadboxHero Thank you so much! the etchant is interesting and now I have to read a bit about what TNPhOH does in the mix. That's funny about the taste! :D Chemists always end up knowing what everything tastes like, too, intentional or not. LOL
Thanks for the info! The geek in one never leaves, old and retired or not... :)

Edit: I KNEW that compound was familiar. And so close to TNT. Picric acid! Boom juice! LOL. Acidic as heck. Now I understand. :)
 
victoroni said:
lots of variables involved in "optimum" condition - maximum hardness would be achieved in a "as quenched" condition but you certainly would not leave it un-tempered - so optimum condition is to be determined by the maker - just referencing Larrins work on AEB-L tempering one can see the trade-offs with different types of cryo as compared to no cryo - so if one is not concerned with achieving an RC of 63 he may just skip the cryo to achieve an RC of 60 because thats what he is trying to achieve - and thus that criteria is his "optimum" condition
Click to expand...
Optimum in stainless is balance between hardness, toughness and stainlessness. It's a narrow gap of 2-3 HRC. One can go beyond with sacrificing one or two. But how one gets to that hardness is the issue. More ways to achieve set hardness. Composition at set hardness will make or brake the optimum. If cryo or even shallow cold treatment is unwanted, there are steels that can reach somewhat optimum without it. Most are not on par with todays standard for custom knifemaking. No one wants to cut lemon and see rust in two hours.
 
Thread consensus (also I agreed based on experiments) - usual knifedom usage, lower RA% is better.

Retained Austenite
Pros: Ductile, localized TRIP, ..
Cons: Ductile, localized TRIP, 'Key consequence of ductility'=lower yield point, ..

For now, let's take reasonable 5% RA (at target hrc & usage) is an acceptable 'better'. There are many routes to achieve objective for steel composition + various ht protocols + minimum quench temp. Often higher aust temp and or high alloy requires lower/colder - than zeroF - quench temp(QT) to reach Mf95 (if possible for temper below 500F). Let's say QT is solved, practical minimum temp constrained by LN2.

What+when+why+how percentage of RA leads to loss of performance to below 'better' ?

There are plenty of observational/empirical/anecdotal studies/testimonies/assertions(stochastically) are qualitatively supports the consensus/edge-wisdom, which loosely/hand-waving association/link between RA% and performance. Of course wisdom is good/useful but thus far missing thereof - OP asked/sought prospective(explicitly quantitative in this case) studies/reasons, whether outcome is within predicted window.
 
DevinT said:
Makers that don’t do cryo tell their customers that it isn’t necessary.

Makers that do cryo tell their customers that it’s a necessary step in the process.

Hoss
Click to expand...
personally I've never had a customer ask if a particular knife had been cryogenically treated, but if a maker should be asked I would hope the maker would explain in detail WHY the knife was cryo"d or not cryo"d
 
DeadboxHero said:
D DevinT
"Makers that don’t do cryo tell their customers that it isn’t necessary."
HC9yd5V.jpeg

"Makers that do cryo tell their customers that it’s a necessary step in the process."
s8ronv6.jpeg
Click to expand...
I prefer the structure in the top image, those carbides at the bottom are huge and I did not realize that deep freeze cooling led to such grain growth.
 
Cryo does not increase grain size. Excessive heat, temperatures above carbide dissolution temps causes grain growth. All that cryo does is change retained austenite into untempered martensite. There are some white papers that claim in helps to precipitate fine "eta" carbide upon tempering, and that it also promotes a better cohesion between the carbides and surrounding matrix, but the main point is getting rid of the RA.
 
Way-Barney said:
I prefer the structure in the top image, those carbides at the bottom are huge and I did not realize that deep freeze cooling led to such grain growth.
Click to expand...
Seems to be some confusion.

Cryo doesn't grow grains and carbides.

The "grains" you are describing in the cryo micrograph are large eutectic carbides not proeutectoid carbides or eutectoid carbides.

Those morphology is a dead give away.

For clarity, both of the carbide morphologies seen are present in both samples.

Understand the micros are at extreme magnification to show you the retained austenite (RA) not the full scope of the carbide bands, primary and secondary carbide concentrations which again are present in both conditions.


While carbide is a "crystal grain" when we think of "grains" we mostly think of the "iron grains" notably the prior austenite grain boundaries (PAGB) in this case.

PAGB are not visible in these micros.
(it's a separate process to reveal them)

Cryo will not affect PAGB nor the eutectic carbides.
 
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