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Heat Treating and Metallurgy Discussion of heat treatment and metallurgy in knife making.

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  #1  
Old 08-29-2016, 09:55 PM
jimmontg jimmontg is offline
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Some Heat Treat Definitions

Hi everybody I was going through some old notes from my machine shop heat treat/TIG welder days. I also did some extra research for some actual science versus anecdotal stories. Thought I put a few things out there.

tribology, tribological
"Tribology is a branch of mechanical engineering and materials science. Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. In knifemaking, tribological studies play a role in determining steel wear characteristics and this is the only scientific, accepted method to determine the relative wear resistance of steel. Cutting tests do not; they are too variable, and knife blades can not be consistently created to any high degree of accuracy. In tribological testing, wear surfaces, indentation, loss of mass, and friction are all considered and calculated. This is ASTM and AISI approved testing of the wear resistance of steels and it is the only recognized standard."

O1: This high alloy oil-hardening tool steel is a standard in the industry for a reason. It's a great hyper-eutectoid tool steel with about .9% carbon and the version I use has high tungsten and vanadium with a bit of chromium, though not enough to be stainless steel. I use it when clients want a great performing black colored blade, because the finish and bluing is excellent on this steel. On one website about steels used in woodworker's tools, the writer claims that because O1 has a higher martensitic conversion than other steels, cryogenic treatment is not effective. This is flat-out wrong. While O1 does perform well with conventional heat treating, cryogenic treatment vastly improves this performance. How does it benefit from cryogenic treatment? O-1 can have up to 8.5% retained austenite when quenched to room temperature (20C). While this does not seem to be a lot, it is significant, and proves that at the very least, O1 should be quenched to sub-zero temps and held there to reduce the amount of retained austenite. So much for the woodworker's assessment of O1. Now here's the really important result and proven by highly specific and controlled technical scientific studies: in treating O1 to shallow cryogenic treatment (SCryo), the wear resistance was improved 221%. In treating O1 to deep cryogenic treatments (DCryo), the wear resistance was improved 418%. Simply put, either of these treatments dramatically and substantially improves the wear resistance while making the blade tougher, and the finish better overall! Why not do this?

I came across this. I never knew the name of the science of cutting basically. I found a wealth of information on carbide formation. I also now know the scientific reason I alway shallow cryo my O1, ie pack in dry ice until it evaporates with my homemade ice chest it takes almost 3 days. It definitely makes a difference as measured scientifically. It held an edge longer, but I couldn't say about toughness as I tell my customers it isn't a pry bar. Leastways I haven't had one come back because it broke. Liquid nitrogen Deep Cryo improves it even more and that's just O1. D2 is through the roof with both types especially with LN cryo. I have to get after my heat treating company, guy told me my steel only needed 2 hours, but it needs at least 6 in LN for 154cm and same for 440C, actually all high chrome knife steel.

I got most of this stuff from Jay Fisher's Knifemaker website.

Just some food for thought. Here's a few references. There's much more than this though.

Cryogenic Quenching of Steel Revisited, Zbigniew Zureki, 2005

Cryogenic Treatment and its Effect on Tool Steel, Yugandhar, Krishnan, Rao, Kalidas,

Effect of Deep Cryogenic Treatment on the Carbide Precipitation and Tribological Behavior of D2 Steel, Das, Dutta, Topo, and Ray, 2007

Comparison of Wear Properties of Tool Steels AISI D2 and O1 With the Same Hardness, Bourithis, Papadimitriou, Sideris, Tribology International 39 (2006)
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  #2  
Old 08-30-2016, 11:42 AM
samuraistuart samuraistuart is offline
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If you austentize O1 at the higher than recommended temp of 1475, as in 1525+, yes RA becomes an issue. However, if you harden at 1475F, recommended temp, then RA is negligible, and the sub zero dry ice isn't going to do much of anything.

Try it yourself. Take coupons, and harden them at 1425, 1450, 1475, 1500, 1525, 1550. You will notice RC will drop about 1 point every 25 degrees (give or take) each side of 1475F. Once you go above 1475, and the hardness drops...then you can take that coupon and cryo or sub zero, and watch the RC bump back up to max. That is the RA being converted back over to martensite. (untempered martensite technically).

Again, the idea with knife steel is to put just enough carbon in solution to attain max RC, leaving the rest of the excess carbon tied up in the carbides...for added wear resistance. Most don't realize...the steel received annealed actually has a higher percentage of carbides than hardened.

I've seen all the cryo studies before, usually funded and put out by.......cryo companies that sell their services.

If you're seeing vast improvements on O1 by employing sub zero/cryo....go back and rethink the HT you are doing.

Wear resistance does not equal edge stability/edge retention. It is but one factor.

D2 is a different story. Same with A2. Your austenitizing temperature is so high that RA is most certainly an issue. Hence the recommended sub zero with these steels....and any and all higher alloy steels. Even AEBL loves to be at least sub zero.

Now full on cryo, LN temps and soak time, it gets a little different. It isn't solely about RA reduction. The LN temps will permit the formation of so called epsilon, or eta, carbides, upon tempering, that would not otherwise be formed in a standard quench, or even sub zero quench. There is very little info about the formation of these super small carbides that, frankly, is WAY more interesting than the straightforward concept of RA reduction.

If O1 were to be improved by 400% by a simple sub zero soak, I promise you, every single knife maker would be doing it. If it offered 10% increase in performance, I promise you every single knife maker using O1 would do it. It simply does not.
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Old 08-30-2016, 01:25 PM
jimmontg jimmontg is offline
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Talking

I know about the argument against cryo on O1. This is just something that I noticed about it. The dry ice cryo didn't bump the hardness up by any significant degree. 0.1 to 0.2 RC isn't significant as there can be that much error in the conditions of testing it. But I did notice a significant change in how it held an edge. I treat all my O1 to RC 60. They will hold that edge all day, but my O1 filet knife with cryo cutting through bone outlasted even my expectations. It was 1/8" thick and I asked our fishing guide to use it on a charter my son and friends took me on in Florida, It never got dull through two days and over 80 fish and to me that's the best I've ever seen except a D2 filet knife I made my son that hasn't gone through 80 fish yet, but still shaves. The fishing guide bought the blued O1 knife by the way. I explained it will most definitely rust, but the sheath is soaked in linseed oil and melted beeswax, I told him to wipe it down every day with an oiled rag and to still not store in the sheath unless he sprayed oil in it from time to time. Will be going down there this winter, I'll see how the knife is holding up. Had cocobolo handle over a hidden tang. Didn't want an exposed full tang.

Nothing scientific, just anecdotal and those references are not just from people that sell cryo services. The .5% tungsten and chrome would react to cryo as will the .2% Vanadium and vanadium makes harder carbides than tungsten does. It was Hinderliter HT in OKC who told me to do that to O1 and 440C. They were very helpful to me back then and we only ever sent them long paper cutting blades of D2 because they wouldn't fit in our kiln.

Also, this is something I know about first hand. I was told more than once not to treat materials beyond "the book" on account that my company wanted the dies and such to wear out faster. It is a kind of standard in the field. Don't use superior materials or HT because of volume sales, it why some cutters of the same size cost two to three times more as they last as long or longer than that. Changinging out cutters in a cnc takes time.

Case in point I used to have to surface weld a stellite alloy on the end of some steel rods. They would last about a month before the alloy fell off. I drilled some small holes on the ends and then welded the stuff on. I got chewed out for doing that because the parts didn't come back until the Stellite wore down, about 3 months and we had to do them like that all the time afterwards. That was about $1000 a month in business, how much profit I don't know, but they lucked out and received other jobs from the same company because they were impressed because of our "higher standard process" so it worked out.

That may partly be why everybody doesn't do it.
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Old 08-30-2016, 01:48 PM
jimmontg jimmontg is offline
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I need a new Heat Treater.

By the way, does anyone know of a heat treater in the Asheville, NC area? Mine just decided to break off from Premium Knives and go it alone, so they won't be available for at least two months. I can use Texas Knifemaker Supply for air quench steels, but I need a kiln and am not allowed one in my apartment for O1 or 15n20. Both ht at 1475-80 and oil quench. No I don't need dry ice as my last HT charged too much for it and wouldn't do it right anyway, which is at least two days or longer in the dry ice. He only would use the LN until he went home and he should have kept the D2 and 154CM in overnight, but he was relatively inexpensive and other than TKS the other big treaters are expensive.
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154cm, 440c, back, blade, blades, carbon, conversion, edge, flat, heat, heat treat, homemade, knife s, knifemaker, knifemaking, materials, o-1, play, quenched, shop, stainless, stainless steel, steel, tools, toughness


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