Austempering Advantages?

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Austempering creates bainite, the hardness will be too low for a good knife blade, although toughness is increased. Marquenching is wonderful for steel. Marquenching is quenching in oil or in salt bath typically at 300-400F. My understanding is you do not want to marquench for too long. Many heat treat shops marquench (also called martempering). Distortion is about 1/2 of conventional quenching, and toughness is greatly increased.
I have wondered for some time, about patent 4,180,420 by Gillette razor blades. This 1979 patent used "isothermal annealing" to create stainless steel razor blades with .3-.4% carbon. They claimed hardness better than conventional stainless steel razor blades with .6% or more carbon - well over Rc60. The low carbon content greatly improved corrosion resistance compared to the higher carbon content blades. They said the isothermal annealing produced "a fine carbide microstructure comprising about 200 to about 500 carbides per 100 square microns, hardening them under conditions in which the fine microstructure will be substantially retained". They give the alloying used in their steel. It looks like the Gillette razor blade material might be an excellent knife steel.
 
As far as carbide microstructure - Crucible's CPM steels such as S30V is made differently An advantage is that it is more homogenious and the carbides are finer and therefore more to the square inch. As far as bainite - I just mowed my lawn and I'll bet the mower blades are bainite - a perfect application.
 
"Austempering creates bainite, the hardness will be too low for a good knife blade, although toughness is increased." - Matt51

For me, it seems to be a highly controversial subject as to what hardness is too soft for a good knife. And what constitutes a "knife" is often the sticking point. From my point of view and in my experience most knives that exceed 5" in length need to be a bit softer to survive hard use....knives over 10" (in my experience and opinion) are too hard and subject to breakage if they are harder than about Rc59-60 in tempered Martensite.

Just for my benefit, what hardness do you consider to be too soft to make a good knife? I'm not disagreeing with you, I'm just always curious when I hear this statement as to what hardness level we are talking about.

Thanks Matt!

Brian
 
Brian,

When I looked in the ASM handbook under austempering, it looks like most steels in bainite are in the Rc45-55 range. It may be possible to get to Rc58, with the right steel and right process. Applied Process is a company that specializes in austempering. appliedprocess.com has a lot of data about austempering on their web site, look under engineering data. They also have a large section talking about austempered ductile iron, which I have never seen applied to knives. I would think you would have to HIP (hot isostatic press) or forge the ductile iron bar before machining a knife blade. This is because the ductile iron bar is cast, and would have porosity. Very clean steel, makes the best knife steels, as toughness is improved with cleanliness (ATS34 and BG42 are both double vacuum melt vim-var?). You do get a lot of improvement in toughness with marquenching (although the steel is not as tough as bainite). Maybe bainite is best for lawn mower blades, I wasn't thinking about that application.
 
Let me explain about hardness tests - they were developed to measure approximate tensile strength - this they do very well. When we try to use hardness as a measure of other things we get some major problems. For example Talonite isn't great as far as hardness but in wear resistance it's exceptional.That's because Talonite has a soft matrix but lots of carbides. We always have to balance properties wear, toughness ease of sharpening cost etc. Wear resistance is related to the amount and type of carbides. The strength of the matrix has a lesser effect on wear. Toughness is related to size and distribution of carbides, grain size and the inherent toughness of the matrix. When Galloglas gets O1 bainite he's getting carbides for wear and very strong but very tough bainite matrix, a combination that's hard to beat. Give it a try.
 
"Toughness is related to size and distribution of carbides, grain size and the inherent toughness of the matrix. When Galloglas gets O1 bainite he's getting carbides for wear and very strong but very tough bainite matrix, a combination that's hard to beat. Give it a try."

Thanks Mete. Through experimentation with a couple of different steels at different quench temps. and hold times coupled with destructive testing I have come to a lot of interesting conclusions but I lack the technical knowledge to explain why it works. Thanks for the enlightenment!:)

For me, edge holding ability has always had little to do with hardness. Wootz is legendary for taking and keeping a good edge and yet this steel, both the modern made ones and the original antique steel, is really very soft compared to modern steels and the hardnesses in use by most modern knifemakers. In my experimentation I have gotten excellent edge holding ability (not quite as good as tempered Martensite)in Bainite O1 and 5160 blades that were in the Rc 54-59 hardness range. This, coupled with the strength, toughness and ability to endure *massive* amounts of shock and impact without damage has sold me on Bainite for large knife blades. There is little reason to austemper small blades in my opinion as Bainite really is about shock and impact resistance and small blades just are not a good choice for these kinds of knife applications.

Again though, we are talking about blades in excess of 10" and in applications where edge holding is of secondary importance to shock and impact resistance. The O1 Bainite I had tested for hardness when quenched and held at 500f was in the 57-59 Rc range...and extraodinarily tough compared to O1 I marquenched and tempered to about the same hardness. The edge holding capacities of both blades was good to excellent and I attribute this to the complexity of O1's alloying elements. Bainite in O1 is a very scary combination of edge holding, massive toughness to the point of enduring unbelievable abuse, and resistance to permanent sets and bends.

Those who like O1 and make camp knives, Bowies, or swords with this alloy would be well advised to try it in Bainite a couple of times. It is an awesome performance steel when austempered.

brian
 
Originally posted by Galloglas
The O1 Bainite I had tested for hardness when quenched and held at 500f was in the 57-59 Rc range...and extraodinarily tough compared to O1 I marquenched and tempered to about the same hardness. <snip>

Bainite in O1 is a very scary combination of edge holding, massive toughness to the point of enduring unbelievable abuse, and resistance to permanent sets and bends.
brian
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Excellent thread! I didn't really appreciate what Bainite was or how it could be exploited.

... and Brian, excellent piece of heat treating on O1 (Rc57-59 with bainite), and great insights into testing & toughness.

Brian, ever tried say W1, W2, L6 with this process? L6 is supposed to be a tough mother (nickel?), so would be interesting to see whether L6's makeup added to what you could achieve with O1.

Just curious: I realize A2 is air hardening, but can be oil quenched also... and it has a significantly higher alloy content than the simpler carbon steels. Any obvious problems, or benefits for that matter, you guys can see in trying these bainite heat treating tricks with A2? You might get more vanadium (grain refinement) and molybdenum, but then you have to deal with 4-5% chromium in the process also.
 
L6 can be very successfully treated to get bainite thats why the swordmakers like it Tough with martensite ,tougher with bainite. A2 is not a good choice for bainite because after 20 hours the transformation to bainite is still incomplete.
 
Originally posted by rdangerer


Brian, ever tried say W1, W2, L6 with this process? L6 is supposed to be a tough mother (nickel?), so would be interesting to see whether L6's makeup added to what you could achieve with O1.

Just curious: I realize A2 is air hardening, but can be oil quenched also... and it has a significantly higher alloy content than the simpler carbon steels. Any obvious problems, or benefits for that matter, you guys can see in trying these bainite heat treating tricks with A2? You might get more vanadium (grain refinement) and molybdenum, but then you have to deal with 4-5% chromium in the process also.
Click to expand...

I have absolutely no experience with A2 or any other air hardening steels, so it would be difficult for me to advise you. But it would seem to me that the "nose" of the curve in an air hardening steel would be about 3 miles to the right side...so you would have perhaps a matter of minutes to get behind it. If A2 can be oil quenched it should work. I have no idea of how long it would take to get complete conversion to Bainite but someone with access to the TTT curves should maybe pipe in here and speculate with us! :D

I have austempered a bit of L6 and it works very well. In fact it may be the best of all of the steel I have tried. But it is virtually unavailable in the thicknesses and sizes that I need as I am stricktly a stock removal kind of guy. This is because my shop is 6'X 6' and my neighbors would not appreciate me banging on steel in the garage. L6 pretty much comes as round bar....you can get some L6 bar stock from Admiral but the sizes are very limited.

The simple 10XX steel do Bainite very well and you can make some excellent springs and stuff from 1084 and 1095. My test and experiment data tells me that 1095, austenitized at 1550f, quench into 500f salt and held for 90 minutes yeilds Rc 56-58. Since the W series steels are water hardening and don't have any alloying elements to push the nose to the right they should yeild similar results. If in doubt, hold it longer at temp. as it won't hurt anything if it is fully converted at 90 minutes and you hold it for 2 or3 hours.

Thin stock works well but with blades thicker than about 1/8" it is hard to get behind the nose. Remember, you have less than a second to get from the critical temp. to under 900f for hardening 10XX steels. It is difficult to get enough heat out of the blade fast enough to beat the clock when quenching into 500f-600f salt if the blade is large and thick. As I said earlier, there are some high tech, high temperature oils that may work well with W1, W2, and the 10XX stuff as the heat extraction characteristics are suppossed to be better than the hot salt. But again, I have no experience with this and I am *scared* to death about quenching into any oil that is held at 500f...I don't care what the manufacturer says. The idea of a whole tray of 500 degree oil flashing over in my basement gives me the willies.:eek:

Experiment heavily is my suggestion. I am curious whether or not anyone has austempered an air hardening steel or if it is even possible. This is something I never considered doing before.

Brian
 
Galloglas,

Congratulations! Feels good to make such an accompliment eh? I commend your efforts and wish you the best with bainitic blades. They fit their niche better than any other.

Jason
 
Great info and thanks for the heads up on the books Fitzo. I just scored Bains book, autographed no less! ; )

Seth Howard
 
Cool, Seth. You're welcome. It is a very techical read, but so chock full of in depth science that it will create a much better understanding of what's going on with the heat treats we do. Hope you enjoy it.
 
I knew someone was doing this. Are any of you still around? I had no idea that bainite could be hardened as high as it can. I just read about it a couple of months ago. Coming from a general industrial background in metallurgy education (that means we had 2 semesters and hit the high points), many specifics were not covered because they dont have widespread application in industry. Lets just say I dont know of anyone using HRc 60 bainite in industry. Galloglas (sp) information is exactly what I was looking for and hoping to duplicate on my own using basically the same steels he uses (1095 and O1, a personal favorite, for the 3 knives I've made). Looks like I've got 2 schools to try. High hardness (>63 HRc) for edge holding in small knives ~8" overall, and high durablility for larger ones. Still gotta try some HRc 65 M2. My buddy sounded disappointed in the edge holding of his ~57-58 O1 hunting blade, though I was tickled ****less for a first try.
 
Been hearing it since grade school. Actually I stumbled on this through a random search for something else. I've forgotten what it was, since HRc 58 blades 1/8" thick capable of 180 degree (horseshoe type) bends is so much more interesting. Thats not from here, but another source.
 
I looked at the date on this and said what the?? but kind of glad it was brought up again, I'll post it on the sticky:)
 
MAN!!... Just when I was starting to feel smart some guy like Mete or Galloglas has to chime in and effectively ruin a for the most part not too bad day! Thanks alot!! Was going to grind a blade today but I think instead I'll just go bury ny head in the sand.:o
Seriously tho it's really great to have people with your guys' grasp on the subject.:thumbup:
 
Say Heah Guys,
Funny I found this tread, but I just bought a knife made from 52100 austempered blade, I just wanted to understand my purchase a little better. All I know is that John Bradley made it and how well it cuts, man this knife is scary sharp and hold the edge like none other knife I own. Its a 4 1/2" camp knife I thought the 1/8" thickness would be too weak, but its not. Even though you guys are way over my head, all I know is that by luck I got a great knife.But Thanx so much for your explanations. Regardless the proof is on the pudding.
Thanx Guys,
Paul'ie
 
paul'ie,
This is a very old thread that has been resurrected once before. I am going to lock it to let it slip away.

Posting a new thread in the General Knife Discussion about your new knife will be a better place to discuss it. Shop Talk is about making knives.

Just a few words from a moderator:
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