Austempering. How is it done? Who's done it? who likes it?

[Smithoncorner]

I read a short piece on a manufacturing website about how they use austempering on most of their parts. i was just curious who here has done this type of HT, the process they used, and if they liked the results.

[Bo T]

I've read a little on this subject also and am interested in the comments.

From what I've read you take your blade (simple steel 1084, 1095) to austenizing and soak. Do an interrupted quench at 600 - 500 F. Immediately transfer to the oven at 450 F. Hold for 3 or more hours. This results in lower Bainite which is a little softer than Martensite tempered at the same temperature but a lot tougher. If I've misconstrued something let me know. Also, I'm not sure about more complex alloys.

[me2]

Certain steels are better for this than others. Its difficult to get full bainite in the simpler alloys because they will form a little pearlite very quickly. They form the same challenges during regular quenching too, but the elevated temperature of the austempering bath makes it worse by slowing cooling. Low alloy steels such as 5160, 52100, O1, O7, L6 (low alloy tool steel), 8670M, etc. will allow better results during austempering. Bainite is tougher than martensite of the same hardness as long as they're both above about 45 to 48 HRc. Below that, tempered martensite is tougher. However, after a great deal of research on bainite (downloaded The Bainite Book, 500+ pages), I decided it's toughness advantages were not worth the extra effort for me. Austempering is mostly done with salt baths, and the hold times are measured in hours, so it gets expensive. It's only real use to me would be in a large chopper or machete, and proper steel with good old tempered martensite works fine. Higher alloy steels can have hold times measured in days if you want lower bainite, so those are out for me. I talked with Gallowglas on here and he makes austempered 5160 swords and likes it a lot. I am mainly interested in knives, so the advantages start to diminish. For knives, I'd rather have the extra strength (hardness) than the extra toughness. And if toughness is the primary goal, I just choose an appropriate steel, knowing that no matter what, I'm giving up some edge holding, whether I get it austempered or not. If 5160 isn't tough enough, I can go even lower in carbon content and hardness, without having to worry about elaborate and expensive heat treatment. There are several smiths/makers that use it, but I haven't been able to justify it in a knife for my own use.

[mete]

You need a TTT diagram for the steel in question.The steel is quenched to just above the Ms and held till transformation is complete. It was a fad for knife makers who didn't know metallurgy. Some steels start to transform to bainite in 20 hours !! Not suitable.Even the shorter time transformations take a fairly log time and the end results [toughness vs hardness ] are hardly worth the effort.

[Smithoncorner]

Yeah, i sorta figured it had more of an industrial application than a personal one.

This is a bit off topic, but while i was researching different HT techniques i stumbled across something called a "super quench" it had a bunch of bizarre ingredients like laundry detergent, salt, and some other things. I guess it was mostly used to harden mild steels. However it wasn't clear as to how it worked, or whether tempering after the quench was part of the process. Anybody wanna take a poke at the "hows" and "whys?

[Bo T]

Again, from the reading that I have done. The detergent acts as a surfactant allowing the salt solution to maintain contact with the hot blade longer than pure water. The salt raises the boiling point of the water. Together they allow a very rapid heat transfer from the steel - a very fast quench. The chemical it replaced, a solution of lye was extremely caustic (dangerous) causing it to be regulated essentially out of existance. I don't recall why the low carbon steel was able to gain a relatively high Rhc but it was not due to martensite formation.

[me2]

Actually it was due to martensite formation. Low carbon martensites are surprisingly hard. However, hardenability is a problem and they need either alloying elements to reduce the required quenching speed, or they need an extremely fast quench, faster than ordinary brine. The toughness of low carbon martensite is quite high, and one might be able to get away with skipping the tempering step, maybe. I wouldn't recommend it though. An interesting thing to try would be to use something like 8620 steel for something like tomahawks/hatchets. I'd need to check my references, but a maximum hardness in the mid to upper 40's HRc is possible. A fast oil quench should work for that steel. Superquench is interesting stuff and allows one to get some use out of A-36 type steels, if one has an experimental streak and understands it's still no match for regular cutlery grade steel.

[Bo T]

Actually it was due to martensite formation. Low carbon martensites are surprisingly hard. However, hardenability is a problem and they need either alloying elements to reduce the required quenching speed, or they need an extremely fast quench, faster than ordinary brine. The toughness of low carbon martensite is quite high, and one might be able to get away with skipping the tempering step, maybe. I wouldn't recommend it though. An interesting thing to try would be to use something like 8620 steel for something like tomahawks/hatchets. I'd need to check my references, but a maximum hardness in the mid to upper 40's HRc is possible. A fast oil quench should work for that steel. Superquench is interesting stuff and allows one to get some use out of A-36 type steels, if one has an experimental streak and understands it's still no match for regular cutlery grade steel.

Now I am confused. This is what I've been reading, "Only steels in which the carbon content exceeds 0.3wt% are heat treatable to improve mechanical properties by formation of martensitic structures. The lower carbon steels can be strengthened by refining ferrite grain from heat treatment...". Does low carbon martensite have a different name?

[me2]

Where did you get that quote? Maybe they mean it's impractical for low carbon martensites? My main reference, Smith's Structure and Properties of Engineering Alloys has a graph of martensite hardness as a function of carbon content. The maximum hardness for something like 1020 is around 48 HRc.

[Bo T]

Journal of Metals, Materials and Minerals, Vol.21 No.1 pp.67-74, 2011
Improving Quench Hardening of Low Carbon Steel
Thanaporn KORAD*, Mana POLBOON, Niphon CHUMCHERY and John PEARCE
National Metal and Materials Technology Center, Klong Luang,
Pathumthani 12120 Thailand
Abstract
The carbon content in steel determines whether it can be directly hardened. If the carbon content is low (less than 0.25wt%) then an alternate means exists to increase the carbon content of the surface. In this study, the mixed quenchant consisting of brine and surfactants known as Superquench was applied in the quench-hardening process on AISI 1015 low carbon steel. The quench results were compared with quenching in heavy brine solution, water and oil which are recognized as the basic quenchants and cannot cause bulk-hardening low-carbon steels. The hardness tests on different points along the radius of cut round bar specimens were performed, and the results exhibited a greater hardness compared to brine quench. The hardness obtained from water quench was below 20 HRC while quenching in heavy brine solution and Superquench gave a hardness of above 40 HRC.

5. Eghbali, B. (2010). Study on the ferrite grain refinement during intercritical deformation of a microalloyed steel. Mat. Sci. Eng. A-Struct. 527(15): 3407-3410.

The paper cited Eghbali, B. as the source for the statement. Not the first time different conclusions have been reached as to the causal effect. I believe that I have read the same or similar statement in at least one other paper. I am hoping I didn't misread the paper(s).

[me2]

I don't think we're saying anything different. All that above pretty much agrees with what I've read. I'll elaborate more later.

[Smithoncorner]

This is great information guys. Thanks for posting

[zaph1]

[humor]Bo, you're almost blasphemous by merely repeating that information. That might lead someone yet to be indoctrinated in the ways of smithing that you could make a usable knife out of any scrap steel![/humor]

Is this saying that with super-quenching you can get a spring temper out of 1018?

[me2]

At the very maximum you can get near mid 40s. But that is before tempering, and steelslike this will soften very quickly. This is also in lab prepped samples, not production pieces. There is a maker here who makes tomahawks out of mild steel and takes them reportedly to 45 to 48 with superquench.

[Bo T]

[humor]Bo, you're almost blasphemous by merely repeating that information. That might lead someone yet to be indoctrinated in the ways of smithing that you could make a usable knife out of any scrap steel![/humor]

Is this saying that with super-quenching you can get a spring temper out of 1018?

I don't have a clue. But those old lawn mower blades might make a useable crossbow.

[Smithoncorner]

How can you test the hardness of a steel? How can you be sure it actually hardened?

[me2]

The sure way is to use a hardness testing machine. Lacking that, a file before and after will tell there is a difference, but not much beyond that. Flexing the blade might tell you something, as a hardened blade will flex further than an annealed one and return to true. I can easily bend a piece of annealed 1095 by hand at 3/32" thick. Doing so after it's hardened is a different animal.

[zaph1]

A Grizzly T10277 6 Piece Hardness Tester File Set sells for $39.99 plus shipping. It is a set of 6 files of different hardnesses, 40, 45, 50, 55, 60, 65. It will tell you the hardness to within 5 points. Not gonna tell you if it's 57 or 58, but it will tell you it's between 55 and 60.

[paul'ie]

Hi,
I recently bought a camp knife that was austempered and I love it, its 52100 it stays sharp much longer than my other knives, I love to bushcraft so alto of use. Its very light weight knife, and scary sharp, just a suggestion, check out John Bradley, you can find him on the net, he's a guru of austempering , a very knowledgeable man, you can also call him, I believe you can find his number there to i enjoy talking to him about his work. The knife is excellent and his work is like art.
Paul'ie

[Bo T]

Hi,
I recently bought a camp knife that was austempered and I love it, its 52100 it stays sharp much longer than my other knives, I love to bushcraft so alto of use. Its very light weight knife, and scary sharp, just a suggestion, check out John Bradley, you can find him on the net, he's a guru of austempering , a very knowledgeable man, you can also call him, I believe you can find his number there to i enjoy talking to him about his work. The knife is excellent and his work is like art.
Paul'ie

Most knives made with a good profile and geometry out of 52100 should give similar results. From my reading austempering offers advantage for the knifemaker in that there is no stress from the conversion to martensite. Therefore, less chance of warping or cracking the blade. I don't know what the disadvantages are for 52100. If you could get a knife from a maker that martempers 52100 (Ed Fowler comes to mind-there are others) you could compare the 2 side by side.