L6 Bainite formation procedure? Is this chart what I think it is?

B

BoltHead

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Oct 11, 2022
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I thought to ask this from here, even though it's not quite knifes I am making, but you people seem to have vast metallurgical knowledge and experience, and I was hoping you could shed some light on this topic.

I have been jumping between S7 and L6 steels for making punches and various other parts that do not contain cutting edges. The work would primarily be cold work, but high shock and at least good abrasion resistance is warranted. On Charpy V notch test, according to matweb, L6 ranks at 5J, while S7 goes for 16J at the same hardness (57), but for unnotched samples, they both rank around 300J. The CVN value is the only thing that troubles me a little here.

I have been turning towards L6 due to availability. For me, it is readily available cheap, but for S7 I would have to order overseas, and pay more shipping than the value of the product, but in case L6 turns out not suitable, I have to pull that off.

I was thinking if I could improve the toughness with isothermal quenching. I have done some tests and found it a potentially good solution, it seems to reduce warping and produce more tougher parts. I have used 50:50 KOH:NaOH as quenchant, it melts at around 160-180C and remains good to at least 350C, haven't tried further, but sources cite up to 500. A salt mix of ZnCl2, NaCl and KCl in ratio of 68.6 + 7.5 + 23.9 produces an eutectic with mp of 200C and should be good up to 850C, but as I happen to have a lot of caustics, I tried it first.

When looking at the attached chart, am I on the right track to see that the "B" section means bainitic region, and in order to obtain bainitic structure with L6, I would have to use at least 250C bath to quench, and soak the parts minimum of 10 minutes and up to 100 minutes to complete the major lower bainitic formation? Are the benefits for impact strength potentially worth the procedure, or is the difference so small I would instead benefit from simple air blast quench and ordinary tempering cycle? I am looking to obtain above 50HRC.

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Hello, are you based in Europe? If yes, the L6 you have access to is probably 1.2714 (0.55% carbon), tougher than the 0.7% one people are used to, so it’s toughness is probably the same as S7 with a good heat treatment. Bainite in that case is overkill, but I would like to hear your results doing so!
According to Larrins tests, Bainitic treatment is not the panacea people think it is, but worth trying!
 
Hi! Thanks for your answer! Yes, the exact composition of the alloy from my supplier is attached. It has slightly lower carbon content, by a point or two. They sell that stuff here as "55NiCrMoV7" or 1.2714. They also sell pre-hardened version of this material, as L6QT, with 1350MPa strength and 40HRC, but for this material I'd like to see a bit more performance.

The whole reason I was looking into bainite is basically because I already have pretty much all the stuff, and along with the same trouble of tempering it separately, it is convenient to just dunk it in and let it soak for good, and, while I haven't performed exact tests for this subject, different texts cite that it very much reduces warping and other issues, so it should essentially bring several pros with only con that you gotta get the austempering bath materials and get it heated up. As the temps are so low, even conventional household oven can be used for the purpose, and could even suit greatly because there will be no hot spots, unlike with direct heating, such as induction or gas. Unagitated fluids tend to form very large temperature gradients, and while the top would be merely molten, the bottom could be way higher than 250 and ruin the part. I get it that for knifes the hardness tends to go to low, low 50's being good for bigger choppers, but way too soft for anything you could call a knife. And yeah, bainite is not magic, it just may bring that extra few % of toughness that will be very beneficial for many mechanical applications.




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Quenching in hotter than usual temperatures (marquenching????) is a common practice among my Polish makers (they quench in 160C-180C insulated oil baths) and if I remember correctly the PHD metallurgist Fredrik Haakonsen quenches his Vanadis 4 extra in 500C molten salt baths. One of the advantages is the warpings are easier to fix and they probably get some extra toughness in their blades doing this. People usually are more worried about hrc numbers, but these guys also care about some extra toughness.

Bainite is just quenching at a higher temperature (250-300C) for some (or a lot) more time. If you have all the setup, just go for it. If I could choose, I would have all my big knives heat treated to bainite, because I’m a toughness junkie. But I tend to use super tough steels where bainite will not matter that much.

For your purposes (high abrasion steel), maybe Hardox 500 or something like that?
 
Issue with abrasion resistant steels is they are generally not very machinable. The parts will go through intricate machining program before heat treatment, and the impact resistance is of critical importance, abrasion resistance is only to prolong their expected life, but as the parts are likely gonna be low duty cycle, this is secondary to toughness. I have compared multiple materials, including but not limited to 4140, 4340, S7, H11, H13, L2, O1, A-series, AerMet 100, P6-P20, 4820/9310 with case hardening, and various others, and considering suitability, availability and cost, have deduced down to S7 and L6, and now to the latter due to availability.


This is some interesting article, where austempering is compared directly with CVN and I notch method, and it shows quite remarkable improvements over impact resistance, and even higher tensile strength. Conclusion quote with the interesting numbers:
The tensile strengths of QT and AT were 1723.1 MPa and 1824.4 MPa, which were
73.8% and 84% higher than untreated AR (991.5 MPa), respectively. In addition,
the bainite AT exhibited a 5.9% higher tensile strength than the martensite QT. The
hardness of QT and AT increased by 84.5% and 102.6%, respectively, compared to
that of AR. In addition, the hardness of AT increased by 9.8% compared to that of
martensite QT.
(4) The maximum impact energy of the KS standard V-type notch specimen increased
in the order of AR < QT < AT, the AT specimen was 85.5% larger than AR, and QT
was 27.6% larger than AR. In particular, the bainite AT made of long and wide laths
had a 45.3% higher impact energy than the martensite QT made of short and narrow
laths. The AT with the V-type notch specimen formed by the bainite structure had the
largest impact energy among the three types of specimen.
(5) The maximum impact energy of the sharp I-type notch specimen increased in the
order AR < QT < AT. The bainite AT was 242.8% larger than the AR, and the martensite
QT was 211.6% larger than the AR. Therefore, bainite AT with a sharp I-type notch
had the highest impact energy of approximately 10% higher than QT. The maximum
impact toughness of the I-type specimen was reduced by approximately 39.6% compared to that of the standard specimen. The impact of the notch shape on the I-type
impact specimens of QT and AT reduced the impact toughness by 20.2% and 39.6%,
respectively, compared to the standard V-type notch specimen
Click to expand...
 
You’re much knowledgeable about steel that I thought! You should ask this question directly to Fredrik Haakonsen, he is a nice guy, you can find him on Facebook.

Did you tested Aermet 100 steel? I can only find it in rounds, unfortunately. I want to have a knife made out of it, but being round it’s just more difficult to work with. These modern steels need much precise forge procedures/temperatures. It’s not for the average guy.
 
Unfortunately I haven't been able to test all the mentioned alloys, only collected all the available and comparable data. Some of them the difference is very marginal and more of an opinion what to use, while some are very different. I haven't seen AerMet stock for sale in such small amounts or prices I'd buy it only to test it, but looking for it, sure.

Heat treatment with special alloys is pretty much impossible without good PID furnace and other accessory, and even then it gets more difficult the bigger the parts are. When you add carburization to that, you'll have another layer of error.
 
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