Background:
I just bought a lovely little k390 puukko from Todor Hristov, and while asking him about his heat treat and perusing the datasheet, something struck me. You never see high alloy bainite, presumably because it takes so long to grow, but there's no reason you COULDN'T force the transition.
I'm a mechanical engineer, so while I'm not a metallurgist or a materials scientist, I am reasonably comfortable with cct and ttt phase diagrams.
[It would surprise me if anyone on this board needed the background, but in case anyone reading this is wondering why I would wonder this, you can read all about bainite and mixed phase bainite here
https://knifesteelnerds.com/2018/07/09/bainite-vs-martensite/
and why it's rarely used in high alloy steels (it's often that it just takes too long, but both carbon and alloys that improve martensite (unlike Boron) can also inhibit bainite growth).]
I'm not sure how k390 compares to other high alloy steels in this regard, but If you look at the K390 datasheet, there's a CCT diagram on page 12. It looks to me like you can continuously cool according to selected cooling rate to attain a wide and controllable mix of martensite and bainite, with a consistent 10-12% RA, independent of the bainite/martensite ratio.
https://www.acerosbohler.com/media/productdb/downloads/K390DE.pdf
K390 seems like an excellent candidate for this because of both its ability to get very tough for its carbide fraction, even as full martensite, as well as the fact that no matter what you do to it, it will precipitate enough very hard carbides that it will exhibit ridiculous wear characteristics at any hardness. You can read about a really interesting (although not useful for cutlery, since we know we need high hardness to get a good edge) approach to K390 here
https://iopscience.iop.org/article/10.1088/1757-899X/118/1/012022/pdf
It looks to me like you could austenitize (in BU's datasheet's CCT plot at 1180 C, so some experimentation might be in order to investigate bainite growth from lower austenitizing temperatures) then marquench to an upper bainite growth holding temperature, here it looks like ~250C to 350C, for a very long time. The longer you can slow the cooling from 350 to 250 here, the greater the fraction of bainite I would expect. It looks like bainite starts to form at 250C when continuously cooled for 15 minutes. The effect of marquenching on bainite growth is unknown to me. You might encounter unexpected behavior by deviating from the charts, but that's why this territory is called uncharted.
I would expect things to get interesting around the 30 min to 2 hours cooling times where you see 21-51% bainite with only ~10% RA, and the balance being martensite (maybe even 4 hours? it's still HV 728, or right about HRC60 at 4 hours, and 72% bainite, but RA starts climbing to 15% and higher after this). This is why things get really interesting. You could control this quench rate/soak time to optimize the balance of bainite/martensite, then attempt to temper to convert as much of the 10-12% RA to martensite as possible.
There are a lot of unknowns (to me) here. The things that would need to be investigated include:
the effect on bainite of processes to temper martensite (Cryo?)
The effect of marquenching vs continuous cooling on the bainite/martensite fraction and grain
Whether bainitic K390 even actually offers toughness improvements over properly tempered, fine grained martensite.
So yeah, if someone that had the equipment, time, and interest (of which I have 2 out of 3, but an apartment is no place for a forge, heat treat oven, or tempering salts) to investigate high carbide, high alloy bainite, I think it would have potential. Even if it performed like a different alloy, it might just open up new uses for it, and I'm sure people (like me) would buy it just for the novelty and interest.
P.S.
It would be fascinating to see a metallurgist develop a steel like K390, but alloyed with boron to enhance bainite growth
I just bought a lovely little k390 puukko from Todor Hristov, and while asking him about his heat treat and perusing the datasheet, something struck me. You never see high alloy bainite, presumably because it takes so long to grow, but there's no reason you COULDN'T force the transition.
I'm a mechanical engineer, so while I'm not a metallurgist or a materials scientist, I am reasonably comfortable with cct and ttt phase diagrams.
[It would surprise me if anyone on this board needed the background, but in case anyone reading this is wondering why I would wonder this, you can read all about bainite and mixed phase bainite here
https://knifesteelnerds.com/2018/07/09/bainite-vs-martensite/
and why it's rarely used in high alloy steels (it's often that it just takes too long, but both carbon and alloys that improve martensite (unlike Boron) can also inhibit bainite growth).]
I'm not sure how k390 compares to other high alloy steels in this regard, but If you look at the K390 datasheet, there's a CCT diagram on page 12. It looks to me like you can continuously cool according to selected cooling rate to attain a wide and controllable mix of martensite and bainite, with a consistent 10-12% RA, independent of the bainite/martensite ratio.
https://www.acerosbohler.com/media/productdb/downloads/K390DE.pdf
K390 seems like an excellent candidate for this because of both its ability to get very tough for its carbide fraction, even as full martensite, as well as the fact that no matter what you do to it, it will precipitate enough very hard carbides that it will exhibit ridiculous wear characteristics at any hardness. You can read about a really interesting (although not useful for cutlery, since we know we need high hardness to get a good edge) approach to K390 here
https://iopscience.iop.org/article/10.1088/1757-899X/118/1/012022/pdf
It looks to me like you could austenitize (in BU's datasheet's CCT plot at 1180 C, so some experimentation might be in order to investigate bainite growth from lower austenitizing temperatures) then marquench to an upper bainite growth holding temperature, here it looks like ~250C to 350C, for a very long time. The longer you can slow the cooling from 350 to 250 here, the greater the fraction of bainite I would expect. It looks like bainite starts to form at 250C when continuously cooled for 15 minutes. The effect of marquenching on bainite growth is unknown to me. You might encounter unexpected behavior by deviating from the charts, but that's why this territory is called uncharted.
I would expect things to get interesting around the 30 min to 2 hours cooling times where you see 21-51% bainite with only ~10% RA, and the balance being martensite (maybe even 4 hours? it's still HV 728, or right about HRC60 at 4 hours, and 72% bainite, but RA starts climbing to 15% and higher after this). This is why things get really interesting. You could control this quench rate/soak time to optimize the balance of bainite/martensite, then attempt to temper to convert as much of the 10-12% RA to martensite as possible.
There are a lot of unknowns (to me) here. The things that would need to be investigated include:
the effect on bainite of processes to temper martensite (Cryo?)
The effect of marquenching vs continuous cooling on the bainite/martensite fraction and grain
Whether bainitic K390 even actually offers toughness improvements over properly tempered, fine grained martensite.
So yeah, if someone that had the equipment, time, and interest (of which I have 2 out of 3, but an apartment is no place for a forge, heat treat oven, or tempering salts) to investigate high carbide, high alloy bainite, I think it would have potential. Even if it performed like a different alloy, it might just open up new uses for it, and I'm sure people (like me) would buy it just for the novelty and interest.
P.S.
It would be fascinating to see a metallurgist develop a steel like K390, but alloyed with boron to enhance bainite growth