L6 Steel Kinetics

This steel is popular for use in knives and swords like the Samurai sword “Bainite Katana”.

Custom Howard Clark L6 Katana
The Bainite Katana is made of a special purpose low-alloy steel. It is very resistant to bending, to the point of near unbreakability. These blades can be made lighter and thinner and still remain stronger than conventional steel or 1086. The blades are also springy rather than soft, they will flex more than a normal blade, but the shape will not be altered. These blades are excellent for tameshigiri as well as general sword work.

Composition; I found the composition of L6 is something like: Fe-0.7C-0.5Si-1.75Ni-0.5Mo-0.25V-0.25Cu.

I calculated the time-temperature-transformation diagram, i’m not too sure about the accuracy bainite start temperature using this program, I would like to be able to find the experimental results for this alloy.

Depending on the bainite start temperature, the kinetics are fast enough to allow isothermal transformation to bainite on a reasonable timescale, at temperature of around 300 C which would give a high strength bainite. Howard Clarke sells the swords with as a martensite/bainite sword. If the bainite start temperature is higher it would be easy to get a mixed microstructure.

I should try to make a prediction of bainite start temperature – I made a neural network model for this.

AISI L6 TTT diagram calculated with MAP program MUCG83

Even without carbide precipitation transformation results in a large volume fraction of bainite.
AISI L6 Volume fraction calculated with MAP program MUCG83

Presumably if the bainite Katana is used in marteniste/bainite condition it is produced by continuous cooling or by quenching. Quenching would be good if it can be done in at a rate which gives martensite on the outside of the blade for sharpness and hardness, and bainite in the centre to give toughness. The other possibility is that the continuous cooling gives a mixture of bainite and martensite at every location. This can have a higher strength or hardness than martensite alone because austenite will get enriched in carbon as the bainite transforms, increasing it’s contribution to the strength.

I’d really like to have a look at the microstructure of these swords to see what structure the bainite has. Also it would be interesting to measure the mechanical properties of the alloy in the same condition – I need to look for some literature on L6 I guess. It looks like they should be strengthened by carbides and by copper precipitation.

Howard Clark who makes these swords has a webpage at mvforge.com.

–edit 6 October 2007–


14 Responses

  1. AISI L6 Tool steel AKA
    # ASTM A681
    # DIN 1.2713
    # SAE J437
    # SAE J438
    # UNS T61206

  2. Interesting blog! Can you calculate the approximate martensite finish point for L6?

  3. Matt>
    I’m not really sure there is a martensite finish temperature. If you look at my other post on Martensite you can calculate the volume fraction of austenite remaining as function of martensite start temperature, and the ambient temperature. If you want to approximate martensite finish to 90 % martensite you could calculate that way.

    I think this would be stretching the calculation above a bit far since I havent been able to find the experimental time-temperature-transformation diagram or values for bainite start or martensite start.

    Thanks for your comment. Mathew

  4. Article on martensite kinetics can be found here: https://bainite.wordpress.com/2007/07/18/martensite-kinetics/

  5. According to a knifemaking book I have here, in order to make Bainite, the steel has to be quenched to 550 degrees in under a second following quenching, sustained at that temperature for 2.75 hours, and then suddenly brought down to room temperature. It would probably require two different quenchants to be used.

    If you could give me some more information on how bainite is made, I might be able to get you a sample – I’ve been thinking about setting up shop here, making knives commercially.

  6. Thanks for the information, that page is about the L6 composition? That’s 550 F or 550C ?

    I don’t understand why the steel would have to be quenched to room temperature from the transformation temperature, but that part isn’t very difficult to achieve.

    Either I can transform the steel myself in a dilatometer to check my TTT diagram or I need to find some papers on this composition. I’m interested to know what strength levels it achieves and if carbide precipitation occurs to form upper or lower bainite.

  7. Whoa, has it been this long since I’ve checked my email? It took me this long to respond? Wow. I chose a poor name too.

    It seems fairly difficult to find information about bainite. The measure is in Fahrenheit, sorry for not clarifying it. I find it surprising that it didn’t use Celsius.

    It doesn’t go into the process of making it. The only other place it mentions it, according to the index, in in the glossary, where it’s defined as “A structure obtained by the transformation of austenite at a constant low temperature. Bainite is usually the result of austempering.” Austempering is defined as “A heat-treating process where the blade is quenched in a medium such as molten lead or special salt. The temperature of the quenching medium is maintained below that where pearlite is formed but above that where martensite is formed.”

    But I’m sure you already knew that. The strength is supposed to be absurdly good when used in knife blades.

  8. It took me a very long time to wander back by here. My apologies for that. Thanks for your reply to my question about martensite finish temperature.

    I’m not a metallurgist, and you clearly understand the subject far better than I do. (That’s why I asked my question of you. 🙂 ) I was hoping you’d have a TTT or I-T diagram for L6. It’s my understanding that the Mf point is very difficult to predict, and is best determined experimentally. But it apparently is common to use M90 as an approximation for Mf.

    Jesus, for more information on bainite you should look at Dr. John Verhoeven’s “Metallurgy of Steel for Bladesmiths & Others
    who Heat Treat and Forge Steel.” In order to figure out how to make bainite in a particular steel, you need to get a TTT diagram for that steel and learn how to read it. The bainite cooling rate will be between the cooling rate needed to form pearlite, and the cooling rate needed to form martensite. You need to quench fast enough to miss the pearlite nose, but hold the steel above the martensite start temperature (Ms). Hold there for a fairly extended time and you’ll form bainite. The 550 F/2.75 hours recipe isn’t necessarily going to work for all steels. Carbon content and alloying elements matter.

  9. Matt B> about the martensite finish temperature, I wrote another article about martensite kinetics. From that equation the MF/M90/M50 would always be a set amount below the MS. M90 would therefore be always about 200 Celcius (~360 F) below MS.

    I’m not able to research too much about that now, but it is something interesting to read about more. There does seem to be a lot of different ideas about martensite kinetics.

  10. There are TTT diagrams here, not sure if they’re exactly what you’re looking for, and I don’t know how to read them yet: http://www.navaching.com/forge/steels.html

    I’m not sure what that recipe was for, it was probably either 9260 or a simpler steel, 1050 or 1060, since the book usually utilizes those. Thank you very much for recommending the book, are there any others of importance on this or related subjects?

  11. Thanks a lot for the link, I had searched before for diagram but didn’t turn anything up.

    The link shows the isothermal transformation diagram, the time after instant cooling that transformation would start. In reality cooling will take some time, during which there can be a small amount of transformation. The start time, is when the amount transformed is first detectable, transformation either occuring slowly before that, or there is an incubation time before transformation starts. From looking at the diagram is seems like you have to cool in around 20 s to the isothermal transformation temperature. You could then transform between 400 and 250 Celcius (500-750 F) the diagram shows the time in seconds for the start and end of transformation. (I presume this is what the diagram shows but I couldn’t see a key/legend).

    Matweb datasheet on L6

  12. Bohler Uddeholm lists a powder metalurgy L6 product called “Grane” –

  13. My unanswered question of the day; witch comes first in the quenching? Dose he produce a martensite edge by quenching the edge then salt pot @800deg f for 40 min OR… Can the martensite edge be produced on a continuos cooling from 30min @1525, quench to 700/800f for 30 min witch will leave a 5min window for the martensite finish quench @ 200f/385f ??????
    Dose he only quench the edge in a extremely shallow oil or find some way of using clay?
    Another possibility is through hardening in multiple salt pots then a claying and reheating too critical on the edge only to finish a differential hardening.
    Sounds like I need to do some experimenting!

  14. What would a sword made with modern science and technology be like?

    Good point, but L6 tool steel actually enables for the formation of martensite/bainite layering (and perhaps some ferrite), dependent upon the heat treatment: https://bainite.wordpress.com/2007/07/12/l6-steel-kinetics/

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