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O1 and Soak Times

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by MBB, Feb 10, 2019 at 9:30 AM.

  1. MBB

    MBB Gold Member Gold Member

    Apr 18, 2014
    Why exactly does O1 need a longer soak time? I have heard this repeatedly but I don't have a distinct explanation for it. Usually I hear "get everything in solution" or "get the carbon in solution". O1 is realistically a low alloy steel with the greatest degree of alloying being Mn. I assume it is not particularly spheroidized, given that it is nearly air hardening. Here are the specs for the O1 Admiral sells:

    C=.95 Mn=1.20 Si=.30 Cr=.50 Va=.20 W=.50

    5160 and 52100 have more Cr than O1. 80 CRV2 has the same amount of V. W1 and W2 have W, although at a lower concentration. None of these require prolonged soak times.

    Is it the W? Is the W somehow clumped and requires prolonged soaks to distribute at 1475F? Does W really migrate around much at 1475F?

    Is it the Mn? Does it need to migrate or does it somehow limit C migration?

    Or is this a historical inaccuracy?

    @Larrin this is for you and the other metallurgy experts.


  2. daizee

    daizee KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Dec 30, 2009
    Hi MBB,

    I just went down the rabbit hole of trying to turn up Kevin Cashen's excellent treatment of O1 in a thread here some years ago. Couldn't find it, and image hosting silliness have wiped all his pictures. But IIRC, the deal is that the carbon in excess of 0.8% gets tied up with the Va and W in carbide arrangements that require more time at temperature to dissolve. So if you just bring O1 to temperature and immediately quench, you're leaving all the potential extra wear resistance in the form of fine, grain-pinning vanadium carbides (and others?) on the table (or in the forge, as the case may be). In that case, you might as well use 1084 instead for the same performance at lower expense. Kevin showed a number of micrographs from O1 soaked for increasing lengths of time, and recommended 10-20min soak, IIRC. After that returns diminished to the point it wasn't efficient to continue.

    But Hopefully Mete, Stacy, or Larrin will be along to confirm or correct.
  3. MBB

    MBB Gold Member Gold Member

    Apr 18, 2014
    Thanks! I do appreciate that Kevin Cashen backs everything up with empiric research. I still am unclear on why O1 is the special case vs say 80CRV2.

    Also, I did a little quick search on this and found this...

    "Tungsten (W) promotes the formation of M6C type carbides [7] where the metal (M) is tungsten, iron (Fe), molybdenum (Mo) or a combination. These carbides dissolve in the austenite matrix at temperatures ranging from approximately 1150°C (2100°F) to the solidus temperature. However, in practice they do not dissolve completely since time at austenitizing temperature is typically only a matter of a few minutes."

    ...suggesting that the W6C carbides are stable to at least 2100F, and thus unlikely to change at O1 austenitizing temperatures.

    Vanadium appears to be soluble at lower temperatures (~1650F) in at least low carbon steels in one article I looked at, but again this is way higher than the typical O1 austenitizing temperature of 1475F.
  4. Willie71

    Willie71 Warren J. Krywko. Part Time Knifemaker Knifemaker / Craftsman / Service Provider

    Feb 23, 2013
    Different alloys move at different rates at different temperatures. 80crv2 benefits from a 10 min soak, but will perform much like 1080 if the soak is short. O1 is a bit higher alloyed, but not much. Being hyper euctoid, rather than euctoid, you want the chromium and tungsten to go where they need to in solution, as there is extra carbon to form carbides with. 80crv2 won’t form a lot of carbides. There isn’t enough carbon there.

    The condition of the steel plays a roll. It’s easiest for steel to go into solution from martensite. Fine spheroidizing is next, then fine pearlite. Course pearlite and course spheroidizing take the longest to go into solution, and course spheroidizing needs higher heat to break up the course spheroids.

    Check out chapters 6,7,8, and 11 here: http://www.hybridburners.com/documents/verhoeven.pdf
    Last edited: Feb 10, 2019 at 12:54 PM
    skillgannon and KillerGriller like this.
  5. Nathan the Machinist

    Nathan the Machinist KnifeMaker / Machinist / Evil Genius Moderator Knifemaker / Craftsman / Service Provider

    Feb 13, 2007
    01 comes in a highly spheroidized condition and is a relatively high alloy carbon steel. The carbon is all balled up in large carbides when you receive it, it takes time to dissolve and equalize that carbon.

    If you don't you will get carbon gradients randomly across the steel. This means highly tetragonal plate martensite near the carbides and carbon lean lath martensite in between. A non-homogeneous structure which may measure normal hardness but when used will have poor edge retention due to mushy crumbly edge.
    skillgannon and daizee like this.
  6. MBB

    MBB Gold Member Gold Member

    Apr 18, 2014

    Verhoeven in Chapter 7 says:

    "What these results show is that when steel is heat treated the Mn and the 3 major alloying elements will not move significantly, whereas the C atoms will move significantly. It is possible to make the larger atoms move smaller distances, in reasonably short times, but only by heating them very hot for long times. For example a Mo atom will diffuse 50 microns when austenite is held at 2200o F for 8 hours. But the corresponding diffusion time for a carbon atom is only 1.7 seconds."

    Which follows that post melt/rolling there is little change in the heavy elements during heat treat in terms of becoming more soluble at the time/temperature used for O1. Also, aren't the heavy elements already in the form of carbides for most steels due to the high temperatures during the initial melt? So during low temperature (<1650F) heat treat, they're wiggling around from thermal kinetic energy but basically just stationary? What I'm getting at is that the temperature for carbide chemical change/dissolution is much higher. So during an O1 heat treat, there isn't a significant change in the carbide structure? Again, I'm not an expert, I'm just trying to assuage my curiosity.

    That does make sense, but why not do a pre-heat treat normalization cycle (1600-1650 F) as you would with other low alloy steels to break up spheroidization? I would assume you would get some martensite formation post normalization due to O1's high Mn concentration, but couldn't it be resolved with grain refinement?

    Thanks you guys for your help!
    I'm really not trying to be a pain in the butt here. These are just questions I've had for a while.
    Last edited: Feb 10, 2019 at 9:56 PM
  7. Willie71

    Willie71 Warren J. Krywko. Part Time Knifemaker Knifemaker / Craftsman / Service Provider

    Feb 23, 2013
    There are primary and secondary carbides. Primary carbides exist as cast, and we can’t change them much. Secondary carbides are the ones we manipulate with normalizing and thermal cycling. I don’t fully understand all of this, and terms for carbides are inconsistently used. Some alloying is in solution as well, and not locked up in carbides.
    comet_sharp likes this.
  8. comet_sharp


    Aug 22, 2016
    My understanding from Verhoeven was the secondary carbides begin to band with forging and thermal cycling because they are not bonded with corresponding alloys and therefore grow a bit fast. What we usually refer to as "banding" from thermal cycling is layers of primary and secondary carbides where the aforementioned one is growing at a faster rate since the plain ol cementite carbides grow at a faster rate than primary carbides that are attatched to whatever alloy is there.
    From my experience blades with tight banding sharpen very toothy and awesome, no banding sharpens as expected and large bands the edge feels odd to me.
    I may also be very off on this as what I get from reading Verhoeven and what he intends may be different things ha.
  9. Larrin

    Larrin Gold Member Gold Member

    Jan 17, 2004
    I see more references to spheroidized material as being a bad thing or undesirable. Any steel that is annealed correctly will have spheroidized carbides. That does not mean they are large or need to be eliminated or broken up. Pearlite does respond faster to austenitizing due to shorter diffusion distances but that doesn’t mean it is a superior starting structure. If the carbides are so large that they do not heat treat as expected using standard protocols that means it is being processed incorrectly.
    Willie71 and comet_sharp like this.
  10. Willie71

    Willie71 Warren J. Krywko. Part Time Knifemaker Knifemaker / Craftsman / Service Provider

    Feb 23, 2013
    Thishas been the issue with a certain supplier’s steel. You have to normalize and thermal cycle before the steel will properly harden. We have had problems with 52100, 80crv2, W2, and some batches of 1084.

    Spheroidized carbides aren’t bad, unless you can’t get the steel to harden with normal temps and soaks. That’s why several of us reference course or fine spheroidized conditions, and ask who the supplier was.
  11. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    A very short answer to why some steels need a soak to get the most out of them has to do with carbon in solution. All the carbon that the iron needs is .84%. The rest has to go somewhere else. It forms carbides and other structures. It takes more time to get that carbon where you want it to form the structures that you wanted/needed to get the knife blade you planned on. You can give most any carbon steel a HT by heating it to 1475F 30 seconds ( to make sure the blade is equally heated) and then quenching inoil. The issue is you will effectively get 1084 with a bunch of carbon and other stuff in it.

    There are complex books that deal with this and it is good to try and understand them, but the best practice is o trust the metallurgists who designed the steel and the many metallurgical bladesmiths who have hones the HT down over controlled testing … use the HT specs given by those people and you will get the most from the steel you chose.

    In short, pretty much every steel above .84 carbon content needs a soak of 5 to 10 minutes. If you can't do the with control against overheating, use 1084. There is nothing wrong with 1084, as it is the eutectoid steel.

    All this shows why doing everything possible to have even and reliable HT control is necessary. Just looking at the color is a pretty poor method. A TC in a muffle pipe is better, but still not exact. A HT oven is the best way to go for doing your own HT. Sending your blades out for HT is the best way for a newer maker to make good knives. Many very experienced makers go back to sending the blades out for HT because it eliminates one variable, and saves time ( which is often worth more than the money).
    KillerGriller likes this.
  12. MBB

    MBB Gold Member Gold Member

    Apr 18, 2014
    Stacy and all,

    Thank you for the replies. I get the hypereutectoid aspect of O1 requiring a 5-10 minute soak, the question I have had is what exactly makes up for the additional time recommended (e.g. "the good long soak"). Based on the fact that most carbides both don't migrate significantly or change chemical structure at 1475 F, why is that necessary? Cashen's own data clearly shows that it is. I was just hoping to find out why on a chemical/atomic level this is necessary.

  13. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    It is a bit complicated, and I would hope Larrin or one of the metallurgists will also respond with a more complete ( and probably more accurate) response.

    It has to do with some things moving to the grain boundaries, redistribution, and forming clusters and structures. This Takes time at 1475F.
    MBB likes this.
  14. samuraistuart

    samuraistuart KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Dec 21, 2006
    5160, 80CrV2, W2 (NJSB W2) are all roughly eutectoid or hypoeutectoid steels. 52100 and O1 are hyper eutectoid steels. They have extra carbon past the eutectoid point. The goal is maximum post quench hardness (65HRC+), and there are 2 ways to go about dealing with that extra carbon. Higher temps and no soak time or lower temps plus the soak time. Using the higher temps causes a few undesirable issues that when using the lower temps and extended soak times doesn't happen. But it also depends on what carbide forming elements the steel has, and in what %. The higher the alloy % and the higher the carbon content past the eutectoid point, you need to free that carbon up somehow that is tied up in carbides as received from the supplier, usually this is a fine spheroidized state.

    If the steel has equal to or less than ~.8% carbon, extended soak times (past 5-10 minutes) aren't all that necessary. Bring up to temp, equalize and short soak if able, quench. But if the steel is hypereutectoid, carbon greater than roughly .8%, it's better to use the lower hardening temperature and extended soak times. So like for O1, if you were to simply bring to 1475f/1500f and quench, the result would be a lower than expected hardness, because you didn't allow time to do it's thing. If you were to use a hotter temp (rather than low temp and time), you could bring in the necessary carbon for the desired hardness (65HRC+), but you also bring in more retained austenite and higher percentage of plate martensite. The better route being to use the 1475F temperature and extended soak times in order to get the necessary .8% carbon in solution for the desired maximum post quench hardness.

    Interestingly, I often hear O1 needs only a 10 minute soak, and this may be enough. I often see the data sheets saying 30 minutes up to the first 1/2" (or 1" whatever it is I don't recall) thickness, which means that even if the piece is only 1/8", it still should get the 30 minutes of soak. If the steel exceeds a certain thickness, time is added. I know some makers do 20-30 minute soak times with O1, and I don't think this would cause any issues concerning grain growth, due to the tungsten and vanadium present, but at some point in time during an extended soak, too much carbon can come into solution, and thus bring the same problems as too high of a temp.

    Then there is the issue of higher % of chromium as in the case with 52100, and exactly what hardening temperature is best suited for that steel, and I have always been taught that this was heavily dependent upon the previous thermal treatment the 52100 underwent, and thus the resulting carbide structure. No normalizing was done? Higher temps are needed to get that carbon in solution. But if the normalizing WAS done, the carbon is ready for solution by using the lower austenitizing temps and ~10 minute soak. While O1 does have a chromium content, it isn't quite as high as the 1.5% in 52100.

    Good question, no doubt about it. What helped me understand better was that when steel is received in the fine spheroidized state, the carbon you need to get the hardness you're after is tied up in the carbides, and must be freed in order to harden the steel. Annealed steel has more carbides in it than hardened steel does.
  15. Larrin

    Larrin Gold Member Gold Member

    Jan 17, 2004
    Manganese is enriched in cementite so I wouldn’t discount the Mn of O1 as contributing to a necessity for soak time.
    MBB likes this.
  16. Willie71

    Willie71 Warren J. Krywko. Part Time Knifemaker Knifemaker / Craftsman / Service Provider

    Feb 23, 2013
    You have to go over 1515f (going by memory here) to cause grain growth in O1. Kevin Cashen has the data that shows this. So 1475f is fine for extended soaks as long as the steel is protected from decarb.
  17. Stacy E. Apelt - Bladesmith

    Stacy E. Apelt - Bladesmith ilmarinen - MODERATOR Moderator Knifemaker / Craftsman / Service Provider

    Aug 20, 2004
    I don't have any charts available, but IIRC, it will take a lot more than 1515°F to have significant grain growth in O-1, and similar blade steels, over a 10 minute soak. I doubt it would be much of an issue at 1615°F over 10 minutes. Grain growth is more significant in the 1800°-2000°F range of forging.
  18. Willie71

    Willie71 Warren J. Krywko. Part Time Knifemaker Knifemaker / Craftsman / Service Provider

    Feb 23, 2013
    I knew I read/heard about it somewhere. Kevin Cashen to,d me about an experiment a certain Phillip Patton had done with O1, soaking for hours (5 in this case.) there was no grain growth even though a temp of 1510f was used. The thread references a chart showing the relationship between time and temperature and grain growth. Kevin commentedon needi g to go above 1500f, and Philip went to 1510f. Kevin might have been saying you have to go to at least 1515f in a tongue in cheek way that I missed, to get grain growth when I spoke with him a few years ago.

  19. MBB

    MBB Gold Member Gold Member

    Apr 18, 2014
    So, an interesting topic for a Knife Steel Nerds post perhaps...
    Willie71 likes this.
  20. Larrin

    Larrin Gold Member Gold Member

    Jan 17, 2004

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