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Memory in Steel? Dealing with Blade Warp

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by Rick Marchand, Jan 3, 2012.

  1. Rick Marchand

    Rick Marchand Donkey on the Edge Moderator Knifemaker / Craftsman / Service Provider

    Jan 6, 2005
    Some time ago, I did a small tutorial on a method for straightening warped blades that involved clamping jigs and tempering cycles. I found that static clamping with little to almost no over-bend would take a set after a tempering cycle and the whole heart-pounding experience of a 3-point straightening jig could be avoided. Not a new technique, by any stretch(we did it all the time on a larger scale in the T&D industry) but the question was raised as to whether I was inducing stress or relieving it. I could not answer it at the time. I have read articles that suggested that the "damage" of warping is irreversible. They even go as far as to suggest that ALL mechanical and thermal events are forever recorded within the steel. Though, I have not seen any data to back that up.

    It is my understanding that during the tempering cycle, as carbon atoms release and alpha martensite gets converted to beta martensite most of the stresses within the blade are relaxed... This is why the blade retains a "set". I have always felt the plastic deformation that results from straightening on a 3-point press is an unnecessary risk. When I clamp a warped blade to a straight piece of steel, there is only a slight preload on it... nowhere near enough to push the elastic limit. If simply running it through a tempering cycle "locks" it in place(even with an 80% success rate), why would anyone resort to a straightening press if they didn't absolutely need to? In the past I've had vice-straightened blades return to a warped state from a temper cycle or grinding operation.

    So I ask...

    Do you believe that steel has a memory of every mechanical and thermal event it's been subjected to?
    Do you believe that any attempt to correct warp only succeeds at inducing more stress into an aready flawed blade?

    Last edited: Jan 3, 2012
  2. mete

    mete Gold Member Gold Member

    Jun 10, 2003
    Sure it remembers - remembers what you did wrong !!

    It can get a bit confusing as to whether you're removing stresses or introducing counteracting ones. Tempering will remove stresses and the higher the temperature the more stresses removed .At about 1200 F you'll remove all of them . Typically a 'stress relieving treatment is done by heating in the range of 700-1200 F.
    Cold bending to straighten things is not a permanent way. When cold bending a rifle barrel to straighten those introduced stresses will disappear when the barrel warms as you fire it ! The target will show changes in point of impact as it warms. We don't see any change in blades as they never get warmed !! BTW don't try straightening a hardened blade below 400 F .
    If you straighten with heat at tempering temperatures then changes would tend to be permanent as the crystal structure has changed.
  3. Darrin Sanders

    Darrin Sanders Knifemaker Moderator Knifemaker / Craftsman / Service Provider

    May 6, 2009
    Very good questions Rick. As to the first question; no, we all know tempering changes steel on an atomic level and I don't believe the atoms involved have a memory. I do however believe steel itself has a "memory" if the atoms have not been rearranged, springs are proof of this. As for whether tempering in a straightening jig induces stress or not, I don't care. I've straightened enough blades to know I couldn't tell one that has been straightened during temper and one that has not by looking at it or performance testing. I guess what I'm saying in short is, I couldn't tell if a blade has stress in it by using it which is what really matters to me. Now maybe somebody can show exactly how much I don't know by proving me wrong. HEHEHE Good thread BTW.
  4. wolffbite


    May 7, 2010
    What about straightening an already warped piece of steel?

    I have had a few pieces of bowed steel I use for stock removal that I just popped in the vise and straightened out as best as I could, then ground them as normal. I had to do some work to get them flat... but I haven't had any of these heat-treated yet... will I end up dealing with some warped pieces when they are returned to me?
  5. MJV


    Apr 27, 2011
    What types of steel does this apply to?
  6. Rick Marchand

    Rick Marchand Donkey on the Edge Moderator Knifemaker / Craftsman / Service Provider

    Jan 6, 2005
    I am refering to the common carbon "forging" steels.... 10xx, 52100, O1, 5160, etc... I have yet to work with stainless alloys.
  7. sunshadow


    Oct 2, 2006
    I have never had a blade take a "set" from "memory" and I have done all of the things at one point or another that certain smiths say will cause all sorts of memory derived warp. My experience with troubleshooting people's warp issues over the years has always come down to either:

    They put their blades in their kiln on their side
    they edge quench
    they have multiple blades in a kiln on a short cycle so the blades heat unevenly
    They have blades leaning against the side of their kiln
    they quench unevenly
    they hit something on the way between kiln and quench or when trying to open a foil envelope with their hot squishy blade

    If you normalize your blade you remove stresses that would cause warp

    There is such a thing as shape memory alloy which is made to keep a shape that was imparted to it and return to that shape with low temperature heating, I used to work at the plant where it was developed, but nobody makes knives out of Nitinol (and nobody would want to, trust me!)

  8. mete

    mete Gold Member Gold Member

    Jun 10, 2003
    The shape memory alloys involve a martensitic transformation.

    Wolffbite , already warped steel should be stress relieved at 1200 F and straightened.
  9. MJV


    Apr 27, 2011
    Thanks for the clarification, I thought that was the steel you were talking about.
  10. Tai Goo

    Tai Goo Banned BANNED

    Apr 7, 2006
    I was going to say something,... but I forgot what we were talking about?
  11. tryppyr

    tryppyr KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Feb 5, 2010
    I am no metallurgist, but for many years I have been interested (as an observer) in the way various sciences look at the same problems and come up with different answers. Not only are the answers different in very fundamental ways, sometimes the explanations change the way we understand the universe around us.

    We can all have a chuckle about the dispute between science and religion about the age of the universe, but it is a little more difficult to understand (let alone find humor in) the dispute between those espousing string theory over those focused on particle and wave theories. In the universe of quantum physics, things we thought we understood well are suddenly challenged.

    So when I see threads like this one where those with very specialized knowledge challenge one another I tend to have a deja vu moment. I make no claim to having specialized knowledge on this subject, but tend to want to caution folks not to be TOO convinced the science they know is comprehensive and complete. Sure, the understanding of chemistry and physics has been fairly static for some time, and many experiments have added to the body of evidence supporting the theories being espoused. But one thing I learned long ago, it is almost impossible to PROVE something to be true, and all too easy to have one data point prove that it is not infallibly true.

    So take this as a word of advice to exercise some humility in your proposals and acknowledge that there are limits to what any of us know. Maybe steel does have a soul... maybe it has a memory that we are just unable to see (even as we are unable to see our own soul or each other's memories). It may not have anything to do with martensite or austentite. It may be a function of quantum mechanics or the flow of chi through the atomic structure. Who knows, it might even be caused by the presence (or absence) of dark energy in the steel. The universe is supposed to be full of dark energy, after all... who knows what properties it possesses and what it might do to steel?
  12. Atlas Knife Company

    Atlas Knife Company KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Feb 16, 2010
    As a beginner, I can attest to the Marchand-method of straightening during the temper being effective and easy with both carbon and stainless. I can also attest, as a beginner, that it is much easier on your blade and less nerve wracking than clamping in a vice.

    As a thinker, it would seem that if iron does retain permanent memory, it should eventually transform back into the shape of the ore that was pulled from the ground and I haven't seen that happen.
  13. Tai Goo

    Tai Goo Banned BANNED

    Apr 7, 2006
    Good points Tryppy.

    … Just forget everything you think you know and it makes perfect sense.
  14. Troop


    Oct 26, 2006
    Rick, Even after reading the post on Kevin's site, I, too believe that the steel retains a "memory" that is resistant to "correction".
    I have especially observed this with O-1, maybe because of all the alloying.
  15. tryppyr

    tryppyr KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Feb 5, 2010
    That response is what would be expected from someone who believes the science they know is the only science that matters. Be careful of that assumption.
  16. Brian Ayres

    Brian Ayres

    Feb 4, 2011
    Mete, Or someone who knows more than I do.

    In the interest of being clear. I thought new barstock often comes "bent", not warped. Is it under stress in that condition? I didn't think so. I thought most stock removal guys just straightened and ground their "bent" stock.
    Wouldn't a normalizing stress relief possibly ruin the annealed state and make the steel harder to grind(depending on steel type)...???
    If it's annealed. I know you can't always know what condition the steel is in when you buy it. I'm mostly asking for clarification
    What would the best step be.
    1. Straighten
    2. Grind
    3. Normalize
    4. Heat treat
  17. sunshadow


    Oct 2, 2006
    1200 degrees is below af so it will not have any impact on the overall condition of the anneal. At that temperature the steel is somewhat plastic so that is (give or take a little since I am not using a kiln for this I do not measure it) around the range I straighten at, then bring it up again after straightening and let it air cool before grinding (note I said air cool not quench or vermiculite cool) then get on with grinding or whatever the next step is on that particular blade

  18. Dave Behrens

    Dave Behrens

    Feb 25, 2011
    ...:confused: Where am I????:confused:....WHO THE HELL ARE YOU PEOPLE!?!?!?!?! WAIT...WHO THE HELL AM I?!?!?!?!?;)

    Akin to Rick's straightening method, the first warped blades I had, I reheated them to LCT(Im guessing) and reversed the bend so it mirrored itself, re heated and the blade would come back straight. I have no idea whether or not this induced or relieved stress, probably not the most stable solution and the blades held up in testing so....? Steel is like a woman sometimes, could talk all day and not make one bit of sense?;):D

    View attachment 250098

    This being said, compared to most on this thread, I have dick for knowledge on HT, still trying to figure out who the hell Austen and Pearl Ite are and why they keep messin with my blades.
  19. Rick Marchand

    Rick Marchand Donkey on the Edge Moderator Knifemaker / Craftsman / Service Provider

    Jan 6, 2005

    Interesting thoughts Greg.
    All we can be certain of is that there are no absolutes. We can however, do the best with the knowledge we have, now. The understanding we had of steel was made more complex with alloying... but never the less, it can be made predictable and controlable, as well. I think we are doing an injustice to the advancement of knowledge to assert opinions as facts, before they are validated by current standards. Opinions and propositions are the seeds of discovery but when they are based in apprehension and assumption rather than proof or reason they can corrupt truth. When discussing groundbreaking science like quantum and string theory, the experts avoid absolutes. There are however, theories, concepts and laws that have withstood challenge and scrutiny to the point that they become fact. The basic principles of metallurgy are among these truths. We know what happens when you heat steel. I think the challenges we face in heat treatment lie more in control(temperature, atmosphere, etc...) and measurement(hardness, strength, etc...) than new breakthroughs metallurgical science... the information is already there for us.

    ETA: As knifemakers and bladesmiths, we have the freedom of interpreting this info as we see fit. Whether you are hammering on a used file, heating it to red and plunging into used motor oil or... using CNC mills, salt pots, cryo chambers and rockwell testers........... there is room for everyone as long as you are being honest to yourself, your craft and your customer.

    ... or I could be wrong.:D
    Last edited: Jan 4, 2012
  20. parbajtor


    Nov 24, 2010

    You've never seen a blade rust? That's the start of the transformation process.

    As a former fencer I have seen many blades take on an "unnatural" set where they bend the wrong way. In order to get the weapon to work correctly, they are usually straightened and overset to reduce that wrong way bending. If they are subjected to the same wrong way bending forces in a short period of time (usually within a month of 3 nights a week training), the unnatural bend returns and is generally very difficult to straighten out. If it manages to last a month without bending the wrong way, they usually "forget" the bend and perform jiust like any other blade.

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