katana lost curve after heat treatment?

Discussion in 'Sword Discussion' started by tacticalrescue, Dec 19, 2017.

  1. tacticalrescue


    Mar 13, 2016
    hello all,

    I had a katana I bought in 2001 and have since become an avid knife maker. but swords are beyond me so i had the blade sent out to be heat treated and see how things went. to make a long story short i choose a horrible place to do this and got my money back but it seems like the blade no longer has a curve and is perfect straight. can anyone explain what might had happened here? there's no hamon(at least as far as i can see). seems like a low end carbon steel. This was not a cheap knock off made in china but it was not a 10,000$ sword either.

    thanks for anyones time and help.
  2. horseclover

    horseclover Basic Member Basic Member

    Nov 21, 2000
    I have a car I bought in 2013. It is a 2004 model and has silver frost paint. Can anyone tell me what the mileage is, when it last had an oil change and what model car I have here?
    keithf and Lapedog like this.
  3. PirateSeulb


    Jun 6, 2017
    To give OP some insight based off some of my recollections on details of how the hamon and the curve are created. One key aspect to Katana is that they are traditionally differentially hardened so that one part is harder than another. This differential hardening is what actually causes the blade to curve and form the hamon. If the blade was sent out for a new HT and came back straighter and w/o a hamon then my guess is the new HT did not differentially harden the blade causing the hamon to erase or become significantly less pronounced and also cause the blade to spring back to a straighter form.

    That is all speculation based on vague memories from a TV documentary on Japanese sword smiths that I only watched pieces of because who can sit through commercials.
  4. Danke42


    Feb 10, 2015
    So what probably happened is the guy who was giving the sweet deal on the price chucked in in an oven on some bricks with a bunch of lawn furniture or car parts.

    With anything post production like your sword would have been having it re-heat treated is going to result in disappointment. A better place would have just said no to the plan.
  5. sandgrouper

    sandgrouper Basic Member Basic Member

    Jan 20, 2007
    PirateSeulb is quite right, the curve in a katana blade is the result of the differential quench due to the claying with the edge cooling first and faster and ending up as harder but less dense martensite with the spine cooling slower and ending up softer but denser pearlite. The edge ends up longer than the spine, the curve results from that. It seems dead straight is a common starting position before going into the quench, so then the right curve also evidences an appropriate microstructure. If your bloke just treated the whole thing as one item, and it all ends up as one microstructure, either one, then straight seems a likely outcome. The hamon is a result of the same thing.

    If you search Google for katana quenching process you should get a bunch of hits showing a straight forging going in the bath, bending down at first from the differential cooling, then back up again as the transformation kicks in.
  6. Triton

    Triton Gold Member Gold Member

    Aug 8, 2000
    I am trying to understand why you would have what was presumably finished sword reheat treated. That aside a hamon would not be visible without an etch.
  7. PirateSeulb


    Jun 6, 2017
    @Triton I agree the reason OP sent this for a HT is confusing but as for visible hamon it doesn't require an etch if it is a differential quench that process actually creates the visible hamon.
  8. Triton

    Triton Gold Member Gold Member

    Aug 8, 2000
    That's certainly true if one polishes it down fine enough, something tells me that hasn't happened here.
  9. David Stifle

    David Stifle

    Nov 20, 2008
    Katana's are made straight. They only obtain the curve, or sori, id they are quenched in water. My guess is the guy you sent it to quenched it in oil. It so, it will not curve up, and might, in fact, end up with a reverse curve.
    keithf and Mecha like this.
  10. samuraistuart

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

    Dec 21, 2006
    Exactly what David said. Without going into the OPs specific reasons why and what not....

    A sword quenched in water will result in an upward curve (it actually curves DOWN first in the quench, then up as the martensite start temperature is reached).
    A sword quenched in oil will result in a downward curve.

    They do not necessarily have to be clayed for this to happen, rather geometry alone can cause the positive or negative sori. I recently HTd a friends BBQ slicing knife in O1 tool steel (medium speed oil quench), the blade bevels had been cut already, and it resulted in a negative sori bend. If the bevels of that knife had not been ground prior to HT, the negative sori would not have happened. The difference in cooling rate between the relatively thick spine and thin edge caused a differential hardening sori curve.

    There is absolutely no way to tell exactly what happened with the OPs sword. But if it had positive sori prior to the re heat treatment, I can almost guarantee that is what happened. It was quenched in oil. You would have a fun time straightening positive sori by any other method than a re heat treatment!
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