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Silver soldering unhardened steel vs brittleness

Discussion in 'Maintenance, Tinkering & Embellishment' started by M4cr0s, May 17, 2019.

  1. M4cr0s


    May 1, 2012
    I have the end of a (hidden) knife tang that is unhardened. Some sort of carbon steel of the "old truck leaf springs" variety. I need to flux up this tang end, preen it as well as I can and then heat it enough for the solder to take and lock it in place and cover it up. Then grind and polish it to a uniform thing.

    It's a very small piece at the end of the tang, but obviously crucial to keep the whole thing together. As this is a huge differentially (edge only) hardened blade meant for chopping, the "springyness" of the unhardened steel is likely crucial for the integrity of the piece. This solder needs about 600 degrees celcius (1100ish farenheit) to start flowing. Although this is far from the red hot hardening range temperature for the steel, I'm still worrying about somehow making the steel brittle...

    Am I worrying needlessly? ;)
  2. Bill DeShivs

    Bill DeShivs KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 6, 2000
    Yes, you are.
  3. David Martin

    David Martin Gold Member Gold Member

    Apr 7, 2008
    M4, that should not hurt the blade's integrity as it's a ways from the edge and soldering goes fast. You can always wrap it with a damp cloth.
    I have silver soldered guards on to heat-treated blades with no ill affects. This is close to the working portion of a blade and requires more
    torch work. I merely cooled the blade down after a guard was securely attached. After doing a few you'll get comfortable with it. Good luck, DM
  4. Wowbagger

    Wowbagger Basic Member Basic Member

    Sep 20, 2015
    Wouldn't the actual concern be heating the blade enough that the edge is further tempered ?

    You can get "crayons" that indicate temperature reached. You could mark the blade near the edge to be sure the crayon has not flowed. In other words be sure to keep the edge area of the blade below two hundred degrees.

    Realistically wrap the blade in wet rags and or heat barrier putty and go for it.
    If the tang is thick it could take a little while to get that much steel to 1100°

    Finally it is a small thing but I would encourage everyone to not use the word "solder" which is around 500°F

    Actually what we are talking about here is silver BRAZING.

    Silver brazing (and brass brazing) thin wall chro molly tubing (1mm or less wall thickness) is one of my "THINGS".
  5. wade7575


    Apr 3, 2013
    Try wrapping the blade and in a wet cold cloth and put the knife in a vice and have some one slowly poring water on the blade while heat up the tang.
  6. Bill DeShivs

    Bill DeShivs KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 6, 2000
    I'm not quite sure what you are trying to do, but it sounds like you need to hard solder something on the end of the knife tang.
    If so, just put the tang in a vise. The vise will act as a heat sink.
    Unhardened steel is not "springy." It bends easily and stays bent.
    Regardless, there shouldn't be enough heat transfer even without a heat sink to harm the blade.
    David Martin likes this.
  7. M4cr0s


    May 1, 2012
    Hi guys. thank you for input!

    Due to the size av the blade and the length of the tang, I am not at all worried about heat doing something to the edge. This is a huge, thick, thick blade "15 long. It is just the very end of the tang that could possibly reach any critical temperature when heating it for soldering and if it somehow was hardened without proper tempering, I fear it could snap off from the stress when chopping. Alas, if there is no risk for any hardening in the 6-700 degree celcius range I don't have to worry.

    I'm just a hobbyist, with no substantial knowledge about steel, hardening and tempering. Trying to think things properly through so that I don't muck it up before I jump at it. I got basically one go at this. I don't have an anvil so it would be tricky to hammer out his tang and make it longer (and thinner) should it go wrong.

    Oh, we call it soldering in my native language, hence the direct translation. I'm sure you are technically correct as to the anglo terminology @Bill DeShivs :)
  8. Bill DeShivs

    Bill DeShivs KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jun 6, 2000
    If you just let the tang cool, it should be soft. If you quench it, it may harden.
    M4cr0s likes this.
  9. M4cr0s


    May 1, 2012
    I will try and see how it works out when I eventually stress test the blade chopping. In the end and either way, I'll learn something. I'm probably overthinking this :) Thank you!
  10. Wowbagger

    Wowbagger Basic Member Basic Member

    Sep 20, 2015
    Interesting side fact :
    With the tubing I work with (which is a different alloy than the knife) if a breeze were to blow over the thin wall tubing while it was at brazing temp it has been known to over harden and later crack / fail in use. Air hardening steel. It should only get a little harder but spring like.

    Early craftsmen learned to do the work indoors in still air rather than outside under an awning or shade tree. Much better for judging the optimum brazing temp by the color of the heated steel and behavior of the fluxed surface as well.
    M4cr0s likes this.

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