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Questions on a failed 3V heat treat

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by Steve_p, Nov 24, 2020.

  1. Steve_p


    Apr 9, 2019
    I did a few 3v blades over the weekend. Normally I use stainless foil wrap, but I was out so I used anti scale compound instead. I definitely noticed way more decarb than I normally get, then when I started grinding it away I noticed tons of small cracks all over the blade.

    I broke both, to my novice eye the grain structure looks ok but what I’m curious/worried about is the dark ring close to the surface. Any thoughts about what I might have done wrong? I’m thinking I didn’t apply the anti scale correctly and I burned up all the carbon. [​IMG]
  2. Steve_p


    Apr 9, 2019
    The second blade looked the same Once broken, just a way more jagged break so it was difficult to get a good picture.
  3. DevinT


    Jan 29, 2010
    Outline your heat treatment for us.

    DeadboxHero likes this.
  4. Steve_p


    Apr 9, 2019
    1975f for 30 min
    Plate quench
    Dry ice/denatured alcohol overnight
    Temper at 400f 2hrs x2

    after the plate quench they were still around 200-250f so I cooled in water briefly before going into the sub zero
  5. Stacy E. Apelt - Bladesmith

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

    Aug 20, 2004
    Just a comment, but there is no need of advantage to the overnight soak in the dry ice bath. Five or ten minutes is sufficient. Once the blade reaches the -90°F temp, the Mf is done. It probably won't hurt to leave it, but isn't necessary. The best method of HT is to do the first temper immediately after the blade reaches the Mf.

    A dry ice bath is to reach the Mf point.
    Cryo, in liquid nitrogen at -320°F requires a longer time to create changes in the structures and form eta carbides. Overnight won't hurt in cryo, but it really only needs an hour or so.
    Last edited: Nov 28, 2020
  6. Scaniaman


    Jun 15, 2012
    Theoretically it is better to go straight to tempering after cold treatment to Mf, rather than waiting over night yes? Or am I remebering wrong?
  7. Larrin

    Larrin Gold Member Knifemaker / Craftsman / Service Provider Gold Member

    Jan 17, 2004
    There can be a minor danger to long cold treatments for the same reason many datasheets recommend tempering right away (the steel is brittle and may crack without tempering). That said, I’ve never heard of a knife cracking in cryo overnight apart from laminated blades, probably because of the thin cross-section used in knives. I often leave knives in cryo overnight to let my furnace cool down to temper in.

    Leaving in liquid nitrogen for extended periods of time does not lead to any benefits either, I have written about that extensively in the past.
    DevinT likes this.
  8. SBuzek

    SBuzek KnifeMaker Knifemaker / Craftsman / Service Provider

    Dec 7, 2006
    If you left them in dry ice slurry over night and the reach room temperature they may crack from the stress waiting to be tempered.
    DevinT likes this.
  9. Nathan the Machinist

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

    Feb 13, 2007
    This is a problem. This steel is forming most of its martensite in this temperature range. You want a relatively rapid quench at first that gradually slows down a little during the martensite formation temperature range. It is usually okay to dunk hot steel into water most of the time, but never during the primary quench.
    stephenvan, Willie71 and SBuzek like this.

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