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Thread: Help with straightening a blade - PLEASE!!!

  1. #1
    Join Date
    Nov 2010
    Location
    Peachtree Corners, GA
    Posts
    216

    Help with straightening a blade - PLEASE!!!

    Hey guys,

    Need your help on this one. I heat treated a large kitchen knife (CPM 154, 12" blade, 2 1/2" wide, 1/8" thick) and it took a slight bend during process. I've tried just about anything I know of and could read up about straightening bends, short of re-heat treating the knife. Let's see, I tried to:
    Straighten in a vice (jig) right after coming out of the oven after the second temper cycle - snapped right back
    Putting a lot of weight (close to 30 lbs) in the middle of the blade when it was still hot (only the blade ends were supported). It bent a lot, but just snapped right back
    Re-heat to 425 F for 30 mins while clamped to a straight piece of angle, over-shimmed so that it bent in the opposite direction (did this three times - once just let it come to room temp by itself, once dunked the whole thing in a big bucket of water right out of the oven) - very slight reduction, but not really noticeable. Did it again, no real effect either)


    After all of this, there is still probably a 1/16"bend in the blade. Should I just normalize again, and re-heat treat?

    BTW, the blade was straight after normalizing and before it went into the oven the heat treat.

    Thanks in advance guys!


  2. #2
    Join Date
    Sep 2009
    Location
    Central LA (Larto lake)
    Posts
    1,096
    The next thing I would try is a thicker shim in the same area that gave the small reduction. Keep increasing the shim thickness until the warp is gone.
    Darrin Sanders Custom Knives
    564 Paulk Rd.
    Jonesville, LA 71343

    318-339-8290

    darrinsanders8666@gmail.com

  3. #3
    Join Date
    Nov 2010
    Location
    Peachtree Corners, GA
    Posts
    216
    Thanks Darrin. Dumb question - won't that affect the overall hardness of the blade eventually? Given that this is a kitchen knife, I would want to keep it as hard as possible (60-ish RC)

  4. #4
    Join Date
    Sep 2009
    Location
    Decatur, Illinois
    Posts
    1,772
    If you do not increase the temperature you would probably have to leave it in the oven for days without effecting the temper. I clamp my blades cold, but only after one complete tempering cycle, over a shim to counter bend. I use an angle iron because the ~90 will keep the bar rigid. Then I put it in the oven for a complete heating cycle. I've only had to increase the thickness of the shim once and cranking the C clamp down on it was a little hairy. I was expecting the dreaded tink with each twist but I have broken a blade yet with this method.

    If that doesn't fix it after a couple of tries it's time to bite the bullet repeat the normalization a couple of times to relieve stress and then re-harden and temper the blade.

    Doug
    Old age and trechery will always overcome youth and ambition.

  5. #5
    Join Date
    Sep 2009
    Location
    Central LA (Larto lake)
    Posts
    1,096
    Doug is correct. Extra tempering heats have no effect on hardness unless the temp. is higher than the original tempering temp. As long as you keep the temp. the same or lower than the original tempering temp. then a few 30 minute cycles for straightening will make no noticeable difference. Let us know how it turns out.
    Darrin Sanders Custom Knives
    564 Paulk Rd.
    Jonesville, LA 71343

    318-339-8290

    darrinsanders8666@gmail.com

  6. #6
    Join Date
    Nov 2010
    Location
    Peachtree Corners, GA
    Posts
    216
    Thanks a lot guys. I really do appreciate it! I'll let y'all know how it turns out!

  7. #7
    Join Date
    Nov 2010
    Location
    Peachtree Corners, GA
    Posts
    216
    So I got fed up after two more temper cycles, and just annealed the blade again. As soon as it came out of the oven - straight as an arrow... I've ground it clean and will will normalize it again before trying to harden it. Hopefully this time it will stay straight ��

  8. #8
    Join Date
    Sep 2009
    Location
    Central LA (Larto lake)
    Posts
    1,096
    Hate to hear that Ted, hopefully you wont have any more trouble with it.
    Darrin Sanders Custom Knives
    564 Paulk Rd.
    Jonesville, LA 71343

    318-339-8290

    darrinsanders8666@gmail.com

  9. #9
    Join Date
    Nov 2010
    Location
    Peachtree Corners, GA
    Posts
    216
    Thanks Darrin. Let me ask a (potentially) silly question. When I take the blade out of the oven, and lay it down on one of the aluminum quench plates, will the 2 or 3 seconds that it takes me to get the other plate on top of it make it warp because it cools down slightly faster on one side than the other? Is there a better way to do this?

  10. #10
    Join Date
    Sep 2009
    Location
    Central LA (Larto lake)
    Posts
    1,096
    Theoretically it could but I doubt it. The steel is still very pliable at this stage and should straighten when the other plate is applied. Here is how I handle foil wrapped blades;
    1. Be sure to place the blade in the foil packet with the edge toward the folded seam.
    2. Be sure the seam is up when you put it in the oven.
    3. When going from oven to plates be sure to keep the blade as straight as possible vertically.
    4. Be sure the spine is on the plate before you turn it on its side.
    5. Let the plates cool the blade to approx. 400 and remove the foil.
    6. Check for warpage and correct as necessary.
    I find it much easier to correct any warpage at this stage but sometimes no matter what you do you still end up with some warpage.
    I find that if you grab the blade and turn it sideways before placing it on the plate its own weight causes it to warp. Especially on longer knives but small ones will do it too.
    I'm sure there is a better way but this is what works best for me.
    Darrin Sanders Custom Knives
    564 Paulk Rd.
    Jonesville, LA 71343

    318-339-8290

    darrinsanders8666@gmail.com

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