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Testing For Toughness Of Blade

Discussion in 'Heat Treating' started by RussGen, Jul 1, 2020.

  1. RussGen

    RussGen New Member

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    I've got to say I,m never quite sure about testing my blades. When I run a file across my blade tempered and untempered it seems to skate okay but if I really press in it does bite. How hard do you press? So it might be crazy but this obsessive hobby has resulted in a bag of knives heavy enough that I finally felt comfortable taking two of my latest builds (one 80crv2 and one 1084) and whacking them with a lot of force into a throwaway deer antler. I was surprised to see no damage of any kind. I've doubted my heat treat because I have never used a lot of force when file testing, just some steady pressure. Just curious how some of you are performing this test.
     
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  2. Griff

    Griff Active Member

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    I think we touched on this in your other post.

    A regular file, metal working, wood working, whatever... should skate after the quench when your steel is at full hardness and therefore harder than the file. This is where this test is done.

    The only file you should even remotely think of touching your tempered knife with (just my opinion) is an HRC file in the relative hardness you where shooting for in the temper cycles (if you must). Outside of that, don’t scratch your beautiful knife after it’s tempered lol!

    But please do continue with your testing of the edge by whacking it into :poop ;) *Thumbs Up*

    And yes skating the file is exactly how it sounds, you’re just Lightly brushing the file along the edge, to hear that skating sound. You’re not trying gouge it!

    Also, sometimes the heat-treatment black-smutch (yes it’s a technical term:D) is all that gets scratched and new makers think thats their edge and the steel is soft...it’s not.

    :beer: Griff
     
    Last edited: Jul 1, 2020
  3. RussGen

    RussGen New Member

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    "Lightly brushing". That's what I wanted to hear. I think my knives are up to snuff. Gotta say say it really is fun testing them with the whacking harder objects. Thanks Griff.
     
  4. Mythtaken

    Mythtaken Staff Member CKM Staff

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    You can also use the brass rod test to check your temper. If you aren't familiar with it...

    Clamp a piece of brass rod in a vice (lengthwise) so that half is sticking up above the vice surface, or glue a piece to a piece of wood or steel and clamp that in the vice.
    Hold the edge of your blade against the brass at around 15 degrees (like you would for sharpening) and using a bit of pressure, run the edge of the blade across the rod.

    If the temper is good, you should see the blade edge deflect over the rod and then spring back. Chips or cracking means it's too hard and bending without springing back means it's too soft.
     
  5. RussGen

    RussGen New Member

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    Sorry I'm trying to picture this but there are so many directions and ways you could move the knife. Vertically, keeping the knife horizontal, horizontally keeping the knife horizontal or just knicking the top of this rod. I have 3/8" brass rod so I will try this. Thanks
     
  6. Griff

    Griff Active Member

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    Here you go! Look text is cold and I say this with all sincerity and helpfulness...YouTube is your best friend for seeing something said here as a visual *thumb up*

     
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  7. dancom

    dancom Dust Maker

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    Without having a CATRA edge testing machine handy, try cutting some material in measured way. Slicing through a chunk of sisal is pretty standard, but who has a bunch of old rope handy?

    I have found that corrugated cardboard will dull a blade pretty quickly. Make about 40 to 50 cuts averaging about 8 to 10" long through corrugated cardboard. Check the edge. If it still slices like a razor through a telephone book page, good, try another 40 to 50 cuts on the cardboard and check again. When the cutting edge wants to tear the paper and not cleanly cut, call it. Of course, results varies by steel, cardboard and cutting edge angle, but should give you an idea of how the heat treatment came out.

    Larrin has some details on a proper test.
    https://knifesteelnerds.com/2018/06/18/maximizing-edge-retention/

    After working with the same steel long enough, one learns what a properly heat treated blade sounds like. I know that seems weird, but there is a certain "shhh-ting" ringing sound that can be heard when carefully plucking at the blade with a fingernail. I know this is 100% non-scientific, and blade shape and edge geometry has something to do with it, but I know the HT is good when I hear the" shhh-ting."

    Dan
     
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  8. RussGen

    RussGen New Member

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    Thanks guys. Just got this info. I'll look it over and give it a try. Just normalizing 4 blades in 32 degree weather. Beats winter though for sure
     
  9. Scott Kozub

    Scott Kozub Active Member

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    I have a hardness tester but more and more I rely on the brass rod test with every knife. The nice thing about it is that it can be done on a finished blade in case your worried you overheated the edge.
     
  10. RussGen

    RussGen New Member

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    I tried skating a Nicholson file over the edge of another Nicholson file and even it caught a little bit with almost no pressure. I tried it both on the teeth of the file and the flat bit that's right next to the teeth. So I think I'm satisfied with my hardening. Not a huge difference.
     
  11. dancom

    dancom Dust Maker

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    Hi Russ. There's a lot going on in this thread, hardness, toughness, edge retention etc. For the viewers, I'd like to make a distinction between hardness and toughness. Hardness is how hard something got, say after you quench. This is where a file test is applicable. After you temper the blade you lose some hardness in exchange for some toughness. Toughness in a blade, like in people, is an ability to withstand abuse. After tempering, your steel may not be as hard as the Nicholson file, so file testing after tempering is not really a good gauge.

    Consider two different materials, glass and aluminum. Glass, when struck with a hard object breaks or chips. Aluminum, when struck with a hard object rolls or deforms. A glass blade might be good for surgical procedures, but be careful not to drop it or it may chip. An aluminum blade might be ok for cutting grass, but when you hit a rock, it will roll. Steel however, has the amazing ability to behave either way, as well as a whole range in between. How it presents depends on the steel and heat treatment. Our aim as knifemakers is to get the heat treatment dialed in to a balance between the two in a way that suits the application.

    In comes the brass rod test. In a way, the brass rod test is a Goldilocks test. This test could potentially chip the blade edge (too hard), or roll the edge (too soft), or do nothing (just right). For almost every application, the do nothing Goldilocks result is very good.

    Dan
     
  12. RussGen

    RussGen New Member

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    Thanks for your reply Dan. It's nice to tap into experience and not learn by trial and error. Saves time. Speaking of tempering I'd like to ask a related question. Given two hardened blades, one 64 and one 62. If I temper them both at 400 degrees for the identical amount of time will they both soften to the hardness that 400 degrees produces, just for example now both are 60 or would the one blade always remain two points above, for example now 64 moves down to 62 and 62 moves down to 60.
     
  13. dancom

    dancom Dust Maker

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    Hey Russ,

    If the two blades are the same steel and one came out of quench at HRC 64 and the other at 62, then yes, tempering the same temp for the same time, will result in one being a higher HRC than the other after tempering. There is a graph here (thanks once again Larrin) that plots the hardness against the tempering temperature. You can see that they start from "as quenched" hardness on the left and the hardness moves up/down according to the tempering temp. Yes, I was surprised to see the hardness go up with very low or very high tempering temps too.

    Dan
     
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