Strange discoloration after hand sanding

Discussion in 'Shop Talk - BladeSmith Questions and Answers' started by killthelights, Jul 9, 2020.

  1. killthelights

    killthelights

    149
    Jan 6, 2016
    Was wondering if anyone has seen this before?(link below) 1/8"1095 from NJSB normalized and heat treated/tempered with Evenheat oven and hand sanded to 400 grit. You can't feel anything when you run your finger over it but you can feel it when sanding. Thanks.
    https://imgur.com/a/gcw3jcr
     
  2. A.McPherson

    A.McPherson KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Jan 27, 2012
    [​IMG]
    Here's your pic...

    Decarb maybe?
     
  3. killthelights

    killthelights

    149
    Jan 6, 2016
    Decarb maybe?[/QUOTE]
    Possibly but I've sanded quite a bit of steel off already. How thick can decarb be?
     
  4. Joshua Fisher

    Joshua Fisher KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    382
    Mar 27, 2018
    Could you run through your heat treating process? Temps you cycled it at and for how long, temp you quenched at and what type of oil was used in the quench and how much oil is in your quench tank, also was any grinding done on the bevels before heat treat or was the blade heat treated full thickness. Also you could try a quick dip in acid to see how the surface looks, if it’s decarb I believe it should stay fairly light compared to the rest of the blade.
     
  5. Drew Riley

    Drew Riley Riley Knife and Tool Knifemaker / Craftsman / Service Provider

    Oct 17, 2007
    Looks like an accidental hamon/differential hardening.
     
  6. Joshua Fisher

    Joshua Fisher KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    382
    Mar 27, 2018
    That’s what I was thinking which is why I’m curious of the heat treating process he used, if it’s decarb it can eventually be ground through but if it’s differential hardened then it will likely be like that all the way through that spot.
     
  7. killthelights

    killthelights

    149
    Jan 6, 2016
    If it is differential hardening, what does that mean for the quality of the blade? Would it be acceptable to sell or is it my new shop knife? I like the idea of acid etching it. I think I'll try that this afternoon.
     
  8. Drew Riley

    Drew Riley Riley Knife and Tool Knifemaker / Craftsman / Service Provider

    Oct 17, 2007
    Chances are that your edge still hardened all the way through. You can etch it if you want, I suppose, but it'll just look like a much more pronounced "splotch" IMO. It may become more pronounced with a patina through use as well, so that may be something to consider.

    IIRC, there was a recent thread about some issues hardening the latest batch of NJSB's 1095 after too many normalizations. I believe the recommendation was to skip the normalizing. This may (or may not) have contributed to what you're seeing. Whether you finish it to sell, keep, or try re-heat treating it with a more purposeful hamon (or to possibly get rid of that spot), well, that's up to you. Some guys will be ok with it, and others may not be.
     
  9. killthelights

    killthelights

    149
    Jan 6, 2016
    So basically just an aesthetic thing. Ok. I can deal with that. As far as the NJSB steel issues it's funny because that's WHY I normalized it. I ususally don't. I've heard his steel could be a little tricky. I guess I'm off 1095. Oh well. Thanks!
     
  10. weo

    weo KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    Sep 21, 2014
    What's the definition of 'quite a bit'? Was this sanded on a grinder or by hand?
    I've had decarb deep enough that there's some still hints after going through 120-220-400 on the 2x72 and then 220, 400, 800, 1500 by hand.
     
  11. Joshua Fisher

    Joshua Fisher KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    382
    Mar 27, 2018
    Personally on a small blade I don’t think it’s a big issue as long as all of the edge is hardened and you have a good amount of hardened material before one would hit the soft spot. On a bigger blade made for hard use I think it’s more of an issue because that spot will not be tempered the same as the rest of the blade which could lead to a bend or set if someone hit it wrong unless you treat the rest of the spine the same by blue backing it so it’s more even instead of one concentrated spot. You could also acid etch then stonewash the blade and I bet you wouldn’t be able to see the spot. It may also be a good destruction test knife to see how the heat treat went if you weren’t happy with the aesthetics.
     
  12. killthelights

    killthelights

    149
    Jan 6, 2016
    I started by cleaning it up with 120 on the SGA. Then 220, 320, and 400 on the 2x72. And lastly hand sanded 220, 320, and 400.
     
  13. killthelights

    killthelights

    149
    Jan 6, 2016
    The knife is 12"OAL chef's knife with a 7" cutting edge and the spot is about 1"x2". Gonna etch later today and see what happens.
     
  14. Joshua Fisher

    Joshua Fisher KnifeMaker / Craftsman / Service Provider Knifemaker / Craftsman / Service Provider

    382
    Mar 27, 2018
    Gotcha, no point blue backing a chef knife, I don’t think it’s an issue then, it shouldn’t be used for batoning or any rough work. Best bet for blending it in would be either the acid stonewash or just a utility belt finish.
     
  15. killthelights

    killthelights

    149
    Jan 6, 2016
    Right on. I agree. I appreciate the help. Thanks!
     
  16. robwil

    robwil

    57
    Aug 18, 2007
    Does the spot show up on both sides of the blade?
     
  17. tkroenlein

    tkroenlein

    155
    Dec 10, 2016
    Looks like that spot wasn't quenched quickly enough to harden, which might be logical given it is present in the area of greatest mass. I'd be afraid that the edge wasn't fully hardened.
     
  18. killthelights

    killthelights

    149
    Jan 6, 2016
    I'm sanding the other side now.
     
  19. killthelights

    killthelights

    149
    Jan 6, 2016
    I'm also concerned. Gonna finish and test it.
     
    tkroenlein likes this.
  20. killthelights

    killthelights

    149
    Jan 6, 2016
    Yep. Same exact spot on the other side. So i guess that solves the mystery. It just didn't harden in that spot. Which is weird.
     

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