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Why is harder/more abrasion resistant steel more brittle with lower grit stones?

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  • #40679
    Mark76
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    This question is based on a number of topics that were recently posted here.

    A number of people observed that hard and abrasion resistant steel is more brittle than softer and less abrasion resistant steel. Someone wrote that such steels are less ductile and that sounds quite logical to me.

    However, what was also observed is that these steels (in particular Maxamet and ZDP 189 steel) start to chip when sharpened with fine stones (after a progression with more coarse stones). There also seems to be a tipping point in terms of stone fineness after which the edge starts to chip. It was suggested that the fine stones make the edge (even) thinner, but this is what I don’t understand.

    Coarse stones remove far more steel than fine stones, so why would the fine stones suddenly cause the chipping? And why (it’s a hypothesis) the chipping point?

    All ideas are highly appreciated!

    Molecule Polishing: my blog about sharpening with the Wicked Edge

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    #40731
    sksharp
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    Hey  Mark,

    The problem as I see it, when your using the coarser stone they don’t take the edge to it’s thinnest state due to the scratching, more metal there because of the deeper scratches. The more and more it’s refined the thinner the actual cutting edge is becoming until it reaches the point that it’s to thin to hold together or keep from chipping. I suspect that the chipping is happening all the way thru but doesn’t manifest until the scratches from the stones are smaller than the chipping that’s probably been occurring all the way thru. If refined even further it will seem to just fall away. This seems to happen on some harder steels sharpened at to acute of an angle and by taking the angle out a bit the problem is greatly reduced. Having said all that I don’t have experience with the so called super steels other than S30V which isn’t considered “super steel” anymore but is considered harder to sharpen that some other steels. Remember 154cm? Used to be considered a super steel in its day but that steel still has the ductility to hold together pretty well and will take a great edge and hold it pretty darn well. I do have some experience with more inexpensive steels that in my opinion were over hardened and not tempered back correctly and those are very difficult if trying to sharpen at to acute of an angle. They are advancing the steels for knives faster than ever these days and I fear that some of them are more hype, to get us buy them, than substance. I do how ever think that some of these steels are legitimate and have proven them selves to be a better steel than in the past. It’s hasn’t been that long ago that steel was not being made for knives specifically so it ‘s a fairly new science in some respects. In the past knives were made from steel that was designed for something completely different and the trick to get those steels to work to there potential for a knife is the heat treating and tempering to get the hardest steel possible but enough ductility to hold together. Now the finer grain steels that they are making help those steels hold together better but all steel has a tipping point between hard and ductile. Most of this post is theory and opinion so take it for what it is worth.

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    #40740
    tcmeyer
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    I’ve probably done 12 – 20 sharpening on my two ZDP knives and have never seen chips appear with finer grits.  I have had chipping problems, but it was traced to an 800-grit diamond stone with clusters of diamonds which stubbornly resisted my efforts at breaking in.  I eventually went to working the stones over a piece of plate glass and within a few strokes, the clusters were gone.  Is t possible that this is what happened in those reported cases?

    Here’s a photo of the chips on my Delica:

    delica chip annotated

    And here’s the cluster that did it:

    800 #3 compressed

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    #40742
    Mark76
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    Thanks, guys. As you may have guessed I really don’t know whether it is a common phenomenon or something due to a problem with stones or pollution. I don’t sharpen “super steels” very often and I also don’t examine every edge through a microscope.

    I cannot wait until I can sharpen again. Then I’ll definitely repeat the experiments of MarcH and others!

    And thanks, skssharp, what you write sounds quite logical. Of course, it is theory or a hypothesis, but it might well be the reason if it is not due to a stone/film problem or pollution!

    Molecule Polishing: my blog about sharpening with the Wicked Edge

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    #40743
    sksharp
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    As I stated above I have not done any of the specific steels mentioned above, not willing to spend $200 or $300 on a knife at this point. I have run into problems of chipping and the edge seeming to fall away after getting past a certain point in the progression and had to work it out. On most of these going from say 30 deg.(inclusive) to 40 deg.(inclusive) pretty much resolved the issue.

    I was not commenting on any one particular steel and, like I said, some of the steels that they are developing and coming out with I to am excited to work with.

    The steels that I’ve run into this problem with are inexpensive and I suspect coarse grain and not heat treated and tempered properly in my opinion.

    My comments were just theory on some problems that others have run into with SOME so called super steels(several different ones) that they were having problems with. Some of those steels have a reputation for being very difficult to sharpen, for what ever reason. I’ve done a couple with S30V , those have warnings when you buy them from some manufacturers, Buck in particular, for being difficult to sharpen. Personally I have not had a problem sharpening those knives what so ever. There are a hundred or more reasons that chipping could occur and so please don’t think I was saying or blaming any one steel in particular for being substandard.

    I do believe that no matter what steel you are working with, if it’s not mixed, heat treated and tempered back properly for that particular steel, the steel will not work well or at least as well for a blade that it should and in my mind is the challenging part for any one developing “new steel”. This problem is more prevalent with inexpensive knives I fear as I do also believe that you get what you pay for, at least in most cases.

    Thank you both, Mark and Tom… I love kicking these things around with all of you guys and in a lot of cases you have changed my view on things.

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