Carbides in K390

I’ve titled this article “carbides in K390” with the idea that it will be part of a series that aims to help understand the role of carbides in knife performance and sharpening. The first article in this series, Carbides in Maxamet, dispelled the myth that carbides are weakly bonded to the surrounding steel matrix. Of particular interest in this article is the poorly understood interaction between relatively coarse (40-50 micron) grit stones and the 1-2 micron carbides found in powder metallurgy steels. As usual, we should be cautious about generalizing these observations to all examples of K390 or similar steels, or jumping to conclusions about how these microscopic observations translate to macroscopic performance. However, the observations I show here are consistent with those I have made in other samples – while they may not be the rule, they are definitely not the exception.

The particular knife used in this article is a Spyderco Endela in K390 that I purchased from Blades Canada. K390 is one of the currently popular high (9%) vanadium powder metallurgy steels. With a carbide volume fraction of around 17%, carbides in or at the apex play a major roll in how a knife made from this steel will perform and will affect the sharpening protocols required to maximize that carbide-enhanced performance.

The factory edge shows the typical morphology of carbides exposed by buffing. Grinding lines are visible in the bevel 20 microns away from the apex. Exposed carbides with matrix shadow trails dominate the last 20 microns of the bevel.

Factory edge of the K390 Endela with exposed carbides and shadow trails of steel.
Factory edge of the K390 Endela with exposed carbides. The carbides create “shadows” in the matrix metal downstream of the buffing direction (edge trailing).
Cross-section view of the factory edge of the K390 Endela shows no evidence of damage to the carbides or the matrix.

The factory edge performed adequately, slicing paper cleanly.

Cotton resume paper cleanly sliced with the Endela factory edge.
Close-up of the severed cotton paper fibres shows texture matching the exposed carbides.

High vanadium steels like K390 can be challenging to sharpen as vanadium carbides are harder than typical aluminum oxide abrasive and grind/wear relatively slowly. In particular, this wear-resistance may lead to difficulty with burr formation as the apex is more likely to bend away from the stone rather than being cleanly abraded. This repeated bending of the apex may also weaken the steel, leading to micro-chipping. It is sometimes claimed that these micron-scale carbides have minimal impact on coarse grinding, with unsubstantiated claims that coarse grit stones can simply scoop out or “pop-out” the carbides. The observations I show here suggest that diamond and silicon carbide are able to abrade the vanadium carbides and remove steel sufficiently well to form a new apex. The cut depth is less than the typical carbide diameter – there is no evidence that carbides are scooped or popped out during sharpening.

Shown below is the K390 blade sharpened on a DMT Coarse diamond plate, where a moderate burr was formed. This the default result where minimal effort is made to avoid/remove the burr. This particular burr can be felt when brushing fingers off the edge. Cracked carbides are visible near the end of the burr, presumably broken during repeated flexing of the apex.

The K390 Endela readily forms a burr when sharpening on a DMT diasharp coarse.
Close-up of the cross-sectioned burr formed by the DMT-C shows damaged carbides in the most flexed region.

Another interesting observation is that the carbide near the surface appears to be pulverized and partially smeared out in the direction of sharpening while carbides one micron below the surface are intact.

Close-up of a carbide near the surface that has been partially ground showing severe cracking.

The knife was sharpened again on the DMT Coarse, but this time with alternating, edge leading strokes, which still results in a triangular burr, although not one that can be felt or perceived in any way. Again, the part of the burr that was flexing on the diamond plate displays cracked carbides. The burnishing action of the coarse diamond plate can produce a keen edge, however the cracked carbides will likely reduce its wear resistance. This is also likely one mechanism by which microchips form.

Cross-section of the K390 Endela sharpened freehand on the DMT-C at low angle (and minimal burr) displays cracked carbides in the region that flexes during sharpening.

The knife was sharpened again, this time on a Shapton Pro 320, freehand at about 33 degrees inclusive. A much smaller burr results, as compared with the diamond plate, with cleanly abraded carbides at the bevel surface. This small burr could not be detected by traditional methods.

Cross-section of the K390 Endela free-hand sharpened on a Shapton Pro 320 at a more substantial angle where minimal flexing occurs shows only a small burr and no apparent carbide damage.
Close-up of a carbide partially abraded by the Shapton Pro 320.

The knife was sharpened again, this time on a Sigma Power Select II 240 grit stone at around 31 degrees inclusive angle. Once again, a relatively small burr is formed, and one that is not detectable by traditional methods. The sharper abrasives and the presence of loose particles in the mud likely minimize the burr formation as compared to the diamond plate.

Cross-section of the K390 Endela sharpened at around 30 degrees (inclusive) on a Sigma Power II 240 hone showing minimal burring and carbide damage.

Fractured carbides at the surface of the bevel suggest that the abrasion mechanism involves fracturing the carbides rather than simply abrading them or “removing them whole.”

Close-up of the K390 Endela sharpened on the Sigma Power II 240 shows partially abraded carbides at the surface with severe cracking.

After sharpening on a Shapton Pro 320, the knife was micro-bevelled using a translucent Arkansas stone. This type of stone generally removes steel by adhesive wear and its silicon oxide composition is much softer than the vanadium carbides in the K390 steel. Not surprisingly, it is relatively ineffective, but it does produce some interesting results. First, as with the factory edge, carbides are exposed as the matrix is wears faster than the carbides (if the carbides wear at all). Based on the amount of metal removed, this procedure has likely just exposed the carbides previously shaped by the coarse stone.

Following sharpening on a Shapton Pro 320, a micro bevel was applied to the K390 Endela with a translucent Arkansas stone.
The surface of the K390 Endela abraded with the translucent Arkansas shows exposed carbides with no shadow trails due to the use of circular motions in honing.

Close examination of the carbides on the micro-bevel surface show some evidence of wear (or flattening) of the carbides. There is no evidence of carbides being dislodged or “popped out.”

Cross-section of the translucent Aransas honed 390 Endela shows carbides that are flattened, but without other apparent damage.
A few carbides exposed in K390 by micro-bevelling with a translucent Arkansas stone.

For comparison, the Spyderco Native in Maxamet steel was sharpened in the same way and micro-bevelled with the translucent Arkensas. Again, I was not able to find a hole or pit that might indicate a “popped out” carbide, but the one shown below did appear mostly excavated.

Carbides exposed by the Arkansas stone in Maxamet

To investigate, I cross-sectioned through this particular vanadium-rich carbide and the tungsten-rich carbide on its right.

Cross-section view of a carbides exposed by an Arkansas stone.
The same cross-section view of the exposed carbides, but in back-scatter contrast, shows the bright tungesten-rich carbide and the slightly darker vanadium-tungsten carbide. The pulverized remains of a vanadium-tungsten carbide are visible on the upper-left side of the image.

Contrary to popular belief, coarse stones do not cut deep enough to “scoop out” 1-2 micron diameter carbides in these very hard steels. Instead, the mechanism appears to be that the carbides are abraded/worn in place, flattening and thinning them until they are thin enough to shatter and be removed along with the metal swarf.

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  22 comments for “Carbides in K390

  1. Edward Ocampo-Gooding
    September 14, 2021 at 8:56 pm

    Just here to say that I love this blog and tell everyone I meet interested in sharpening about it.

    Thanks for the article!


  2. John harper
    September 14, 2021 at 9:50 pm

    Hi Todd,

    Thank you for yet another very informative article.

    How did you create the cross sections shown?

    Keep up the good work, and



    • Antonio Takimi
      September 15, 2021 at 8:25 am

      Focused Ion beam


  3. Leith Murray
    September 14, 2021 at 9:53 pm

    I own spydercos in Maximet and K 390 in Australia were I live these knives are expensive.
    However the superior edge I feel is worth it.
    Your writings are absolutely brilliant, ThankYou for all the dedication.
    I look forward to further articles


  4. Phil Vardy
    September 14, 2021 at 10:11 pm

    Sent from my iPhone


  5. Mark Stump
    September 14, 2021 at 10:32 pm

    Thanks again


  6. September 15, 2021 at 8:33 am

    You are our Myth Buster, LOL. Wow. I guess I’ll keep both my DMT and my Shapton Pro for these high Vanadium steels. It would seem there is a place for both, especially in repair / reprofiling.

    Just for the sake of my curiosity, I wonder about the edge leading or trailing stroke when you sharpened on the Shapton and Sigma stones?

    The surface fracturing of the carbides puzzles me, and I wonder if heat is a contributing factor?

    As always, you do not disappoint!



    • September 15, 2021 at 9:21 am

      I ended with edge leading, alternating sides to minimize the burr.

      Heat is not a cause of damage, it is the energy liberated by mechanical damage. The carbides are like glass – they will break rather than bend.


  7. Scott Guthrie
    September 16, 2021 at 9:22 am

    Hey Todd:

    I just was able to give the article a
    good reading – which for me is about
    4 times through – and
    think this is one of your best !

    The photo’s are particularly nice:
    they depict just what we need to see,
    they are sharp with detail, and they
    display the carbides so well.

    The distinction between vanadium
    and tungsten has never been so clear!

    Once again, we are stimulated to
    experiment with diamond AND
    natural stones, and to think about
    leading and trailing strokes……

    Cheers, Scott


    • Scott guthrie
      September 16, 2021 at 1:29 pm

      Todd: I ran your article past a friend of some 50 years that is also a motorcycle racer, and he had a couple of questions that you might be able to help with.

      Here’s the first:

      “Another question I have is about corrosion.

      I have often seen knives become dull from
      sheer exposure to the planetary atmosphere.

      The sharper the edge, the more totally
      surrounded by oxygen are the few atoms
      that make it up. And so they eat the metal,
      which yearns to return to the status of ore
      in the embrace of loving oxygen.

      Gradually the tip radius grows.

      Those cracked carbides look like
      prime locations for oxygen infiltration, too.”


      • Scott guthrie
        September 16, 2021 at 1:33 pm

        Todd: Here’s the other question.

        “The carbides in these photos are not
        precipitated from the metal by heat treatment,
        but are part of the initial composition of the powder, I think.

        This is called “dispersed phase hardening”
        and something similar has been does by
        intimately mixing aluminum oxide into piston alloys
        (typically by high pressure extrusion) resulting in material
        with roughly double the properties
        of the present day piston alloys.

        F1 started down that road but the FIA
        ruled that materials hardened by
        a dispersed phase are composites.

        So, back to reinforcing piston alloys only
        with precipitated phase hardening, such as 2618.

        Wonderful stuff – thank you.”


  8. September 18, 2021 at 3:27 pm

    Thanks Todd

    Awesome images and discussion.
    If anyone here could point me to info dispelling the toothy edge myth I would appreciate it.




    • September 23, 2021 at 1:10 pm

      I’m working on a piece that will address this more directly.


      • Edward Ocampo-Gooding
        September 25, 2021 at 2:17 pm

        Very excited for this. Thank you!


  9. Vic
    September 23, 2021 at 12:33 pm

    I remember Cliff Stamp once microbeveled a K390 knife with an Arkansas stone and was surprised that it worked (the knife got sharp). He hypothesized that the reason for that is that quartz is quite a bit harder than is commonly assumed. However, it seems that superior hardness is not required and the carbides can be worn and reduced by mechanisms other than cutting.


    • September 23, 2021 at 1:08 pm

      I don’t believe these images show evidence that the Arkansas stone cut the undamaged carbides, but I did find vanadium on the surface of the arkansas stone. I would speculate that the Arkansas stone can wear away the damaged layer (including crushed carbides) left by the coarse stone, but once it reaches “whole” carbides it stops working.


      • Vic
        September 24, 2021 at 8:47 pm

        I have a natural stone (not Arkansas, but a similar thing – microcrystalline quartz) that works for microbeveling a 440C knife. I was always curious why that’s the case. Somehow the stone must be reducing very large chromium carbides. It does seem to me that high pressure during microbeveling is important.
        If you intend this article to be a part of a series, that’s great. Chromium carbides can be very large, so investigating 440C, D2 or the like should clarify what’s going on.


  10. Vic
    September 26, 2021 at 8:40 pm

    “Fractured carbides at the surface of the bevel suggest that the abrasion mechanism involves fracturing the carbides rather than simply abrading them or “removing them whole.”

    Speaking of which, sharpening crystalline materials by striking them or pressing hard into them has a very, very long history…


  11. October 1, 2021 at 2:11 am

    Very interesting results. Diamond fracturing hard carbides is not really a surprise, but in this very case a testament to the excellent bond of the carbide and the matrix.
    As to the wearing of the carbides by the Arkansas stone that is somewhat unexpected. They do show some wear with the flattened tops and it is hard to say that it was done by the silicon oxide and not some VC on VC wear by removed carbides in the slurry.
    That VC is quite the material and few things might polish it that are readily available. Titanium carbide might work.


  12. Maxim bellehumeur
    November 8, 2021 at 11:50 am

    These are very interesting, each new research you make on carbide seems to change how I understood many things about sharpening.

    How do you think sharpening VC steel on diamond vs softer abrasive change the edge retention.
    And I’m curious about the newer resin bonded diamond stone we are seeing recently, do you think they fracture the carbide less since they are supposedly less aggressive.


    • Edward Ocampo-Gooding
      November 8, 2021 at 11:54 am

      Happy to sponsor research around the resin bonded diamond stone issue. I’ve a friend who uses them in kezuroukai competitions and I’m curious!


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