Carbides in Maxamet

Carbides in steel are often compared to gravel in concrete. While there are some parallels, such an improving wear resistance, the analogy fails in the prediction that carbides can “pop out” of the matrix.

Worn concrete sidewalk with exposed gravel.

We are all familiar with the surface voids produced in concrete when a stone from the mix falls out after beings exposed by wear, and it is tempting to believe that carbides in steel will behave the same way.

Worn concrete sidewalk where a piece of smooth gravel has come loose, leaving a hemispherical cavity.

Under the category of Myths Busted, this article presents a few images of a knife blade fabricated from carbide-rich Maxamet Alloy (Carpenter Technology). These images will demonstrate that the small smooth carbides, typical of this type of tool steel, are well adhered to the matrix and do not “pop out” when exposed.

The manufacturer provides the following weight percentages of constituent elements. Because Tungsten has an atomic mass of more than three times that of iron (183.85 vs 55.85) it should be understood that the atomic percentage is closer to 4%.

Carbon2.15%Manganese0.30%
Sulfur0.070%Silicon0.25%
Chromium4.75%Cobalt10.00%
Vanadium6.00%Tungsten13.00%
IronBalance   
Elemental mapping of a small region of polished Maxamet showing that several types of nearly spherical carbides in the range of 1-2 microns in diameter occupy about 1/4 of the volume of the steel.

I purchased a Spyderco Native 5 in Maxamet steel from Blades Canada and used it for various urban tasks including cutting down cardboard boxes until it was obviously dull but still able to slice copy paper.

SEM image of the factory edge after sufficient use to dull the blade.

The blade appears to have been sharpened with a grinder an then buffed to slightly abrade the matrix around the carbides.

The apex is blunted as the iron matrix has been eroded, leaving carbides exposed.

Although the blade was obviously blunted (I could drum my fingers on the blade with little concern) it still retained good slicing aggression. It was able to slice copy paper reasonably well, other than catching on the few micro-chips that had formed. Microscopic examination indicates that the residual slicing aggression is likely due to the exposed carbides along the apex.

The blade was easily sharpened with Wicked Edge diamond stones (200,600,1200) at 17 degrees per side and the apex polished with 1 micron diamond lapping film at 17.5 degrees per side to produce a thin foil-burr.

A foil burr is produced by the diamond lapping film, as it normally does.
The carbides, slightly darker circular regions, remain in place even in the foil, which has flexed back and forth under the lapping film.
Two carbides (lighter circular regions) in the foil burr show cracking, but little evidence of “popping out” from the surrounding matrix.

This polished apex is quite unimpressive, and obviously does not expose the carbides which should be the goal if we want a blade with long lasting slicing aggression.

To “de-burr” the blade and expose the carbides at the apex, the blade was micro-bevelled with a Shapton Glass 8k stone (by hand). The fine aluminum oxide erodes the iron matrix around the much harder carbides.

The blade was then used for various cutting tasks and imaged at three stages; after light, moderate and heavy use.

Shapton Glass 8k micro-bevelled Maxamet after light use.
Shapton Glass 8k micro-bevelled Maxamet after moderate use.
Shapton Glass 8k micro-bevelled Maxamet after heavy use. Minimal blunting is observed with carbides still remaining at the apex. In the upper part of the image, the bevel is galled with aluminum.

Some time ago, I was asked a question by Stefan Pusch about an observation discussed at http://kochmalscharf.freeforums.net/ regarding an observed improvement in edge retention for high carbide steels when finished with a JNat hone. Below, I show that a JNat has the same effect as the Shapton Glass 8k, to expose the carbides near the apex. Perhaps this helps to explain that earlier observation.

Maxamet blade sharpened with Wicked Edge diamonds (600 and 1500) at 17 degrees and polished with 1 micron diamond lapping film at 17.5 degrees. The blade was then micro-bevelled with a Japanese Natural Asagi finishing hone. The silica abrasive in the hone is incapable of abrading the carbides, but removes the matrix around them, exposing them at and near the apex.
JNat micro-bevelled Maxamet blade. The carbide is obviously very well adhered to the matrix and in no danger of “popping out”
JNat micro-bevelled Maxamet blade. Carbides are exposed and firmly adhered.

To demonstrate how well adhered the exposed carbides are, the blade was used to make 20 slicing cuts in to hard plastic and the edge imaged again.

JNat micro-bevelled Maxamet blade following 20 cuts into hard plastic. Carbides at the apex remain in place.
High magnification image of carbides in the JNat micro-bevelled Maxamet blade following 20 cuts into hard plastic.
High magnification image of carbides in the JNat micro-bevelled Maxamet blade following 20 cuts into hard plastic.

This article has presented a few images from one particular knife, and is obviously not intended to be a systematic study. These images do provide clear evidence that, unlike gravel in concrete, the carbides in Maxamet steel are very well adhered to the surrounding matrix.

  15 comments for “Carbides in Maxamet

  1. Ben Spaloss
    November 3, 2019 at 1:54 pm

    You mentioned a polished edge on maxamet is unimpressive due to exposed carbide being needed for slicing aggression. I’m assuming you chose the shapton 8k in the progression you did so you can best see the carbides. I hypothesize that a coarse edge will have a more aggressive edge and will hold that aggression longer. Do you think aggression is really the cause of carbide or the type of edge that is used?

    Like

    • November 3, 2019 at 2:19 pm

      There are several micro-geometries that produce an aggressive slicing edge; however, based on the way this particular type of steel dulls, exposing the carbides seems like an obvious approach.

      Like

  2. Big Brown Bear
    November 5, 2019 at 12:20 am

    Let’s see the edge with a bonded CBN/Diamond Stone finish, shapton glass and jnats are not the right tools for this steel.

    Like

  3. Big Brown Bear
    November 5, 2019 at 12:26 am

    That was a great read Todd, I want MORE! More steels, more stones

    Like

  4. Rick
    November 5, 2019 at 8:58 am

    Where does the idea that carbides pop out come from? Is there any evidence of it happening in other steels?

    Like

    • OhioApexing
      November 6, 2019 at 11:42 pm

      Great read — I second what BBB said; more abrasives, more steels!

      I wonder if the stability of these carbides is impacted by the relatively high HRC of the surrounding matrix found in Maxamet and furthermore, if the same effect would be witnessed in something like M390, S30V, etc. at say, 59 HRC?

      Like

    • November 7, 2019 at 8:25 am

      R. Landes proposed that in his book back in 2006 and from what I have seen with my (optical!) Zeiss Epiplan metallurgical microscope it seems that it really depends on the hardness of the surrounding matrix whether or not the carbides will stay “anchored” within the matrix or not when sharpened with too soft abrasive like in this case Shapton Glass 8k. I think the comparison between diamond and Shapton here is beautiful and I would love to see CPM S125V. Because, if you care, on Instagram TV (my Instagram handle is @kknives_switzerland) I show on video the effects of sharpening with diamond versus shapton glass stones on CPM S125B at 64HRC. While admittedly my resolution of course is not like an SEM, we still see how the edge remains with chunks missing after the Shaptons. If you want to watch; it is my first and second video, so you gotta scroll. Maybe the blog author would also like to watch and maybe show us some CPM S125V

      Like

    • November 7, 2019 at 3:57 pm

      I have never seen any evidence that carbides “pop out.” However, we may need to argue about the semantics of what “popping out” means.

      There are some images (not mine) that show micro-chips in the apex, and it has been claimed that these are the result of carbide pop-out. I am fairly confident that these are actually showing broken carbides, or carbides in a matrix that has been heavily damaged/softened, not carbides that have simply come loose from the matrix.

      What I have shown here is that even with 85% of the surface area exposed, exposed carbides can hold on the to the matrix under harsh cutting conditions.

      Vanadium carbides are quite hard and brittle, and can easily be crushed into tiny bits with the “wrong” sharpening technique, for example using a ceramic honing rod or a Spyderco Sharp Maker.

      Like

      • Ben Spaloss
        November 7, 2019 at 11:22 pm

        Vanadium carbides can be crushes? That’s incredible. I’ve never heard of this. Can you elaborate on this please?

        Like

      • November 20, 2019 at 4:28 am

        That is a very good input! By softened you mean overheated from dry ginding/sharpening?
        Now, as a baseline: Would agree with the tenor of most sharpeners (me included) that using ceramics or aluminum oxide on high vanadium carbide steels will give inferior initial sharpness due to reasons yet to be scientifically identified? It would be amazing if you could do a BESS sharpness test on both these edges you showed in the images and plot the initial sharpness of differen sharpening media against each other. (Assuming that they are sharpened at the same angle.)
        Thank you for all your work which I follow with great interest!
        Roman Kasé
        http://www.kase-knives.com

        Like

        • November 20, 2019 at 8:13 am

          Understand that a keen edge requires a dimension of around 100nm (1/10 of a micron) and the vanadium-rich carbides in Maxamet, or S110V, or even the chromium carbides in ZDP189 are 1-2 microns in size. So sharpening these knives to a keen edge with ceramic involves crushing those carbides (near the apex) into “nano-dust” and burnishing the matrix around them. (This is not heat-related).

          I would argue that it makes no sense to sharpen these steels into a keen edge, but rather to “celebrate” the wear-resistant micro-texture of carbides exposed at the apex, just as we do with the micro-texture of ceramic knives.

          Like

          • November 21, 2019 at 8:55 am

            Thus, a high-carbide knife is a saw, not a razor.

            Like

          • Ben Spaloss
            November 22, 2019 at 3:01 pm

            Exposing the carbides to maximize edge retention entirely depends on what edge is being held. Exposing the carbides in maxamet will not help when holding a keen edge since a keen edge will never be achieved in the first place. High carbides steels can hold a high level of sharpness edge if they are sharpened and more importantly deburred and derooted (outlined in Vadim’s book) properly. As a result, in my case, exposing the carbides does not maximize edge retention because I look for edge longevity in my knives. Maybe if I’m cutting a ton of rope or cardboard the situation would change

            Like

  5. November 5, 2019 at 10:53 am

    Thank you for this Test!

    I am one of the KMS guys using steel like Vanadis 23, Р12М3К5Ф2-мп and so on in the kitchen.

    I use Shapton Pro till 12k, then switching to Dia Lapping Film 1my, after this a short Visit on Shapton 30k and some very light passes on Jnat… Sometimes especially for push cutting i use surgical Black Arkansas or Jaspis as finisher. Works well.

    Greets Sebastian.

    Like

  6. keyresults
    November 11, 2019 at 6:17 pm

    As always, your article is ever so enlightening! My biggest regret is that I didn’t have your excellent library available before succumbing to so many “myths” and misunderstandings that drove my “investing” in equipment and materials and techniques that are simply unnecessary or non-optimal use of time.

    I really appreciate your consistent and objective presentation of results. I also appreciate your interpretations from time to time also. Your website has become very important to a whole bunch of people past, present, and future. Thank you so much for all the incredible amount of work and commitment to excellence.

    Kenny

    Like

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