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