Carbide Size and Sharpening: Why Some Steels Feel Different

Two steels with similar hardness numbers can feel completely different under a sharpening stone, one glides along smoothly, another feels like it’s dragging against sandpaper. That difference almost always comes down to carbides: how many there are, how big they are, and how they’re distributed through the steel.

What Carbides Are

Carbides are hard compounds that form when carbon bonds with alloying elements like chromium, vanadium, or molybdenum during the steel’s manufacture and heat treat. They’re significantly harder than the surrounding steel matrix around them, which is exactly what makes them useful: carbides are what actually resist wear and hold a working edge as a blade cuts through abrasive material.

Why Carbide Size Matters for Sharpening

Larger carbides are harder for an abrasive (a sharpening stone or grinding belt) to cut through cleanly, since the abrasive grit has to actually be harder than the carbide to remove it rather than just skating over or around it. Steel with large carbides, common in some higher-carbide conventional (non-powder-metallurgy) steels, can feel gritty or resistant under lower-grit abrasives, and take noticeably longer to sharpen than a steel with the same hardness but finer carbides.

Why Powder Metallurgy Steels Sharpen More Predictably

As covered in Carbon Steel vs. Stainless vs. Tool Steel vs. Powder Metallurgy, the PM manufacturing process produces much finer, more evenly distributed carbides than conventional casting does, even at the same overall alloy content. This is a big part of why steels like CPM MagnaCut or CPM S35VN can carry high vanadium content (which would otherwise mean large, difficult carbides) while still sharpening more predictably than a conventionally-cast steel with similar vanadium levels.

Matching Abrasives to Carbide Hardness

Standard aluminum oxide abrasives handle simple carbon steels and lower-vanadium stainless steels well. High-vanadium steels benefit significantly from diamond or CBN (cubic boron nitride) abrasives for sharpening, since those abrasives are hard enough to actually cut through vanadium carbides rather than just wearing down around them, which is why sharpening kits marketed for “super steels” usually lean on diamond stones rather than standard oxide or ceramic stones.

Carbide Volume vs. Carbide Size

These are related but separate factors: a steel can have a high total volume of carbides (more wear resistance overall) while still having small, fine individual carbides (easier sharpening) if the manufacturing process supports it, which is exactly the PM steel advantage. Conversely, a steel with the same carbide volume but larger individual carbides will generally be harder to sharpen and can leave a less refined edge, even at comparable overall hardness.

What This Means for Edge Retention vs. Sharpenability

Carbides that resist wear during use are the same carbides that resist abrasives during sharpening, it’s the same hardness working for you in one case and against you in the other. This is the practical root of why high-edge-retention steels are often described as “harder to sharpen”: it’s not really the overall hardness number causing that, it’s the size and hardness of the carbides embedded in the steel.

Why does my high-end steel feel harder to sharpen than my simple carbon steel?

Almost certainly carbide hardness and size, not the base hardness (HRC) number. Vanadium-rich steels form carbides hard enough to resist standard abrasives, requiring diamond or CBN stones to sharpen efficiently, while simple carbon steel has few or no hard carbides standing in the way.

Do I need diamond stones for every steel?

No. Simple carbon and low-alloy stainless steels sharpen fine on standard aluminum oxide or ceramic stones. Diamond or CBN becomes genuinely worth it once you’re regularly sharpening high-vanadium PM steels, where standard abrasives struggle and take much longer to get results.

Does carbide size affect edge toughness, not just sharpening?

Yes. Very large carbides can act as stress concentration points, potentially making an edge more prone to chipping under lateral stress, which is another reason the fine, even carbide structure from powder metallurgy tends to perform well: not just easier sharpening, but often better edge stability too.