Why are some blades harder to sharpen than others?

Not all knife steels are created equal. Two knives of different steels (or even two of the same steel tempered differently) can behave very differently under the stone. The key factors are steel composition, hardness (HRC), and wear resistance, which largely comes from hard carbide particles in the steel. In general, a higher hardness and more alloy carbides mean better edge retention but more effort to sharpen. For example, modern “super steels” like CPM-S30V, M390 or VG-10 contain lots of vanadium, chromium, or tungsten carbides. These carbides are extremely hard and resist wear (helping the edge stay sharp), but they also tear the sharpening stone and make grinding slower. As one metallurgy guide observes, “blades with higher carbon and carbide contents are more difficult to sharpen”.

By contrast, simpler steels such as 1095 carbon steel or AUS-8 stainless have fewer alloying elements. They reach a lower final hardness and have fewer hard carbides. Those knives sharpen quickly and easily. In one widely cited sharpening difficulty scale, 1095 and AUS-8 earn a very easy 8–9 (on a 1–10 ease-of-sharpen scale), while high-alloy steels like S30V score around 5 and exotic powders (S110V, S90V) down to 1. Another way to see this is by hardness: knives tempered to 58–60 HRC (common for quality kitchen knives) are significantly harder to sharpen than blades at 55 HRC. Once you go above about 61–62 HRC (as with many premium alloys), the steel becomes very brittle and can only be reshaped effectively with diamond abrasives.

Carbide size matters too. Many powdered steels intentionally precipitate tiny vanadium carbides, which lets them achieve a keen apex but makes sharpening harder. Smaller carbides allow a finer edge, but any hard inclusion means the stone can chip out little pieces of metal (so-called “carbide tear-out”), slowing progress. Harder steels can even glaze or clog softer stones. In practice, wider, softer stones (like silicon carbide or diamond) are used on these steels. For example, a high-vanadium steel will cut very slowly on an ordinary synthetic waterstone, but a diamond stone will “not care” if the steel is 55 HRC or 70 HRC.

Examples: At the easy end of the spectrum, simple steels like 420HC, 1095, and AUS-8 are very user-friendly. They typically heat-treat to the mid-50s–low-60s HRC and contain modest carbon/chromium. These blades sharpen quickly on waterstones or ceramic rods with little strain. On the other hand, high-performance steels like CPM-S30V, CPM-M390, or VG-10 often run in the 60–62 HRC range. These will retain an edge much longer, but you’ll notice they require a bit more time and diamond abrasion to reach apex sharpness. Even tougher powders (CPM-S90V or S110V) push hardness toward 65 HRC; hand-sharpening those is famously tedious and generally reserved for experts or pros.

Heat treatment: It’s worth noting that two knives of the same steel can sharpen differently if one is tempered harder. A higher Rockwell hardness (say 61–62 HRC vs 58–59 HRC) means the steel is more brittle and harder on abrasives. Many makers purposefully temper blades around 57–60 HRC to balance sharpness with sharpenability. If a blade is tough and soft, it’s easy to grind but its edge dulls quickly. If it’s very hard, it cuts exceptionally well but asks the user to put in more work sharpening.

Practical advice: For everyday sharpening, most stainless kitchen knives do fine on good synthetic waterstones (Naniwa, Shapton, etc.), which can take either 8k–10k grit for polishing or coarse grits for re-profiling. Simple carbon knives also do well on such stones, though some enthusiasts prefer natural finishing stones (like Japanese granites) to exploit carbon steel’s “toothy” edge. When you encounter a truly hard alloy (high vanadium steels), switch to diamond or CBN stones for the coarser grits. Diamond stones are great for initial stock removal on super steels, after which you can refine on finer waterstones. Ceramic bench stones (e.g. Al₂O₃ ceramics) work decently for many steels but can glaze if used on alloys loaded with tungsten/vanadium. Finally, always finish with a very fine stone or strop: no matter how hard the steel, the finest abrasives (10k+ grit or leather strops) will put the final polished edge on the blade.

In summary, easy-to-sharpen steels (e.g. 1095, 420HC, AUS series) are lower hardness and lower carbide, making them forgiving. High-end stainless or powder steels (e.g. S30V, M390, ZDP-189) contain more hard phases and usually run rockwell 60+, so they demand tougher stones and more time. Heat treating above ~60 HRC turns any steel into a sharpening challenge. Understanding your knife’s steel and temper will help you pick the right stones: softer steels ⇒ conventional waterstones; extremely hard steels ⇒ diamond/CBN abrasives for initial work followed by fine polishing.