Carbide Grades Explained: K10, K20, P30, and What They Actually Mean

June 28, 2026 · 6 min read · Materials & Grades

Look at any carbide insert box and you'll see a grade designation: K10, K20, P30, M20, and so on. These aren't random numbers. They're part of the ISO 513 classification system that tells you exactly what that insert is designed to cut — and more importantly, what it absolutely should not cut.

The ISO Grade System in One Sentence

The letter tells you the material group. The number tells you the hardness-vs-toughness balance. Lower numbers are harder and more wear-resistant (for finishing). Higher numbers are tougher and more impact-resistant (for roughing).

P-Grades: Steel and Alloy Steel

P stands for steel — low-carbon, alloy, tool steel, and cast steel. This is the most common grade group because steel is the most commonly machined material.

The challenge with steel is that it produces long, continuous chips that generate heat at the cutting edge. P-grade carbides are formulated with higher cobalt content for toughness, plus TiC and TaC additives to resist crater wear at high temperatures.

M-Grades: Stainless Steel

M stands for stainless — austenitic, ferritic, martensitic, and duplex grades. Stainless is gummy and work-hardens easily. M-grade inserts need high hot hardness plus a sharp cutting edge to shear rather than push the material. Edge preparation is critical: a light hone or sharp edge is preferred over a heavy hone, which would increase cutting forces and generate more heat.

K-Grades: Cast Iron

K stands for cast iron — gray, ductile, nodular, and malleable. Cast iron produces short, discontinuous chips that are highly abrasive. K-grade carbides prioritize wear resistance over toughness. They typically have lower cobalt content and finer grain size than P-grades. CBN (cubic boron nitride) is often preferred over carbide for high-volume cast iron machining.

N-Grades: Aluminum and Non-Ferrous

N covers aluminum, copper, brass, plastics, and composites. These materials are soft and don't generate much heat, so the primary concern is built-up edge (BUE) — material welding itself to the cutting edge. N-grade inserts typically use uncoated, highly polished carbide with sharp edges. PCD (polycrystalline diamond) is the gold standard for high-volume aluminum production.

S-Grades: Superalloys and Titanium

S covers heat-resistant superalloys (Inconel, Waspaloy, Hastelloy) and titanium alloys. These are the most challenging materials to machine. They have terrible thermal conductivity — heat stays at the cutting edge instead of flowing into the chip. S-grade carbides need extreme hot hardness and toughness simultaneously, which is a contradictory requirement that makes this the most expensive grade category.

H-Grades: Hardened Steel

H is for hardened steel above HRC 45 — die steel, hardened tool steel, case-hardened components. At this hardness, conventional carbide struggles. H-grade is where CBN takes over. A CBN insert can machine hardened D2 at HRC 60 at 400-600 SFM with excellent tool life, while carbide would fail in seconds.

The Trade-Off That Defines All Grades

Every carbide grade sits on a spectrum: hardness vs. toughness. You can't have both. A harder grade (lower number) resists wear better but chips more easily under interrupted cuts. A tougher grade (higher number) handles impact but wears faster. Matching the grade to the operation is the single most important tooling decision you'll make.

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