Carbide Grades Explained for Buyers: What P25, K20, M30 Actually Mean

July 24, 2026 · 6 min read · Procurement Basics

If you're responsible for buying carbide tooling and a supplier sends you a quote for "P25-grade CNMG inserts," you need to understand whether that grade is right for your application — or whether the supplier is just quoting whatever they have in stock.

This guide explains carbide grades in plain English. No metallurgy degree required.

The ISO System: Six Letters, Six Material Groups

Every carbide insert and end mill falls into one or more ISO application groups. The letter tells you what material it's for. The number tells you whether it's for roughing or finishing.

The Numbers: Roughing vs Finishing

The two-digit number after the letter indicates the toughness-to-wear-resistance balance:

• 01-15: Very hard, wear-resistant. For finishing cuts at high speeds. Think smooth surface, light depth of cut. These grades are brittle — don't use them for roughing.
• 20-30: Balanced. The "general purpose" sweet spot. Covers 80% of machining operations. This is the range you want for most orders.
• 35-50: Tough, impact-resistant. For roughing, interrupted cuts, and unstable setups. These grades can take a beating but wear faster at high speeds.

So a P25 grade is a general-purpose steel-cutting carbide. A K10 is a finishing grade for cast iron. An M40 is a tough grade for roughing stainless steel.

What This Means for Procurement

When you're sourcing carbide tooling, specifying the grade is just as important as specifying the geometry. Here's what to ask suppliers:

1. "What ISO grades do you offer for this insert?" — A legitimate factory should be able to name specific grades (e.g., "P25-P35, M20-M30") with actual test data, not just "good for steel."
2. "Is this your own substrate or rebranded?" — Many small factories buy carbide blanks from a larger producer and just grind the geometry. This isn't necessarily bad, but you need to know so you can verify the substrate quality.
3. "What coating do you recommend for this grade?" — The grade + coating combination is what determines performance. CVD for turning steel. PVD for milling and stainless. A supplier who can't explain the coating choice doesn't understand their own product.
4. "Can you provide a competitor cross-reference?" — Ask them which Sandvik/Kennametal/Iscar grade their grade is equivalent to. If they can't answer, they haven't benchmarked against industry standards.

Common Grade-Related Mistakes Buyers Make

• Buying by price only: A $2 insert that lasts 10 parts costs more per part than a $3 insert that lasts 20 parts. Always compare cost per edge, not cost per insert.
• Not matching grade to material: A P-grade (steel) insert used on stainless (M-grade) will wear 3-5x faster. The geometry looks the same. The substrate is completely different.
• Ignoring coating: An uncoated P25 insert costs less but lasts a fraction as long as a CVD-coated P25. The coating is not an "extra" — it's an integral part of the grade.
• Assuming all P25 grades are equal: Sandvik GC4325 and a generic "P25" from an unknown factory are not the same thing. The ISO code defines the application range, not the quality level within that range.