ISO P/M/K/N/S/H Carbide Grade Reference Guide: Cutting Parameters, Coatings and Application Tables Compared

Understanding the ISO Carbide Grade Classification System

Every machinist and tooling engineer encounters the ISO carbide grade classification system on a daily basis, yet few fully understand how the six letter groups — P, M, K, N, S, and H — translate into real-world insert selection and cutting parameter decisions. This reference guide breaks down each ISO group, explains the underlying metallurgy and coating strategies, and provides practical cutting parameter tables that you can use directly on the shop floor.

The ISO 513 standard classifies cemented carbide cutting materials according to the workpiece material they are designed to machine. Each letter corresponds to a workpiece group, and within each group a numeric suffix (typically 01 through 50) indicates the position on the hardness-to-toughness spectrum. A lower number means higher hardness and wear resistance (ideal for finishing), while a higher number means higher toughness and plastic deformation resistance (ideal for roughing and interrupted cuts).

ISO P Group — Steel Machining Grades

The ISO P group is the most widely used category in general machining. It covers carbon steels, alloy steels, and cast steels in the hardness range of approximately 150-350 HB. Steel generates long, continuous chips and produces high cutting temperatures at the tool-chip interface, so crater wear (diffusion wear) and flank wear are the dominant failure modes.

Recommended Grades by Application

Cutting Parameters for ISO P (C45 Steel, ~200 HB)

Tip: For alloy steels above 300 HB or steels with high silicon/manganese content, reduce Vc by 20-30% and move one step tougher in grade (e.g., from P20 to P30). CVD Al2O3-coated grades are preferred for steel turning because the alumina layer acts as a thermal barrier, while PVD TiAlN grades are preferred for milling due to superior edge toughness.

ISO M Group — Stainless Steel Grades

Stainless steels (austenitic AISI 304/316, duplex, superduplex, martensitic 410/420, ferritic 430) are notoriously difficult to machine. Their high work-hardening rate, low thermal conductivity, and tendency to form built-up edge (BUE) require grades with excellent crater resistance and low-friction coatings. PVD TiAlN or AlTiN coatings are generally preferred over CVD for ISO M because the thinner PVD coating maintains a sharper cutting edge.

Cutting Parameters for ISO M (AISI 304, ~180 HB)

Critical rule for stainless: Never dwell the tool in the cut. A stationary cutting edge against work-hardening stainless steel causes severe notch wear. Use a positive chip former (e.g., Walter MP3, Iscar IC807 geometry), keep fn above 0.15 mm/rev, and use flood coolant or high-pressure coolant (70+ bar) to break the chip and suppress BUE. For duplex and superduplex steels, reduce Vc by another 20% due to their higher strength and work-hardening rate.

ISO K Group — Cast Iron Grades

Cast iron (grey cast iron GG20/GG25, nodular cast iron GGG40/GGG60, compacted graphite iron CGI) is abrasive due to free graphite and silicon carbide particles. Flank wear from abrasion is the primary failure mode. Grey cast iron produces short, discontinuous chips and runs relatively cool, while nodular cast iron behaves more like steel with longer chips and higher temperatures.

Cutting Parameters for ISO K

For high-volume grey cast iron machining, Si3N4 ceramic inserts can push Vc to 600-1000 m/min, but they require rigid setups and are sensitive to thermal shock. CVD Al2O3-coated carbide remains the most versatile choice. For CGI, a tougher K20-K30 PVD grade is recommended because CGI has higher tensile strength and produces more abrasive, longer chips than grey iron.

ISO N Group — Non-Ferrous Metals

Aluminum, copper, brass, bronze, and plastics fall into the ISO N group. The key challenge here is built-up edge caused by the low melting point and high ductility of these materials. Uncoated polished carbide inserts (K10/K05 micro-grain carbide) with sharp edges are the traditional choice. For high-silicon aluminum (>12% Si) or high-volume production, PCD (polycrystalline diamond) inserts are superior due to their extreme hardness and chemical inertness.

For aluminum, always use a highly polished insert with a large positive rake angle (15-20°). Avoid coated carbide for aluminum because the coating chemistry can react with the workpiece. Keep the edge sharp — a honed or chamfered edge will cause BUE immediately.

ISO S Group — Superalloys and Titanium

ISO S is the most demanding group, encompassing nickel-based superalloys (Inconel 718, Waspaloy, Hastelloy), cobalt-based alloys (Stellite), and titanium alloys (Ti-6Al-4V). These materials have low thermal conductivity (heat stays in the tool), high hot hardness, and a strong chemical affinity for carbide, leading to notch wear, diffusion wear, and plastic deformation of the cutting edge.

Cutting Parameters for ISO S

Coolant strategy is critical for ISO S materials. Use high-pressure coolant (70-100 bar) directed at the cutting edge to reduce temperature and improve chip evacuation. For roughing Inconel and titanium, ceramic inserts (SiAlON or Al2O3-SiCw) can increase Vc to 150-300 m/min, but they require rigid setups and continuous cuts — never use ceramics for interrupted cuts or thin-walled components. Always maintain a constant chip load to avoid work-hardening the surface.

ISO H Group — Hardened Materials

ISO H covers hardened steels (45-65 HRC), chilled cast iron, and sintered metals. Traditional carbide inserts cannot economically machine materials above 55 HRC, so ISO H applications typically use CBN (cubic boron nitride) for hardened steel and chilled iron, or ceramic inserts (Al2O3-based) for steels in the 45-55 HRC range.

For hard turning with CBN, use a rigid machine with minimal runout, a light depth of cut, and a small feed rate to generate a good surface finish (Ra 0.4-1.6 µm is achievable). A wiper insert geometry can improve surface finish at higher feed rates. Never use flood coolant with CBN in interrupted cuts — thermal shock can fracture the insert; use compressed air or mist instead.

Cross-Group Selection Summary

The following table provides a quick reference for matching the workpiece material to the correct ISO group and starting cutting parameters. Always start at the conservative end of the range and increase Vc once the process is stable.

Coating Strategy: CVD vs PVD

The choice between CVD (chemical vapor deposition) and PVD (physical vapor deposition) coatings is governed by the application, not personal preference.

• CVD coatings (TiN/TiCN/Al2O3 multilayer, 8-12 µm thick) are ideal for steel and cast iron turning where crater wear and thermal load dominate. The thick Al2O3 layer acts as a thermal barrier. The trade-off is a slightly less sharp edge and lower toughness.
• PVD coatings (TiAlN, AlTiN, TiSiN, 2-5 μm thick) are ideal for milling, stainless steel, and interrupted cuts. The thinner coating maintains a sharper edge, and the compressive residual stress improves edge toughness. Modern high-aluminum PVD coatings (AlTiN with 60-70% Al) offer excellent hot hardness up to 1000°C.
• For finishing, prefer PVD for a sharp edge and good surface finish; for roughing steel, prefer CVD for crater wear resistance.

Common Grade Selection Mistakes

• Using too hard a grade for interrupted cuts. A P10 grade in a roughing operation with interrupted cuts will chip immediately. Move to P30-P40 with a tough PVD coating.
• Using too soft a grade at high speed. A P40 grade run at 300 m/min in steel will suffer plastic deformation (cratering and edge rounding) within minutes. Match the grade hardness to the cutting speed.
• Ignoring the work-hardening layer. In stainless steel and superalloys, the first cut must penetrate below the work-hardened surface (ap > 1.5 mm) or the tool will wear rapidly.
• Wrong coolant for the material. High-pressure coolant is essential for stainless steel, superalloys, and deep hole drilling but can cause thermal shock with ceramics and CBN in interrupted cuts.

Conclusion

The ISO P/M/K/N/S/H classification is the universal language of carbide insert selection. By understanding the dominant failure mode for each workpiece group and matching it to the correct grade hardness and coating type, you can maximize tool life, improve surface finish, and reduce machining costs. Start with the conservative parameter ranges in the tables above, monitor tool wear on the first few parts, and optimize Vc and fn based on observed wear patterns. The goal is to achieve controlled flank wear of 0.2-0.3 mm (or crater wear depth of 0.1 mm) rather than catastrophic chipping or plastic deformation.