ISO P/M/K/N/S/H Carbide Grade Complete Parameter Guide: Cutting Data, Grade Equivalents, and Application Selection

Introduction to ISO Carbide Grade Classification

The ISO carbide grade classification system (ISO 513) is the global standard for categorizing cemented carbide cutting tool grades based on their intended workpiece material application. Understanding the six main ISO classes — P, M, K, N, S, and H — is fundamental to selecting the right insert grade for any machining operation. Each class represents a specific workpiece material group and comes with defined ranges for cutting speed (Vc), feed per tooth (fz), and depth of cut (ap).

In this comprehensive reference guide, we break down each ISO class with detailed cutting parameter tables, cross-brand grade equivalents from Sandvik, Mitsubishi, Walter, and other leading manufacturers, and practical application guidelines for turning, milling, and drilling operations.

ISO Grade Classification Overview

The ISO 513 standard assigns each carbide grade to one or more application classes based on its composition (WC grain size, Co binder content, and cubic carbide additions) and coating type. Each class is further subdivided into numerical subgrades (01 through 50) indicating the grade’s position on the toughness-wear resistance spectrum:

• Low numbers (01-15): High wear resistance, lower toughness — for finishing operations at high cutting speeds
• Medium numbers (20-30): Balanced wear resistance and toughness — for general-purpose machining
• High numbers (35-50): High toughness, lower wear resistance — for roughing and interrupted cuts at lower speeds

ISO P Class — Steel Machining Grades

ISO P-class grades are designed for machining carbon steels, alloy steels, and cast steels. These grades typically contain WC with additions of TiC and/or TaC/NbC (cubic carbides) to improve crater wear resistance at the high temperatures generated during steel machining. CVD-coated grades with multi-layer TiCN/Al2O3/TiN coatings dominate P-class applications.

P-Class Cutting Parameters (Turning)

P-Class Grade Equivalents Across Brands

Key application note: For steel turning with continuous cuts, always select the highest wear resistance grade (lowest ISO number) that can handle the cutting conditions without chipping. P10-P20 grades are the workhorses for most general steel turning applications, offering the best balance of crater wear resistance and edge toughness.

ISO M Class — Stainless Steel Machining Grades

ISO M-class grades are formulated for machining stainless steels — austenitic (304, 316), ferritic (430), martensitic (410, 420), and duplex alloys. Stainless steels present unique challenges: they work-harden rapidly, generate high heat, and produce long, stringy chips. M-class grades combine good hot hardness with sufficient toughness to resist the built-up edge (BUE) and notching that commonly occurs with these materials.

Both CVD and PVD coatings are used in M-class applications. PVD TiAlN coatings are preferred for finishing and operations requiring sharp cutting edges, while CVD multi-layer coatings excel in roughing where crater and flank wear resistance are paramount.

M-Class Cutting Parameters (Turning)

M-Class Grade Equivalents Across Brands

Key application note: When machining austenitic stainless steels (304/316), always use the highest cutting speed the grade can support without excessive tool wear. Higher speeds reduce contact time and help minimize built-up edge formation. Use M20-M30 grades for most stainless steel applications; reserve tougher M30-M40 grades for interrupted cuts or heavily work-hardened surfaces.

ISO K Class — Cast Iron Machining Grades

ISO K-class grades are designed for machining cast irons — gray cast iron (GCI), ductile cast iron (DCI/NCI), and compacted graphite iron (CGI). Cast iron machining presents a different set of challenges compared to steel: the material is abrasive (especially gray iron with its free graphite), generates discontinuous chips, and the cutting edge experiences repeated impact loading.

K-class grades are typically straight WC-Co grades without cubic carbide additions, as these additions would reduce thermal conductivity and increase the risk of thermal cracking in cast iron’s interrupted cutting environment. Both CVD and PVD coatings are used, with Si3N4-based ceramic grades also finding application in high-speed gray cast iron machining.

K-Class Cutting Parameters (Turning)

K-Class Grade Equivalents Across Brands

Key application note: Ductile cast iron is significantly more abrasive than gray cast iron and generally requires a grade with higher wear resistance (lower ISO number) at the same cutting speed. When switching from GCI to DCI, expect a 20-30% reduction in achievable cutting speed for the same tool life.

ISO N Class — Non-Ferrous Metal Machining Grades

ISO N-class grades are used for machining non-ferrous metals including aluminum alloys, copper, brass, bronze, and magnesium alloys. These materials are generally soft and machinable at very high cutting speeds, but they have a strong tendency to form built-up edge (BUE) on the cutting edge, which degrades surface finish and can cause dimensional inaccuracies.

N-class grades are typically uncoated straight WC-Co grades with fine to medium grain size, or grades with diamond-like carbon (DLC) or polycrystalline diamond (PCD) coatings for high-volume production of aluminum alloys. The sharp, smooth cutting edge of an uncoated or DLC-coated carbide insert helps prevent BUE formation and ensures excellent surface finish.

N-Class Cutting Parameters (Turning)

N-Class Grade Equivalents Across Brands

Key application note: For high-volume aluminum machining, PCD-tipped inserts can achieve 10-50 times the tool life of uncoated carbide, with cutting speeds exceeding 3000 m/min. However, PCD inserts are significantly more expensive and require rigid setups. DLC-coated carbide offers a middle ground with better BUE resistance than uncoated carbide at a fraction of the PCD cost.

ISO S Class — Superalloy and Titanium Machining Grades

ISO S-class grades are engineered for machining heat-resistant superalloys (HRSA) such as Inconel, Hastelloy, Waspaloy, and titanium alloys (Ti-6Al-4V). These materials are among the most difficult to machine due to their high hot strength, low thermal conductivity, and strong work-hardening tendency. The cutting zone temperatures can reach 1000-1200°C, placing extreme demands on tool material performance.

S-class grades are typically fine-grain WC-Co substrates with high cobalt content for toughness, coated with advanced PVD coatings such as TiAlN, TiSiN, or AlTiN. The fine grain size provides the necessary hot hardness and wear resistance, while the high cobalt content provides the toughness to resist chipping in the interrupted cuts common in aerospace component machining.

S-Class Cutting Parameters (Turning)

S-Class Grade Equivalents Across Brands

Key application note: Inconel 718 and other nickel-based superalloys should be machined at the lower end of the recommended speed range for roughing operations, as the work-hardening effect becomes more pronounced at higher cutting speeds. Always ensure adequate coolant delivery — high-pressure coolant (70+ bar) can improve tool life by 30-50% in S-class applications.

ISO H Class — Hardened Material Machining Grades

ISO H-class grades are designed for machining hardened steels and cast irons with hardness above 45 HRC. This includes through-hardened steels, case-hardened steels, and hardened tool steels. Hard turning with carbide or CBN inserts has increasingly replaced grinding operations in many applications, offering higher flexibility and lower setup costs.

H-class carbide grades are ultra-fine grain WC-Co substrates with high hardness, coated with wear-resistant PVD coatings such as TiAlN or TiSiN. For very high hardness materials (>55 HRC) or high-volume production, polycrystalline cubic boron nitride (PCBN) inserts offer significantly higher cutting speeds and tool life but at a higher cost per insert.

H-Class Cutting Parameters (Turning)

H-Class Grade Equivalents Across Brands

Key application note: For hard turning applications requiring surface finishes better than Ra 0.8 µm, use wiper geometry inserts with low feed rates. The feed rate should not exceed the wiper flat width, typically around 0.10-0.15 mm/rev for standard wiper inserts. Ensure the machine tool has sufficient rigidity — any vibration will immediately show as chatter marks on the finished surface.

Grade Selection Decision Framework

Selecting the optimal carbide grade for a given application involves balancing multiple factors. Use this decision framework to narrow down your choices:

Step 1: Identify the Workpiece Material ISO Class

Start by determining which ISO class your workpiece material falls into. This immediately narrows the field to grades designed for that material type. When in doubt, consult the material’s data sheet or use the manufacturer’s grade selector tool.

Step 2: Define the Operation Type and Severity

Consider whether the operation is finishing, general-purpose, or roughing, and whether the cut is continuous or interrupted. Finishing operations favor wear-resistant grades (lower ISO numbers), while roughing and interrupted cuts require tougher grades (higher ISO numbers).

Step 3: Evaluate Machine Tool Rigidity and Power

Rigid setups with high horsepower machines allow the use of more wear-resistant grades at higher cutting parameters. Less rigid machines or setups with long tool overhangs require tougher grades to resist chattering and edge chipping.

Step 4: Consider Production Volume and Cost per Part

For high-volume production, premium grades with longer tool life are usually justified by reduced tool change time and higher material removal rates. For low-volume or prototype work, general-purpose grades offer the best balance of performance and cost.

Step 5: Start Conservative and Optimize

Always start with cutting parameters at the lower to middle of the recommended range, then optimize upward based on observed tool wear. The target tool life for most turning operations is 15-30 minutes per edge, while milling operations typically target 30-60 minutes per cutter set.

Common Grade Conversion Pitfalls

When converting between brands based on ISO grade equivalents, be aware of these common pitfalls:

• ISO classification is approximate: The ISO 513 classification provides a general guide, but two grades with the same ISO designation can perform differently in the same application due to differences in substrate composition, coating technology, and edge preparation.
• Coating technology varies: Two M20-grade inserts from different manufacturers may have different coating architectures (CVD vs PVD, coating thickness, layer sequence) that significantly affect performance in specific applications.
• Edge preparation matters: The same grade with a different hone (T-land) or chamfer can behave like a different ISO subgrade. Always verify the edge preparation when comparing grades across brands.
• Test before production commitment: Whenever switching brands or grade families, always run a qualification test with representative cutting conditions before committing to a full production run.

Conclusion

The ISO P/M/K/N/S/H carbide grade system provides a valuable framework for understanding and comparing cutting tool grades across different manufacturers. While the classification system is not perfect — and grade equivalents should always be verified through practical testing — it remains the most widely used and recognized system in the metalworking industry.

By understanding the characteristics, cutting parameters, and cross-brand equivalents for each ISO class, you can make more informed decisions when selecting tooling for your machining operations. Remember that the optimal grade choice always depends on the specific combination of workpiece material, operation type, machine capability, and production requirements.

Use the parameter tables in this guide as a starting point for your machining operations, and always optimize based on your specific equipment, setup rigidity, and quality requirements.