Understanding ISO Material Groups P/M/K/N/S/H for Cutting Tools
Ask two machinists how to classify a workpiece material and you may get two different answers — one will say "it is a 17-4 PH stainless", the other "it is a hardened alloy". Both are correct descriptions, but neither tells you how to select a cutting tool.
The ISO 513 classification exists precisely to solve this. It groups workpiece materials not by their metallurgical family but by how they behave during cutting — the wear mechanism they impose, the chip form they produce, and the cutting temperature they generate. Two materials from completely different metallurgical backgrounds can sit in the same ISO group because they attack a cutting edge in the same way.
Why Classification by Wear Mechanism Matters
A cutting tool fails through a limited number of mechanisms: abrasive wear, diffusive wear, adhesive wear, chemical wear, plastic deformation, mechanical fracture and thermal cracking. Each ISO group is dominated by one or two of these.
Because a grade's resistance to each mechanism is set by its composition — grain size, cobalt content, cubic carbide additives, coating — knowing the dominant wear mechanism tells you which composition will survive. That is the practical value of the ISO system: it converts a materials question into a tooling answer.
The Six Groups
P — Steel (Blue)
Materials: Carbon steels, alloy steels, tool steels, ferritic and martensitic stainless steels, cast steels.
Behaviour: Continuous, long chips. The chip-tool interface reaches the highest temperatures of any common material group, often 800–1000 °C at the rake face. The dominant wear modes are crater wear from diffusion and, at excessive speeds, plastic deformation of the cutting edge.
What the tool needs: Maximum hot hardness. Grades are medium to coarse grain with 6–10% cobalt and TiC/TaC/NbC additives that resist diffusion. Coatings are typically CVD multilayer with an alumina (Al2O3) top layer for thermal insulation, or PVD TiAlN where interrupted cuts demand edge toughness.
Practical notes: Steel is the most forgiving group overall, which is why it is often used as the baseline for grade testing. Hardened and alloyed steels in the P group (P3–P4) behave almost like H and benefit from tougher substrates.
M — Stainless Steel (Yellow)
Materials: Austenitic, duplex and super-duplex stainless steels, and some high-alloy steels.
Behaviour: The most awkward of the common groups. Austenitic stainless steel work-hardens rapidly — the cut surface becomes significantly harder than the bulk material, so each subsequent pass cuts a harder surface than the last. Thermal conductivity is low, so heat concentrates in the cutting edge. Combined, these produce built-up edge, adhesive wear, severe notch wear at the depth-of-cut line, and a strong tendency to chatter.
What the tool needs: A balance of toughness (to survive built-up edge detachment) and hot hardness (to resist the concentrated heat), with good thermal conductivity in the substrate. Higher cobalt content than P grades is typical. PVD TiAlN coatings are generally preferred — CVD alumina coatings can flake under the adhesive wear conditions.
Practical notes: Never let the tool rub. Cutting speed must be high enough to stay ahead of work-hardening but low enough to avoid rapid diffusion wear — a narrow window, typically 100–160 m/min for austenitic grades. Generous coolant directed at the cutting zone and positive rake geometries are essential.
K — Cast Iron (Red)
Materials: Grey cast iron, ductile/nodular iron, malleable iron, chilled iron.
Behaviour: Short, broken chips. Because the material fractures ahead of the edge rather than forming a continuous chip, cutting temperatures are markedly lower than steel. The dominant wear mechanism is simple abrasion from the hard, discontinuous structure. Crater wear is minimal.
What the tool needs: Maximum hardness. Grades are fine grain with low cobalt content (4–6%) and no cubic carbide additives, since diffusion resistance is irrelevant. A CVD Al2O3 or TiAlN coating extends life further by adding an abrasion-resistant layer.
Practical notes: Grey iron is often machined dry — the graphite content provides useful lubrication and the dust from coolant is a disposal problem. Speeds can be run 20–40% higher than steel for equivalent tool life. Nodular and ductile irons are tougher and sit at the lower-speed end of the K range.
N — Non-Ferrous (Green)
Materials: Aluminium and its alloys, copper, brass, bronze, magnesium, plastics and composites.
Behaviour: Soft, gummy and highly prone to built-up edge and smearing. Cutting forces are low but the soft material adheres to the edge, and the periodically detached built-up edge takes fragments of coating with it. Abrasion from hard inclusions (particularly in cast aluminium) adds a second wear mode. Thermal load is low.
What the tool needs: Maximum edge sharpness and freedom from adhesion. Grades are fine grain with high cobalt content (10–12%) for edge strength. They are used uncoated with a polished top surface, or with a very thin PVD coating that does not blunt the edge. High positive rake angles and large chip clearances are standard for aluminium-specific tooling.
Practical notes: Aluminium with high silicon content (above 12%, as in many automotive castings) behaves closer to an abrasive K-type material and benefits from thin coatings and diamond-like or PCD alternatives for very high production.
S — Superalloys and Titanium (Brown)
Materials: Nickel-based superalloys such as Inconel 718, cobalt-based alloys, titanium alloys, and some high-strength stainless.
Behaviour: The lowest machinability of any group. These materials retain strength at high temperature, which means the cutting edge does not get the thermal softening assistance it relies on in steel. Cutting forces are high, heat is concentrated in a narrow band at the edge, and the combination produces severe notch wear at the depth-of-cut line, depth-of-cut notching, and rapid edge chipping. Titanium adds a further problem: it is chemically reactive and can ignite in fine chip form.
What the tool needs: Fine grain, high cobalt (10–12%), cubic-carbide-free substrates — diffusion resistance is secondary to edge strength and toughness. Sharp, positive geometries with a light hone rather than a heavy T-land. PVD AlTiN or AlCrN coatings capable of resisting high temperatures.
Practical notes: Speeds must be very low (25–60 m/min) and the depth of cut limited to about 25% of the tool's cutting width to control notch wear. High-pressure coolant directed at the cutting zone is not optional — it is a requirement. Where possible, use ramping rather than plunging to avoid concentrating heat.
H — Hardened Steel (Grey)
Materials: Hardened steels above 45 HRC, through-hardened tool steels, case-hardened and nitrided surfaces.
Behaviour: Combined abrasive and deformational wear. The material is hard enough that chip formation involves plastic flow of the workpiece surface, generating significant heat despite modest cutting speeds. Wear appears as rapid flank wear, and the high contact stress can cause edge chipping if the setup lacks rigidity.
What the tool needs: Ultra-fine grain or submicron substrates with low cobalt for maximum hardness. For the softer end of the range (45–55 HRC), coated carbide with PVD TiAlN works well. Above 55 HRC, cubic boron nitride (CBN) becomes the standard choice, cutting at 120–250 m/min.
Practical notes: Total system rigidity is critical. On hard materials, a small amount of machine or workpiece vibration translates directly into edge chipping. Depths of cut are typically shallow (0.3–1.0 mm) with light feeds.
The Three Sub-Groups Within Each Group
ISO 513 further divides each colour group by difficulty. For group P, materials P1–P2 are easy low-carbon and free-cutting steels, P3–P4 are alloyed and harder steels, and P5–P6 are the most difficult — austenitic, high-temperature and highly alloyed steels.
The logic is the same for every group: the higher the number, the tougher the grade required and the lower the cutting speed. This is why a grade chart appears as a band rather than a single point — a correctly specified grade covers a range of sub-groups, and the recommended speed depends on which sub-group you are machining.
Practical Application of the ISO System
- Identify the ISO group from the workpiece material specification, using the nearest equivalent if the material is unusual.
- Identify the sub-group from hardness, alloy content and heat treatment.
- Read the grade's application band from the manufacturer's chart and confirm the group is covered.
- Select the coating by dominant wear mechanism — abrasion favours CVD, adhesion favours PVD.
- Set the cutting speed within the range for the sub-group, at the low end for the first trial.
- Check the geometry against whether the cut is continuous or interrupted.
One caveat worth stating plainly: ISO 513 describes machinability, not convenience. A material can be "easy" in a metallurgical sense but troublesome in practice because of its condition — castings with scale, forgings with hard skin, or clamped parts with poor rigidity. The ISO group is the starting point, not the complete specification of the problem.
Where the ISO System Helps Most
The ISO material group system earns its place when standardising tooling across a workshop. Instead of accumulating a drawer of single-purpose grades, a small number of well-chosen grades covering P, M, K and N handles the majority of general production, with dedicated grades for S and H where the demands are genuinely different.
If you are rationalising grades across a shop floor, or have a material that resists straightforward classification, send us the material specification and operation details. Our engineering team can map your applications to the right grades and identify where a single grade can serve several material groups.