ISO Material Groups: What Do P, M, K, N, S and H Mean?
The ISO 513 material groups: steel (P), stainless (M), cast iron (K), non-ferrous (N), superalloy (S) and hardened steel (H). Machinability, cutting speed range and coating recommendation for each group.
Short answer
- ISO 513 divides materials into six groups according to their machining behaviour: P, M, K, N, S and H.
- The group colour is standard across the industry: P blue, M yellow, K red, N green, S orange, H grey.
- The group directly determines the cutting speed range and the coating choice, which makes it the first question to be asked in tool selection.
The letters P, M, K, N, S, H in tool catalogues come from the ISO 513 standard.
This standard groups materials not by their chemical composition but by their behaviour
during machining. The first question to be asked when selecting the right tool and the right
parameters is "which group is this material in?".
Short answer
| Code | Colour | Group | Typical materials |
|---|---|---|---|
| P | Blue | Steel | S235, C45, 42CrMo4, 1.2312, 1.2738 |
| M | Yellow | Stainless steel | AISI 304, 316, 2205 duplex |
| K | Red | Cast iron | GG25, GGG50, malleable cast iron |
| N | Green | Non-ferrous | Aluminium, copper, brass, plastic, composite |
| S | Orange | Heat-resistant superalloy | Ti6Al4V, Inconel 718, Nimonic |
| H | Grey | Hardened material | 45–65 HRC steel, cold-work tool steel, chilled cast iron |
P: Steel (blue)
The widest group. From structural steels through alloyed quenched and tempered steels to quenched and tempered mould steels.
- Chip: Tends to be long and continuous. A chip-breaker geometry or the correct
fzis required. - Heat: Moderate. It is shared between the tool and the chip.
- Coating: TiAlN (wet) or AlTiN (dry/MQL).
- Vc: 70–200 m/min (falls as hardness rises).
As hardness increases (above 28 HRC) the behaviour approaches that of group H.
M: Stainless steel (yellow)
Austenitic and duplex stainless steels. Their two hardest characteristics:
- Work hardening: The cut surface hardens. If the tool dwells in the material, the following pass is far harder to cut.
- Low thermal conductivity: The heat cannot be carried away with the chip and accumulates in the tool.
Practical rules:
- The feed must not be reduced; work has to proceed with a constant and sufficient
fz. - Letting the tool turn idly inside the material has to be avoided.
- AlCrN coating is preferred (1,100 °C oxidation resistance).
- Vc: 45–110 m/min.
K: Cast iron (red)
- Chip: Short, brittle, in the form of dust.
- Heat: Low; the cut runs "cool".
- Problem: Abrasive wear. Graphite and free carbides wear the cutting edge.
- Coating: TiCN (wet) or AlCrN.
- Vc: 100–220 m/min.
Because the chips come off as dust, dust extraction is recommended; dry machining is possible.
N: Non-ferrous metals (green)
Aluminium, copper, brass, bronze, plastic, composite and graphite.
- Chip: Long, soft, adhesive (aluminium and copper).
- Heat: Low, but the risk of tool adhesion is high.
- Coating: Uncoated polished or DLC. TiAlN/AlTiN shortens tool life in this group.
- Flute count: 2–3 (chip evacuation is critical).
- Vc: 200–800 m/min.
Cast aluminium alloys with a silicon content above 10% are highly abrasive; here tool life is extended markedly by a CVD diamond coating or a PCD tip.
S: Heat-resistant superalloys (orange)
Titanium and nickel-based alloys.
- Heat: The most critical problem. Thermal conductivity is very low and the heat remains almost entirely in the tool.
- Titanium: It is also chemically reactive; at high temperature it adheres to the tool.
- Inconel: Notch wear is typical; spreading the notch by periodically varying the
apvalue is recommended. - Coating: AlCrN or AlTiN.
- Coolant: Flood or high-pressure coolant.
- Vc: 20–80 m/min.
H: Hardened material (grey)
Hardened steels above 45 HRC, cold-work tool steels, chilled cast iron.
- Chip: Very small, red-orange in colour (this is normal).
- Heat: It makes cutting easier, because the material softens in the cutting zone.
- Work is therefore carried out dry or with MQL. Flood emulsion causes thermal cracking (comb cracks).
- Coating: nACo / AlTiSiN / TiSiN / AlCrN.
- Geometry: Corner radius or ball nose, negative edge preparation.
- Vc: 40–110 m/min.
Comparison table
| Group | Vc (m/min) | kc (N/mm²) | Main difficulty | Coating | Coolant |
|---|---|---|---|---|---|
| P: Steel | 70–200 | 1,500–2,000 | Long chip | TiAlN / AlTiN | Wet or MQL |
| M: Stainless | 45–110 | 2,000–2,300 | Work hardening, heat | AlCrN / AlTiN | Flood coolant |
| K: Cast iron | 100–220 | 1,100–1,400 | Abrasive wear | TiCN / AlCrN | Dry + dust extraction |
| N: Non-ferrous | 200–800 | 400–900 | Adhesion | Uncoated / DLC | Wet |
| S: Superalloy | 20–80 | 2,100–2,600 | Heat, reactivity | AlCrN / AlTiN | Flood / high pressure |
| H: Hardened material | 40–110 | 2,600–3,100 | Cutting force | nACo / TiSiN | Dry or MQL |
The Cutting Parameter Calculator can be used to calculate parameters according to the material group.
Frequently Asked Questions
What is an ISO material group?
It is the set of six classes into which the ISO 513 standard divides workpiece materials according to their behaviour in machining: P (steel), M (stainless steel), K (cast iron), N (non-ferrous), S (heat-resistant superalloy), H (hardened material). Tool manufacturers publish their cutting data according to these groups.
Why is stainless steel a separate group?
Austenitic stainless steels are prone to work hardening: the surface hardens during cutting and the following pass is harder to cut. Their thermal conductivity is also low, so heat accumulates in the tool. These two behaviours make them entirely different from ordinary steel (P).
What is the difference between group H and group P?
Group H covers hardened materials (above 45 HRC). At this hardness the chip is very small, cutting forces are high and heat plays a role that makes cutting easier. For that reason work in group H is carried out dry or with MQL; flood emulsion causes thermal cracking.
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