ISO 513 sorts every workpiece material into six application groups by colour: P (blue) for steel, M (yellow) for stainless, K (red) for cast iron, N (green) for non-ferrous, S (brown) for superalloys and titanium, and H (grey) for hardened material above ~45 HRC. The letter tells you the dominant wear mechanism the tool must survive — crater wear for P, work-hardening and notch wear for M, abrasion for K — which in turn drives cutting speed, carbide grade, and coating choice. This article decodes the workpiece-material side of ISO 513; for choosing the insert grade itself, see the carbide grade selection guide.
A common point of confusion: ISO 513 is read in two directions. The companion carbide grade selection guide uses P/M/K/N/S/H to classify the insert or tool grade (a "P25 grade"). This article reads the same six letters as workpiece-material families — what each group physically contains, how it behaves under the cutting edge, and why that behaviour dictates the parameters you reach for. The standard intentionally links the two sides: you match a workpiece group to a tool group designed for it.
What Does ISO 513 Actually Classify?
ISO 513:2004 (third edition, prepared by ISO/TC 29/SC 9) classifies hard cutting materials — carbides, ceramics, diamond, and boron nitride — for machining by chip removal. Its most-cited element, Table 5, is the application-group system that maps workpiece materials to six lettered groups, each with a standardised colour and a range of two-digit application numbers.
| Group | Colour | Workpiece materials (per ISO 513 Table 5) | Application numbers |
|---|---|---|---|
| P | Blue | All steel and cast steel except austenitic stainless | P01–P50 |
| M | Yellow | Austenitic and austenitic-ferritic (duplex) stainless steel and cast steel | M01–M40 |
| K | Red | Cast iron — grey, ductile (spheroidal), malleable | K01–K40 |
| N | Green | Non-ferrous: aluminium, copper, brass, and non-metallics | N01–N30 |
| S | Brown | Heat-resistant special alloys (iron, nickel, cobalt based), titanium and titanium alloys | S01–S30 |
| H | Grey | Hardened steel, hardened cast iron, chilled cast iron | H01–H30 |
ISO 513 assigns each workpiece-material family one of six colour-coded groups, and the colour is part of the standard rather than a vendor convention. The colours appear on insert packaging across manufacturers, so a blue band signals a P-group (steel) grade regardless of brand.
Key Data Point
ISO 513:2004 Section 3.2 defines the number convention directly: within a group, lower numbers (P01, P10) are harder and run at higher cutting speeds, while higher numbers (P40, P50) are tougher and favour higher feed rates and interrupted cuts. The standard visualises this with arrows — speed and wear resistance increasing upward, feed and toughness increasing downward.
How Each Group Behaves Under the Cutting Edge
The reason ISO 513 groups materials the way it does is wear mechanism: each family attacks the tool differently, so each needs a different defence. The ISO 513 letter is, in practice, shorthand for the dominant tool-wear mode a workpiece produces.
| Group | Example materials | Dominant wear mode | Machinability behaviour |
|---|---|---|---|
| P (steel) | 1045, 4140, 4340, mild & alloy steel | Crater wear on the rake face | Long continuous chips; high heat at chip-tool interface; generally predictable |
| M (stainless) | 304, 316, duplex 2205, cast steel | Notch wear, built-up edge, work-hardening | Gummy chips; surface work-hardens 1.5–2x base hardness; low thermal conductivity traps heat |
| K (cast iron) | Grey iron GG25, ductile GGG60 | Abrasive flank wear | Short brittle chips; abrasive graphite/carbide phases; lower heat than steel |
| N (non-ferrous) | 6061, 7075 aluminium, brass, plastics | Built-up edge, adhesion | Soft, low cutting force; sticks to the edge unless geometry is sharp and polished |
| S (superalloys) | Inconel 718, Ti-6Al-4V, cobalt alloys | Heat, plastic deformation, notching | Very high cutting-zone temperature; low thermal conductivity; abrasive and chemically reactive |
| H (hardened) | Hardened steel >45 HRC, chilled iron | Abrasion + thermal, demands hot hardness | Very high cutting forces; thin red-hot chips; finishing-only depths typical |
P-group steels typically generate crater wear on the rake face because long continuous chips slide across it at high temperature, which is why P grades emphasise crater-wear resistance. By contrast, K-group cast irons typically cause abrasive flank wear from hard graphite and carbide phases rather than crater wear, so K grades emphasise abrasion resistance instead. Austenitic stainless in the M group tends to work-harden the cut surface to roughly 1.5–2x its base hardness under typical conditions, which promotes notch wear at the depth-of-cut line.
The machinability index above uses the long-standing AISI 1112-based percentage scale (Machinery's Handbook), where higher percentages mean easier machining. Nickel-based superalloys such as Inconel 718 sit at roughly 10–20% machinability on the 1112 scale, making the S group the most demanding family in typical turning and milling. Treat these figures as comparative starting points rather than fixed constants — actual values shift with heat treatment, hardness, and cutting conditions.
How the Group Drives Speed, Grade, and Coating Decisions
Once you know the group, the standard's number convention and decades of manufacturer practice point you toward a parameter window. The group does not give you a speed — ISO 513 deliberately publishes no cutting speeds — but it sets the direction of every downstream choice.
| Group | Typical carbide grade window | Typical coating direction | Indicative turning speed (carbide) |
|---|---|---|---|
| P | P10–P40 | CVD multi-layer (continuous), PVD (interrupted) | ~150–400 m/min |
| M | M10–M30 | PVD TiAlN / AlCrN | ~120–250 m/min |
| K | K10–K30 | CVD Al₂O₃ (continuous), ceramics for high speed | ~80–500 m/min (grade dependent) |
| N | N10–N20 | Uncoated polished or DLC; PCD for volume | ~500–1,500 m/min |
| S | S10–S25 | PVD AlCrN; ceramics/CBN for nickel alloys | ~30–80 m/min (nickel), ~50–120 m/min (Ti) |
| H | H10–H20 | CVD multi-layer; CBN for hard turning | ~80–200 m/min (hard turning) |
Treat the speed columns as broad order-of-magnitude windows for carbide tooling under stable conditions — they shift with depth of cut, rigidity, coating, and the specific alloy. Non-ferrous N-group aluminium typically runs an order of magnitude faster than nickel-based S-group alloys with carbide tooling because aluminium generates far less cutting-zone heat per unit of material removed. The coating column follows the wear mode: TiAlN coatings are preferred for M-group stainless and S-group milling because their oxidation resistance (stable to roughly 800°C) limits crater wear at high cutting-zone temperatures, while AlCrN is often chosen over TiAlN in high-feed interrupted cuts for its slightly higher hot hardness. DLC coatings are used on N-group aluminium because their low friction prevents the soft alloy from welding to the edge as built-up edge. For the grade-side detail behind these windows — substrate grain size, cobalt binder content, and CVD-versus-PVD trade-offs — see the carbide grade selection guide and the broader cutting tools complete guide.
The speed–life relationship within each group follows Taylor's tool-life equation, VT^n = C, where V is cutting speed, T is tool life, and n is the material-dependent Taylor exponent. Feed and speed do not trade off equally against tool life: speed dominates because tool life is far more sensitive to the exponent n than to feed, so a 10% speed cut can extend carbide life by roughly 1.5–2x in steel under typical conditions (Taylor exponent n ≈ 0.14–0.25 for steel per Machinery's Handbook 31st Edition Table 5b). For parameter optimisation across groups, see the CNC machining optimization guide.
A group of application is not identical to a cutting material grade. Grades from different manufacturers which are in the same application group could be different as far as application range and performance level are concerned.
— ISO 513:2004, Section 4ISO 513 explicitly states that the application group is not a grade-equivalence chart, so two vendors' P25 grades may perform differently in the same cut. Use the group to narrow the field, then validate the specific grade against your operation rather than assuming cross-brand interchangeability.
Material-Family Reference: Each Group in Detail
This section is the decoder. Each group below links to its dedicated machining guide where one exists, so this article functions as the material-side hub for the cluster.
P — Steel (blue). Carbon and alloy steels (1045, 4140, 4340) and cast steel, excluding austenitic stainless. The largest single group by tonnage machined. Long ductile chips and high rake-face temperatures make crater wear the limiting mode. See the steel machining parameters guide and the umbrella material machining complete guide.
M — Stainless (yellow). Austenitic (304, 316) and duplex (2205) stainless plus austenitic-ferritic cast steel. Austenitic stainless work-hardens rapidly and conducts heat poorly (roughly a third of carbon steel's thermal conductivity), so heat concentrates at the edge and built-up edge forms readily at low speeds. See the dedicated 304/316 stainless steel machining guide.
K — Cast iron (red). Grey, ductile (spheroidal-graphite), and malleable iron. Chips break short and brittle, and abrasive constituents wear the flank. Grey cast iron's graphite flakes act as a built-in chip breaker and mild lubricant, so K-group machining produces dust-like chips rather than the continuous chips of P-group steel. See the cast iron machining guide.
N — Non-ferrous (green). Aluminium (6061, 7075), copper, brass, and non-metallics. Low cutting forces but a strong tendency to form built-up edge on soft, sticky alloys. Aluminium machining favours sharp, polished, uncoated or PCD edges because any built-up edge degrades both finish and dimensional accuracy. See the aluminum high-speed machining guide and plastics & composites machining guide.
S — Superalloys (brown). Nickel-based (Inconel 718), titanium (Ti-6Al-4V), and cobalt alloys. The most demanding group: high cutting-zone temperatures, low thermal conductivity, and chemical reactivity drive aggressive notching and rapid edge breakdown. Titanium alloys retain strength at temperature and conduct heat poorly, so cutting speeds are typically held to a fraction of steel speeds to keep the edge from overheating. See the titanium alloy machining parameters guide and Inconel & superalloy machining guide.
H — Hardened (grey). Hardened steel above ~45 HRC, hardened and chilled cast iron. Very high cutting forces and thermal load demand hot hardness; hard turning with CBN often replaces grinding at finishing depths. See the hardened steel machining guide.
Outlook and How to Use the Classification
ISO 513 has been stable since its 2004 third edition, and its colour-coded six-group framework remains the universal cross-vendor language for matching tools to materials. As coated carbides, ceramics, and CBN continue to widen the speed windows within each group, the classification's role shifts from prescribing parameters toward providing a shared starting vocabulary that pairs cleanly with manufacturer-specific grade data.
The practical workflow is the same on every job: identify the workpiece, map it to its ISO 513 group (P/M/K/N/S/H), let the group flag the dominant wear mode, and then select a tool grade and coating engineered for that mode — validating the specific grade rather than assuming brand interchangeability.
Read ISO 513 from the material side first, then choose the grade.
ISO 513's six colour-coded groups — P (blue) steel, M (yellow) stainless, K (red) cast iron, N (green) non-ferrous, S (brown) superalloys, H (grey) hardened — encode the dominant wear mechanism each workpiece family imposes on the tool. Identify the group, infer the wear mode, then pick a carbide grade and coating built to resist it, treating the standard as a starting vocabulary rather than a cross-vendor equivalence chart.
