Carbide Grade: WC Grain Size, Cobalt Content and ISO 513 Classes
In cemented carbide, the cobalt content, the WC grain size and the ISO 513 application class together set the hardness–toughness balance of a tool. The technical meaning of the class codes, the grain size ranges and the mechanical property values.
Short answer
- In cutting tool carbides the hard phase (WC) accounts for more than 80% by weight; the remaining cobalt binder carries the toughness.
- As the cobalt content rises from 3% to 20%, hardness falls from roughly 90–93 HRA to 83–86 HRA and transverse rupture strength rises from 1.0–1.2 GPa to 2.0–2.6 GPa.
- ISO 4499-2 grain size classes: nano < 0.2 µm, ultrafine 0.2–0.5 µm, submicron 0.5–0.8 µm, fine 0.8–1.3 µm, medium 1.3–2.5 µm, coarse 2.5–6.0 µm.
- In ISO 513 a higher class number means more toughness and more feed capacity, lower hardness and lower cutting speed; P10 sits on the finishing side, P40 on the roughing side.
- Young's modulus is almost independent of grain size and falls with the cobalt content: roughly 668 GPa at 3% Co and roughly 500 GPa at 20% Co.
The word "carbide" in an end mill catalogue describes not a single material but a broad family. The difference in tool life between two tools carrying the same name usually comes not from the geometry but from the cobalt content of the substrate and the tungsten carbide grain size. This guide covers the structure of cemented carbide, the ISO 513 application classes and the numerical meaning of the hardness–toughness trade-off. A general comparison between the tool material families is found in the tool materials guide.
1. The structure of cemented carbide
Cemented carbide is a composite produced by powder metallurgy. The hard phase consists of tungsten carbide (WC) grains; these grains are held within a metallic cobalt (Co) binder. In grades intended for metal cutting applications the hard phase accounts for more than 80% by weight.
The behaviour of the material is adjusted through two independent variables:
- Binder content: the weight percentage of cobalt. It determines the toughness and the resistance to plastic deformation.
- WC grain size: the mean grain diameter. It determines the hardness at a given binder content.
In some grades cubic carbonitrides (TiC, TaC, NbC) are added alongside WC. These raise the hot hardness and allow gradient structures to form; in solid carbide end mills, however, the basic composition is predominantly WC-Co.
On the role of cobalt: Cobalt is not an "impurity" but the load-carrying phase of the material. Reducing the cobalt raises the hardness while leaving the material defenceless against crack propagation.
2. The effect of the cobalt content
As the cobalt content increases, hardness and wear resistance fall while toughness and bending strength rise. The values below belong to typical WC-Co compositions reported in manufacturers' technical publications.
| Cobalt (by weight) | Density (g/cm³) | Hardness (HRA) | TRS (GPa) | Young's modulus (GPa) |
|---|---|---|---|---|
| 3% | 15.1–15.2 | 90–93 | 1.0–1.2 | 668 |
| 5% | 14.8–15.0 | 90–91 | 1.6–1.8 | 620 |
| 10% | 14.3–14.5 | 88–90 | 1.5–1.9 | 584 |
| 15% | 13.8–14.0 | 86–88 | 1.8–2.2 | 548 |
| 20% | 13.1–13.3 | 83–86 | 2.0–2.6 | 500 |
For comparison: the hardness of pure WC is 92–94 HRA and its Young's modulus is roughly 720 GPa, while the Young's modulus of pure cobalt is roughly 209 GPa. The elastic behaviour of the composite shifts between these two extremes in a near-linear fashion with the composition.
The substrates used in solid carbide end mills mostly lie in the 6%–15% cobalt band. The lower end of this band works in favour of wear resistance, the upper end in favour of impact and edge chipping resistance.
| Cobalt band | Character | Typical use |
|---|---|---|
| 6%–8% | High hardness, low toughness | Hardened steel, cast iron, high-speed finishing |
| 8%–10% | Balanced | General purpose steel and stainless milling |
| 10%–12% | High toughness | Interrupted cuts, small-diameter tools, unstable clamping |
| 12%–15% | Very high toughness | Heavy roughing, impact loading, low-rigidity machines |
Transverse rupture strength does not rise without limit with cobalt. According to manufacturer data, TRS peaks at around 15% cobalt and falls again at higher binder contents. That peak does not appear in the table above, because the table rows are compiled from grades whose grain sizes differ from one another, and grain size influences TRS at least as strongly as the cobalt content does. For this reason the simplification "more cobalt means more toughness" loses its validity once the cutting tool range is left behind.
3. Grain size classes
The mean WC grain size is measured metallographically under ISO 4499-2 using the linear intercept method. The standard also defines the class names under which the measurement result is reported.
| Class | Mean grain size | Character |
|---|---|---|
| Nano | < 0.2 µm | Highest hardness; difficult to produce and to sinter |
| Ultrafine | 0.2–0.5 µm | Very sharp edge, high compressive strength |
| Submicron | 0.5–0.8 µm | Balance of sharpness and toughness; the standard for small-diameter tools |
| Fine | 0.8–1.3 µm | General purpose high performance substrate |
| Medium | 1.3–2.5 µm | Toughness-oriented; heavy cutting |
| Coarse | 2.5–6.0 µm | High thermal shock resistance, low hardness |
| Extra coarse | > 6.0 µm | Wear parts and mining rather than cutting tools |
Grain size is the second lever that determines hardness at a given cobalt content. Fine grain gives higher hardness at the same binder content; this is explained by the shortening of the mean free path of the binder. Compressive strength and fatigue strength also increase as the grain becomes smaller: in grades with small grain size and low binder content the compressive strength approaches the order of 7,000 N/mm², while the fatigue strength at 2×10⁶ cycles remains at 65–85% of the static compressive strength.
Submicron carbide and small-diameter end mills
There are three concrete reasons why a submicron substrate is preferred in small-diameter tools.
- Edge radius. The smallest roundness a ground cutting edge can reach is limited by the grain size. A sharp and regular edge cannot be obtained on a substrate with 2 µm grain.
- Chip thickness scale. In the 1–3 mm diameter class the feed per tooth is mostly on the order of a few microns. When the edge radius approaches the chip thickness, ploughing begins instead of cutting; heat and force increase.
- Edge micro-chipping. In a small-diameter tool the cross-sectional area is already small; a large grain torn out of the edge is a large defect relative to the diameter scale. Fine grain reduces the size of the particle that breaks away.
A submicron substrate does not provide absolute superiority. On a machine that is not rigid, or on a workpiece with scale or an irregular surface, fine grain is more prone to edge chipping; under these conditions a grade with coarser grain and higher cobalt gives longer tool life.
4. ISO 513 application classes
ISO 513 divides hard cutting materials for tools with defined cutting edges into six main groups according to the workpiece material group. Each group has a letter and an identification colour.
| Group | Colour | Workpiece material | Typical class number range |
|---|---|---|---|
| P | Blue | Steel and cast steel (long chipping) | P01–P50 |
| M | Yellow | Stainless steel, austenitic and duplex | M01–M40 |
| K | Red | Cast iron (short chipping) | K01–K40 |
| N | Green | Non-ferrous: aluminium, copper alloys, plastics | N01–N30 |
| S | Brown / orange | Heat resistant superalloys and titanium | S01–S30 |
| H | Grey | Hardened steel and chilled cast iron | H01–H30 |
The division of workpiece materials into these groups is described in more detail in the ISO material groups guide.
ISO 513 also codes the hard cutting material itself. The three codes that matter for solid carbide tools are the following:
- HW: uncoated cemented carbide, predominantly WC, grain size ≥ 1 µm
- HF: uncoated cemented carbide, predominantly WC, grain size < 1 µm
- HC: the coated versions of the above
The meaning of the class number
The two-digit number following the letter indicates the position within the group. As the number increases, toughness and the feed that can be carried increase; as the number decreases, hardness, wear resistance and the cutting speed that can be carried increase.
| Class | Hardness / wear resistance | Toughness | Cutting speed | Feed | Typical operation |
|---|---|---|---|---|---|
| P01–P10 | Highest | Lowest | Highest | Lowest | Precision finishing, continuous cutting |
| P15–P25 | High | Medium | High | Medium | General purpose milling |
| P30–P40 | Medium | High | Medium | High | Roughing, interrupted cutting |
| P45–P50 | Low | Highest | Low | Highest | Heavy roughing, unstable conditions |
The same logic applies to the M, K, N, S and H groups. A class number does not give an absolute hardness value; it indicates only the relative position within the same group. For this reason P30 and K30 are not directly comparable.
5. The hardness–toughness trade-off
Selecting a carbide grade amounts to choosing a position on a single curve. Lowering the cobalt content or reducing the grain size moves in the same direction, that is, towards hardness; raising both together leads towards toughness. These two levers can be used so that they partly balance each other: the combination of submicron grain + 12% cobalt is common for this reason in small-diameter tools that call for both a sharp edge and high edge toughness.
Young's modulus stands outside this trade-off. Measurements show that the elastic modulus is almost independent of the grain size and changes essentially with the binder content. The practical meaning is this: the deflection behaviour of the tool is determined not by the grain size but by the cobalt content and the overhang length. A substrate with higher cobalt bends somewhat more at the same geometry.
The mechanism by which wear progresses also affects the grade selection. In work where abrasive wear dominates the decision falls in favour of low cobalt, and in work where the risk of adhesion and edge chipping dominates in favour of high cobalt. The distinction between the mechanisms is covered in the tool wear guide.
Producing comparable figures requires the measurement standards to be the same: ISO 3878 (Vickers) or ISO 3738-1 (Rockwell A) for hardness, ISO 3327 for transverse rupture strength and ISO 3369 for density. Comparing TRS values taken with different specimen geometries directly gives an erroneous result.
6. Selecting the substrate together with the coating
The coating and the substrate are not two separate choices but two layers of a single grade. The coating provides hot hardness, the diffusion barrier and the friction behaviour; the substrate provides the mechanical support beneath the coating. A hard coating on a soft substrate rests on a base that collapses under load and lifts off by flaking.
| Workpiece material | Substrate tendency | Coating tendency |
|---|---|---|
| Hardened steel (H) | Low cobalt, submicron/ultrafine | AlTiN/AlCrN class with high hot hardness |
| General steel (P) | Medium cobalt, fine grain | TiAlN class |
| Stainless (M) | Medium–high cobalt, fine grain | Coating with high resistance to adhesion |
| Titanium and superalloys (S) | High cobalt, fine grain | Thin, low-diffusion coating that does not spoil the sharp edge |
| Aluminium (N) | Medium cobalt, submicron | Uncoated polished surface or DLC/diamond class |
The detail of the coating families is given in the coatings guide. For the practical consequences of substrate–coating matching in hardened materials, the hardened steel milling guide can be consulted.
Critical point: The presence of only the coating name in a tool catalogue says nothing about the substrate. Tools offered by two suppliers under the same coating name can give markedly different tool life because of differing cobalt content and grain size.
Once the substrate and coating decision has been made, the choices on the geometry side can be assessed through the square end mill category.
Sources
- ISO 513:2012 — Classification and application of hard cutting materials for metal removal with defined cutting edges — Designation of the main groups and groups of application (iso.org)
- ISO 4499-2:2020 — Hardmetals — Metallographic determination of microstructure — Part 2: Measurement of WC grain size (iso.org)
- ISO 3327:2009 — Hardmetals — Determination of transverse rupture strength
- ISO 3878:1983 — Hardmetals — Vickers hardness test · ISO 3738-1:1982 — Hardmetals — Rockwell hardness test (scale A) · ISO 3369:2006 — Impermeable sintered metal materials and hardmetals — Determination of density
- Sandvik Coromant — Cutting tool materials (sandvik.coromant.com)
- Hyperion Materials & Technologies — Mechanical strength of cemented carbide (hyperionmt.com)
- A.L.M.T. Corp. — Features of cemented carbide (allied-material.co.jp)
Frequently Asked Questions
Is a carbide with a high cobalt content better?
On its own it is neither better nor worse; it is a different balance point. As cobalt increases, toughness and transverse rupture strength rise while hardness and wear resistance fall. High cobalt is preferred for interrupted cuts, unstable clamping and roughing; low cobalt is preferred for high-speed finishing and abrasive materials.
What does submicron carbide mean?
According to ISO 4499-2, a carbide with a mean WC grain size in the 0.5–0.8 µm range falls into the submicron class. The 0.2–0.5 µm range is called ultrafine and below 0.2 µm nano. In the market the term 'micrograin' is mostly used for fine-grained grades in the 0.8–1.3 µm band.
What is the difference between P10 and P40?
Both are for the steel group (P); the difference lies in the hardness–toughness balance. P10 is harder and more wear resistant, and is used at higher cutting speeds and in finishing work. P40 is tougher, carries higher feed and interrupted cuts, and is preferred in roughing.
Which grain size is preferred for small-diameter end mills?
Submicron and ultrafine substrates are preferred. The smaller the grain, the sharper and more regular the ground cutting edge; in the 1–3 mm diameter class this becomes decisive because the edge radius drops to the same order of magnitude as the chip thickness itself. Fine grain also gives higher hardness at the same binder content.
Are the carbide grade and the coating selected separately?
They are not selected separately; the substrate and the coating together form a 'grade'. The coating provides hot hardness and chemical stability, while the substrate provides the mechanical support beneath the coating and the edge toughness. A good coating on a weak substrate ends in edge collapse or coating flaking.
What does TRS (transverse rupture strength) measure?
TRS is the bending strength measured according to ISO 3327 by loading a standard specimen at mid-span while it rests on two supports until it breaks. It gives a practical indication of the toughness of the carbide. It is sensitive to specimen geometry: values taken from cylindrical specimens come out roughly 20% higher than those from rectangular-section specimens, which is why values from different sources are not directly comparable.
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