Cemented Carbides

§1Carbide in a metal binder

Cemented carbide is a composite: hard ceramic tungsten-carbide grains, far harder than any steel, held together by a tough metallic cobalt binder that cements them into a solid.

Neither constituent alone would do. Tungsten carbide is hard enough to cut almost anything but, like all ceramics, brittle; cobalt is tough but soft. Sintered together — the carbide powder and cobalt pressed and heated until the cobalt melts and binds the grains — they combine the carbide’s hardness with enough of the cobalt’s toughness to survive cutting. This is the same carbide idea that gives tool steels their wear resistance (the materials section), taken to its limit: here the tool is mostly carbide by volume, not merely seeded with it. The result is dense — around 14.5 to 15.6 g/cm³, nearly twice the density of steel — and very hard.

§2Cobalt: hardness against toughness

The single most important variable in a carbide grade is how much cobalt binder it contains. More cobalt means more toughness and less hardness; less cobalt, the reverse.

§3The ISO application groups

Carbide grades are classified for use by an international letter-and-colour system that tells you what material a grade is meant to cut, rather than its exact composition.

§4Coatings

Almost all modern inserts are coated: a few micrometres of even harder ceramic laid over the carbide substrate, combining a tough core with a super-hard, heat-resistant surface.

The coating is only 5–20 µm thick but transforms performance, and it is usually layered — each layer doing a job (the hero). A titanium carbonitride (TiCN) layer resists abrasion; an aluminium oxide (Al₂O₃) layer resists heat and acts as a thermal barrier, letting the tool run faster; a thin titanium nitride (TiN) top layer, the familiar gold colour, lowers friction and doubles as a wear indicator, its gold rubbing off where the tool has worn. Coated inserts routinely cut faster and last longer than uncoated, which is why coating is now standard. The coating gives the tool the best of both: a tough carbide body that resists breakage, under a ceramic skin that resists wear and heat.

§5Why carbide cuts faster

The decisive advantage of carbide over high-speed steel is red hardness — carbide keeps its hardness at temperatures that would soften any steel — so it can run several times faster.

The tool-steel page showed that even high-speed steel softens above about 600 °C, capping its cutting speed. Carbide stays hard far hotter, so the cutting edge survives the greater heat that faster cutting generates. In practice a carbide tool turns steel at perhaps 150–250 m/min where high-speed steel manages 30 — roughly five to eight times the speed, and correspondingly more metal removed per hour. Through Taylor’s law from the cutting-tools page, that higher sustainable speed is the whole economic case for carbide: it shifts the tool-life curve bodily to higher speeds, making cuts that would destroy a steel tool in seconds routine.

§6Inserts and holders

Because carbide is hard, brittle and expensive, it is not made into whole tools but into small replaceable inserts clamped into a steel holder — the indexable insert system.

An insert is a small carbide tile with several cutting edges — a triangle has three per face, a square four — and when one edge dulls the insert is simply indexed round to a fresh edge, then flipped or replaced when all are used. This wastes no carbide on a tool shank that does no cutting, keeps a sharp edge always a few seconds away, and standardises tooling: one holder accepts many insert grades and geometries. The insert’s shape, size, nose radius and clearance are captured in a standard code, so the same seven-element geometry idea from the cutting-tools page reappears as a catalogue designation. Brazed carbide tips on solid shanks survive for special forms, but the indexable insert is the modern norm.

§7Quick reference

The working core of the page on one card rack.