Cemented Carbide

One line: hard grains carry the cutting, a tough binder carries the load. Neither component alone would work.

Weigh the tungsten carbide round blank on the 0.01 g scale. Record the mass.

Fill the graduated cylinder to 50 ml, lower the blank in, and read the new level. The difference is its volume in millilitres.

Density = mass ÷ volume. Expect roughly 14-15 g/cm³ for a cobalt-bonded carbide.

Weigh and displace a piece of the mild steel bar. Steel comes out near 7.85 g/cm³.

Carbide is roughly twice as dense as steel. Tungsten's atomic mass is doing that, and it is your evidence that the blank really is what it claims to be.

Drag a corner of the carbide blank across the mild steel bar with light pressure. It marks the steel easily.

Now try to scratch the carbide with an HSS cutter edge. It will not mark. Hardness is one-way, and that asymmetry is the whole ranking.

Test steel, HSS, carbide and glass against each other in pairs and write them in order. You have reconstructed the logic of the Mohs scale from your own bench.

Inspect an indexable insert under the loupe. Note the blunter included angle compared with HSS — hardness bought with brittleness must be supported by geometry.

Count the usable corners on one insert. When one dulls it is rotated, not resharpened — carbide is too hard to grind on an ordinary wheel.

Rest the blank on the wooden board and press its edge firmly against the steel bar. Note how unforgiving it feels — carbide chips where steel would deform. Do not strike it.

Interrupted cuts and flexible setups favour HSS; continuous cuts, hard material and high speed favour carbide. Write the rule in your own words.

German patent DRP 420,689, Osram Studiengesellschaft, filed 30 March 1923 and published 30 October 1925; method by Karl Schröter. Two things are claimed: a composition — tungsten monocarbide with 10-20 wt% of an iron-group metal — and a manufacturing route, pressing and sintering with a liquid phase present. Krupp acquired the patent in 1926 and launched WIDIA (wie Diamant) in 1927.

Why a lamp company invented a cutting tool. Osram drew tungsten wire for filaments through diamond dies, which were costly and wore out. A synthetic die material was the goal; machining was an application nobody had asked for. The pattern — a material developed for one industry that reshapes another — recurs constantly in materials history.

Cobalt won. The patent lists iron, nickel or cobalt as binder. Cobalt proved best because it wets tungsten carbide grains almost perfectly during liquid-phase sintering, giving a dense body with no voids. Modern grades are still overwhelmingly WC-Co, tuned by grain size and cobalt percentage: more cobalt is tougher and less hard, less cobalt is harder and more brittle.

Why this blueprint does not sinter anything. The process needs powders at 1,400 °C, and fine cobalt powder is a genuine inhalation hazard. Solid sintered carbide — the blank and inserts used here — is inert and safe to handle. The measurements in steps 2-9 establish everything the patent claims about the material without ever making it: density proves the tungsten content, and the scratch order proves the hardness ranking.

What it displaced. High-speed steel (Taylor and White, US 668,270, 1901) gave a tool that survived at 600 °C. Carbide survives past 1,000 °C and cuts several times faster again. It did not replace HSS — drills, taps and end mills for general shop work are still largely HSS, because it can be resharpened and it tolerates flex.