Cobalt in Cemented Carbide Facts

The ability to cut metal faster and faster is to a great extent at the heart of the economic growth in the 20th Century. Up until World War I, cutting tools were made from high carbon steels and cutting speeds of 25 ft/min were the norm. 1896 saw the start of cemented carbide manufacture when Moissan in France melted/fused tungsten and carbon together to make diamonds. He didn't but WC resulted. Although mixtures of WC and MoC did get used for cutting, the great leap forward came when Schroeter and Osram produced a carbide material consisting of crushed cemented carbide in cobalt. Iron was the first choice but it was cobalt for reasons which only became clear subsequently, which was the most successful binding material. The need for a binder is paramount as carbide alone is brittle and has little impact strength. The actual driving force however was not for cutting tools but as wire drawing dies. Osram was cut off by a blockade from its sources of diamonds for dies and the carbide route was the alternative they developed. The cutting properties however were quickly exploited and by the 1920's, 150 ft/min cutting speeds were commonplace.

Although nickel has also been used as a binder, cobalt reigns supreme. Why should this be? There are several criteria which govern the performance of a binder for carbides:
a) It must have a high melting point - Cobalt: 1493°C
b) It must have high temperature strength - Cobalt does
c) It must form a liquid phase with WC at a suitable temperature - Cobalt does at 1275°C.Thispullstheinterred part together by surface tension and eliminates voids.
d) It must dissolve WC - Cobalt forms a eutectic with WC at 1275°C/1350°C and at that temperature issolves 10% WC.
e) On cooling, WC should reprecipitate in the bond-in cobalt it does, giving hardness combined with toughness.
f) The binding agent should be capable of being ground very finely to mix with the hard carbide particles- cobalt can be produced very finely and grinds down to << 1µ. On grinding, it reverts to the close packed form which is brittle although in the carbide product, it retains the more ductile cubic form at room temperature.

Cobalt fulfils all the needs of a binder whilst others, like Ni, Fe, etc., only fulfil some. It is this fact that has kept it irreplaceable in carbides.

Production Methods

The accompanying flow chart outlines the production of sintered carbide parts. Cobalt powder itself is as vital ingredient and its size/shape/form, etc., influence the final properties. Originally, powders were made by reducing cobalt oxide in hydrogen. Ultra-fine powders regarded as essential for carbide bonding are made either by direct precipitation or by thermal decomposition and reduction of cobalt oxalate. The powders are around 2µ in size but during milling with the carbide break down even more finely. This is partly due to an induced phase change from FCC to CPH that occurs almost as soon as milling starts. This is the final requirement which cobalt fulfils, it can be produced as a fine powder (10 times finer than nickel) and milling reduces it further to .001µ. This allows intimate mixing and coating of the hard carbide particles. The sintering part of the cycle is carried out in a vacuum or a reducing atmosphere of hydrogen. The cobalt dissolves tungsten and carbon from the carbide and produces a liquid phase around 1320°C. The surface tension of the liquid pulls the pressed part together and causes shrinkage of around 20%. On cooling, carbide is re-precipitated partly as a (WC + Co + C) eutectic.

From the original binary carbides, many other mixed carbides have developed -NbC, TaC, TiC, Cr3C2 - all aimed at giving certain properties allied to specific applications. More recently cutting tools have been coated with TiC to further improve cutting speeds (other coatings used are TaC, TiN, TiCN).