An increase in cobalt content has which of the follow-ing effects on WC—Co cemented carbides: (a) decreases hardness, (b) decreases trans-verse rupture strength, (c) increases hardness, (d) in-creases...

Microsoft Word - COBALT FACTS Cemented Ca
ides.doc
Cobalt Facts, © 2006 CDI 41
7 Cobalt in Cemented Ca
ides


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 ca
on steels and cutting speeds of 25
ft/min were the norm. 1896 saw the start of tungsten ca
ide manufacture when Moissan in France
melted/fused tungsten and ca
on 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 ca
ide material consisting of crushed tungsten ca
ide 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 ca
ide alone is
ittle 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 ca
ide 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 ca
ides:

a) It must have a high melting point – Cobalt: 1493°C
) 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. This pulls the
sintered 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
dissolves 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 ca
ide particles –
cobalt can be produced very finely and grinds down to
1µ. On grinding, it reverts to the close packed
form which is
ittle although in the ca
ide 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 i
eplaceable in ca
ides.


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