Subject: Is tungsten carbide ferrous?
From: "Ed Huntress"
Date: 08/02/04 07:49 GMT Standard Time
Message-id:

"Don Wilkins" wrote in message
.. .
On Sat, 07 Feb 2004 17:58:50 GMT, "Ed Huntress"
wrote:

,;"Don Wilkins" wrote in message
...
,; On 6 Feb 2004 17:29:55 -0800,
,; (benwoodward.com) wrote:
,;
,; ,;Thank you for your replies. TC is a ceramic then. I was totally
,; ,;unaware just how alien to steel tc is. I'm classifying it under
'alien
,; ,;technology successfully reverse engineered' and leaving it at
that.:-)
,;
,; I certainly would not classify WC as a ceramic.
,;
,;The you would certainly be wrong, Don. WC is a ceramic.
Tungsten-carbide
,;tools are made of a powder-metallurgy composite of WC ceramic and metal
,;powders.

Well I disagree WC is not a ceramic.

Take a look at a dictionary definition.

HOkay, sometimes I lose track of the confusion that's propagated about this
material, and the common uses of the term "ceramic," as well as the vague
terms used in metallurgy to describe WC and TaC, which are interstitial
carbides. On top of it all, the metalworking- and other industrial fields
have added their own jargon. This is going to be one of my lengthy
dissertations, so I'll apologize in advance and warn off anyone who doesn't
really give a damn. On the other hand, they may find this interesting,
because it's the result of research I did on the history of carbide tools in
1977, plus years of exposure as Materials Editor of American Machinist, and
more recently as Tooling Editor of Machining magazine. You won't see it put
together quite this way anywhere else, I believe.

The term "ceramic" is old and vague as hell. It can include any non-metallic
material, or compound of a metal and a non-metallic material, that's hard
and brittle. You will sometimes see plain sand- and lime-putty mortar, which
has a compression strength of only 75 - 350 psi, described as a "ceramic" in
the building-restoration literature. This is not as ridiculous as it sounds;
when lime putty (hydrated lime, Ca(OH2)) picks up CO2 from the atmosphere,
it becomes CaCO3 -- limestone. That fills the bill: non-metallic and
somewhat hard, somewhat brittle. As reconstituted limestone goes, lime
mortar pretty lousy, but it's still within the definition. The definition of
"ceramic" actually is that broad. Fine-clay dishware is generally described
as "ceramic," as another example.

There is no absolute technical definition, although engineers of advanced
ceramics from around the world got together in 1993 and wrote one for their
own use. It doesn't apply to anything we're interested in. It applies only
to the newest, most advanced engineering ceramics. In Europe, engineers
define plain glass as a ceramic. We're not interested in that definition,
either.

What we're interested in is something that will help us sort out the often
contradictory uses of the term in industry. Here's a good one from
Encyclopedia Britannica:

"Industrial ceramics are commonly understood to be all industrially used
materials that are inorganic, nonmetallic solids. Usually they are metal
oxides (that is, compounds of metallic elements and oxygen), but many
ceramics (especially advanced ceramics) are compounds of metallic elements
and carbon, nitrogen, or sulfur."

There's tungsten carbide for you: a metal (tungsten) compounded with carbon.
It's hard, it's brittle, it's refractory, and it doesn't even melt like a
metal. That's a ceramic. The bonding of W and C fit well within the more
technical chemical definitions, which allow for both ionic and covalent
bonds, plus various combinations thereof.

WC is a particular type of ceramic: an interstitial carbide. Here's
Britannica again:

"Interstitial carbides are derived primarily from relatively large
transition metals acting as a host lattice and the small carbon atoms
occupying interstices of the close-packed metal atoms...Several have
industrial importance, including tungsten carbide (WC) and tantalum carbide
(TaC), which are used as high-speed cutting tools because of their extreme
hardness and chemical inertness."

Now for some background on the confusing, often contradictory uses of the
terms.

Back in the 1920s, some Germans came up with a way to make tools out of this
hard stuff by mixing some WC powder and some cobalt powder, and then
pressing and sintering them together into a composite material. It was NOT a
compound. The carbide particles remained distinct in the composite. It was
something like concrete in that regard.

At about the same time, Philip McKenna (Kennametal) was experimenting with
WC for cutting tools, too, trying to make them out of solid WC. You do this
by taking straight WC powder and pressing it, and then sintering it at
extremely high temperatures. I think WC sinters at around 4700 deg. F. Very
difficult, and quite expensive (what do you make the pressing tools out of,
for example?). He made the WC into billets and then sliced them into cutting
tools with diamond saws.

McKenna had some success but the result was too brittle and too expensive.
It *was* capable of handling higher temperatures than the German stuff, and
it was harder. But it was a no-go commercially. (Aside: the US Army is
making big pieces of solid tungsten carbide again, much like McKenna's
billets he made in the 1920s. Only the Army is using it for artillery
shells.)

So McKenna reverted to the German method, and these carbide/cobalt (and
sometimes nickel) composites became available around the world, and were
commonly called "cemented carbides." The cement, also known as the matrix,
is the cobalt.

First terminology problem: People got sloppy and started dropping the
"cemented" part of "cemented carbide." It was just "carbide," or "tungsten
carbide." Only it wasn't, really. It was some tungsten carbide mixed with
some metal and sintered into a composite material.

Second terminology problem: Metallurgists are more contrary than chemists,
and they sometimes call tungsten carbide "metallike," or even a "metal."
Chemists object. Metallurgists don't care. Machinists wind up confused. It's
metallike, only it allows almost no slippage of crystals along slip planes:
it has virtually no ductility, in other words. It behaves like a ceramic. It
fits the definitions of ceramics that chemists and other scientists use.
Metallurgists still don't care. They have their own little world.

Third terminology problem: As newer ceramics came into play in the emerging
world of "hard materials," they, too, got mixed with metal binders or
matrices to form composites, and were given the name "cermets." Snappy,
descriptive, and neat. However, somebody forgot about
tungsten-carbide/cobalt composites. They're cermets, too. Marketing people
in the cutting tool business didn't care. They had a snappy new name to
apply to their new products, and they weren't going to let lowly old
tungsten carbide horn in on the action. Machinists wind up confused. Tool
companies don't care.

So, does that help? There are no WC tools on the market that I know of.
They're all cemented, metal-matrix composites made of ceramic WC, refractory
metals, and sometimes odds and ends of other metals and ceramic compounds. A
pure mass of WC is a pretty rare thing, and is not used for cutting tools
today. It never was, except for some experiments.

'Sorry for the length. I didn't have time to write it shorter. g

Ed Huntress

Excellent dissertation and very helpful. Thank you.


Dave Baker - Puma Race Engines (www.pumaracing.co.uk)
I'm not at all sure why women like men. We're argumentative, childish,
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