refractory metals

Stanley Glukh

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Apr 30, 1997, 3:00:00 AM4/30/97

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I would appreciate if fellow experts can help to a non-metallurgists.

I need to use some parts made of refractory metals in a vacuum furnace
at 2400 degrees C. The furnace is heated by graphite heating elements.

Questions:

1) Is it possible that carbon vapour could interact with metals such as
tungsten, molybdenum, or tantalum? I know that tungsten carbide is used
to make tools but I have no idea how it is made. Assuming that graphite
elements do not touch any metal would carbon vapour be an efficient
source for carbonization?

2) What is the maximum heating or cooling rate that allows to avoid any
changes in the metals' microstructure? If such changes are unavoidable,
is it possible at least to keep the metal parts non-brittle?

TIA,

Stanley
--

cmw

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May 5, 1997, 3:00:00 AM5/5/97

to Stanley Glukh

> 1) Is it possible that carbon vapour could interact with metals such as
> tungsten, molybdenum, or tantalum? I know that tungsten carbide is used
> to make tools but I have no idea how it is made. Assuming that graphite
> elements do not touch any metal would carbon vapour be an efficient
> source for carbonization?

Absolutely,
While "pure" carbon sublimes well above 2400 C, I think you will find
that 1) heating elements are far from "pure", and 2) at lower vapor
pressure (vacuuum) the sublimation temperature drops dramatically. I am
not really familiar with tantalum, but I process tungsten and molydenum
regularly. In the case of tungsten, you will most assuredly form
carbides in this situation. Molybdenum will contaminate to a somewhat
lesser degree.

My recommendation would be to replace the carbon elements with tungsten
elements. The most major change required is that it will be necessary
to throttle back controls during initial heating to limit the amperage
going to the "cold" elements of there will certainly be overdraw and
breakers tripped. The tungsten elements should hold up for long periods
(several years) at 2400 C, but care must be taken to provide proper
support on tungsten hangers.

> 2) What is the maximum heating or cooling rate that allows to avoid any
> changes in the metals' microstructure? If such changes are unavoidable,
> is it possible at least to keep the metal parts non-brittle?

If dealing with the "pure metals" there is always grain growth at the
elevated temperatures you are considering. I really don't believe there
is any particular "effect" of heating/cooling rates. Recrystallation is
promoted by straining the material prior to heating.

James R. Woodruff
Sr. Materials Engr.
CMW Inc.
visit our web pate at:
http://www.cmwinc.com/cmw

Mike

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May 6, 1997, 3:00:00 AM5/6/97

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Stanley Glukh wrote:
: I would appreciate if fellow experts can help to a non-metallurgists.

: I need to use some parts made of refractory metals in a vacuum furnace
: at 2400 degrees C. The furnace is heated by graphite heating elements.

: Questions:

: 1) Is it possible that carbon vapour could interact with metals such as

: tungsten, molybdenum, or tantalum? I know that tungsten carbide is used
: to make tools but I have no idea how it is made. Assuming that graphite
: elements do not touch any metal would carbon vapour be an efficient
: source for carbonization?

I regularly carburize niobium, tantalum and Ta-W alloys at 1600-2000C in
our vacuum furnace. I don't offhand know what the vapor pressure of the
carbon gases are at that temperature, but they may not be negligible. It
should be easy to figure out from a good set of thermodynamic tables.

Moly is a pretty traditional furnace hardware material -- my furnace is
moly and graphite felt in the hot zone. That seems to work well to 2000C,
who knows about 2400C. Like the graphite elements, oxygen can cause
problems with moly and tungsten. I also see it forming a pretty good
transport species for the carbon, carburizing the metals without direct
contact. If you have any specific questions, I may be able to give you
some better answers -- ar at least point you in the right direction.
Try checking with some of the applications engineer types at a few vacuum
furnace manufacturers. You should be able to find them in the Thomas
Register (now online, too).

: 2) What is the maximum heating or cooling rate that allows to avoid any

: changes in the metals' microstructure? If such changes are unavoidable,
: is it possible at least to keep the metal parts non-brittle?

I think time at temperature would be a bigger problem. Metals are nicely
resistant to thermal shock, at least until they are carburized. I'll be
able to answer that part AFTER I finish my thesis :). There might be some
dimensional changes/warping if heat is applied too quickly, but I am just
pulling that out of thin air. My gut sense is that total time of service
would be the main concern. When the grain size grows too large, you may
get thin sections that are only one grain thick. I had this happen to some
platinum crucibles (perfectly normal). Be clean with everything. I
contaminated the Pt and ended up with some very expensive shiny flakes. I
had to stay out of sight of the boss for a while after that.

I am learning that the hotter you need to go, the work necessary increases
exponentially. This lazy kid thinks working at 2000 is a pain in the butt,
then again what isn't?

Good luck, maybe I can pull some info out for you on something more
specific.

: TIA,

: Stanley
: --

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