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Re: seaint Digest for 12 Jan 2000

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ON 1/12/99  Dave Meney wrote:

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My client has some existing mining structures (typically conveyor trusses
and trestles, bins and open braced structures) which are currently in
Australia.  He wants to re-use these structures in a new gold mine in
Mongolia.  Some structures currently outdoors will be housed within an
insulated building at the new site.  Other structures, particularly
conveyors, will need to remain outdoors.

Our Australian steel is rated at -10 degrees C (14 degrees F) or -20 degrees
C (-4 degrees F)depending on its thickness.  In Mongolia, temperatures can
get as low as -40 degrees Celsius.

What are the consequences of using the existing steel?  Is brittleness an
issue when stresses are low?  What would be a suitable threshold?  Or should
we simply reject the existing structures and provide new structures built
using suitable low-temperature steel?
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Dave-

I am not sure what the Austrailian "ratings" of =10 degress C or -20 degrees
mean.  Typically, suitability of steels for cold weather service isjudged by
evaluating the Charpy V-Notch toughness characteristics of the material as a
function of termperature.  Typical structural steels display an "S" shaped curve
of toughness vs. temperature.  The portion at low temperatures represents
brittle behavior in which the steel may have toughness on the order of 10 ft-lbs
or less and behaves in a very brittle manner (some describe it as glass).  At
some temperature (different for each steel) the "S" swings upward.  Where this
occurs is called the "transition" temperature.  Above this transition
temperature, the steel's toughness improves significantly.  Eventually you get
to a flat portion at the top of the "S" curve, termed the upper shelf.  This is
where you would really like to operate.  Some steel have upper shelf toughness
on the order of 100 ft-lbs or more.  Others do not.  The tougher the steel at
the service temperature, the better off you are and less likely to fracture.
The toughness of individual steel depends on the chemical composition and more
importantly the type and fineness of the grain structure.  Most standard
structural steels can be produced with outstanding toughness, when "fine grained
practice" is used by the rolling mill.

I presume that the Australian rating of -10C or -20C is an indication of the
transition temperature.  If this is the case, the steel may not be suitable for
service at -40C. Since there is considerable variation in steel, the best way to
tell would be to sacrifice some of the existing structure, and take a series of
Charpy specimens, so that a transition curve ("S" curve) for the material can be
developed.

If you want more information on this, I can send you a pending SAC writeup on
fracture issues for your information. Let me know.

List serve members in the US, please do not request this, it will be available
from FEMA, shortly.

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