I'm sorry about this but I've arrived here half-way through
Not really. I just subscribed to that list myself, and joined
in what appears to be a thread in progress already.
Perhaps you might enlighten me as to the advantages of
finer grain in silver castings as I'm always looking for ways
to improve them.
Here's the deal. This applies, by the way, to virtually any
metal, not just silver. The strength, malleability, tensile
strength, ductility, and other desireable properties of any metal
we use is much greater with a single crystal of the metal, as the
crystal boundaries are much weaker than the crystals themselves,
In addition, impurities will segregate during solidification,
concentrating themselves at the grain boundaries.
As I said, the best metal would be actual single crystal
objects, and in, for example, aerospace use, some items are
actually manufactured to be single crystal objects. The titanium
turbofan blades in jet engines, for example. Needless to say, the
technology needed to get an object to solidify as a single
crystal is complex and way beyond what we can, or should try,
So the next best idea is to minimize the harmful effects of
grain boundaries. When metals have large crains, then stresses
can concentrate at the boundaries, and with large grains, the
actual total surface area of a grain boundary path accross an
object can be fairly small. Have you ever seen a cracked casting
where it seemed the metal was crystalized in the crack? What
you were seeing was fracturing following the grain boundaries.
If the metal has smaller, more uniform grains, then the
boundaries form more of an intertwinded net, allowing stresses to
more uniformly distribute themselves through the mass. Also,
with smaller grains, the total area of those boundaries is
greater, meaning that the impurities which concentrate at the
boundaries will be more dilute. Since these are part of the
reason why the boundaries are weaker, this dilution increases the
strength of the metal mass. And, because the overall surface
area of the net of boundaries is increased, working of the metal,
such as forming or work hardening of any sort, which occurs due
to slip planes in the crystals being deformed, causes less
overall stress at any one crystal boundary, since there is now
more surface area of boundaries over which to distribute the
strain induced by working.
To put it in simpler terms, smaller grained metal will always,
with gold and silver and platinum alloys, at least, be stronger,
take greater working without showing "orange peel effects", or
before cracking or other failure. It can be (if the alloy allows
it) be heat treated or age hardened to a greater degree, and will
show less problems with polishing due to porosity or other such
flaws, which are also often concentrated at grain boundaries.
The only ways to reduce grain size is to either cast such that
the initial crystalization of the metal has lots of nucleation
sites, leading to lots of crystals, and thus smaller ones (this,
I think, is the function of grain refiners) or physical working
and then annealing. When the metal is worked, such as rolled or
drawn or forged, individual crystals are deformed. Annealing
causes these deformed crystals to realign, but into several
smaller cryastals instead of each larger one. If the annealing
temp or time is raised or extended, then these smaller crystals
start to grow together, and grain size increases again. You can
see this with sterling, if you overanneal too hot, you'll find
the resulting metal then will show orange peel textures when
bent or stretched, where the initial piece, before annealing
didn't do it.
Hope this helps.